View source for Water on Mars

{{short description|Study of past and present water on Mars}}
{{About||the ''Doctor Who'' special|The Waters of Mars{{!}}''The Waters of Mars''|the band|Water on Mars (band)}}
{{Use mdy dates|date=July 2018}}
[[File:Plan_view_of_Korolev_crater.jpg|thumb|Mars contains water, though mostly as [[permafrost]]. As top surface layer water appears readily visible at some places, such as the polar [[Korolev (Martian crater)|Korolev Crater]].]]

Almost all [[water]] on [[Mars]] today exists as [[Martian ice caps|polar]] [[permafrost]] ice, though it also exists in small quantities as vapor in [[Atmosphere of Mars|the atmosphere]].<ref name=":1">{{cite book |last1=Jakosky |first1=B. M. |last2=Haberle |first2=R.M. |date=1992 |chapter=The Seasonal Behavior of Water on Mars |title=Mars |editor-first=H. H. |editor-last=Kieffer |display-editors=etal |publisher=University of Arizona Press |location=Tucson, Arizona |pages=969–1016}}</ref>

What was thought to be low-volume liquid [[brine]]s in shallow [[Martian soil]], also called [[recurrent slope lineae]],<ref name="Torres 2015">{{cite journal |title=Transient liquid water and water activity at Gale crater on Mars |journal=Nature Geoscience |date= April 13, 2015 |last1=Martín-Torres |first1=F. Javier |last2=Zorzano |first2=María-Paz |last3=Valentín-Serrano |first3=Patricia |last4=Harri |first4=Ari-Matti |last5=Genzer |first5=Maria |doi=10.1038/ngeo2412 |volume=8 |issue=5 |pages=357–361|bibcode=2015NatGe...8..357M }}</ref><ref name="Ojhaetal2015">{{cite journal |last1=Ojha |first1=L. |last2=Wilhelm |first2=M. B. |last3=Murchie |first3=S. L. |last4=McEwen |first4=A. S. |last5=Wray |first5=J. J. |last6=Hanley |first6=J. |last7=Massé |first7=M. |last8=Chojnacki |first8=M. |date=2015 |title=Spectral evidence for hydrated salts in recurring slope lineae on Mars |journal=Nature Geoscience |doi=10.1038/ngeo2546 |volume=8 |issue=11 |pages=829–832|bibcode=2015NatGe...8..829O |s2cid=59152931 }}</ref> may be grains of flowing sand and dust slipping downhill to make dark streaks.<ref name=":2">[https://www.nasa.gov/feature/jpl/recurring-martian-streaks-flowing-sand-not-water "Recurring Martian Streaks: Flowing Sand, Not Water?"] {{Webarchive|url=https://web.archive.org/web/20211208070556/https://www.nasa.gov/feature/jpl/recurring-martian-streaks-flowing-sand-not-water |date=December 8, 2021 }} NASA, November 20, 2017</ref> While most water ice is buried, it is exposed at the surface across several locations on Mars. In the mid-latitudes, it is exposed by impact craters, steep scarps and gullies.<ref>{{Cite journal |last1=Byrne |first1=Shane |last2=Dundas |first2=Colin M. |last3=Kennedy |first3=Megan R. |last4=Mellon |first4=Michael T. |last5=McEwen |first5=Alfred S. |last6=Cull |first6=Selby C. |last7=Daubar |first7=Ingrid J. |last8=Shean |first8=David E. |last9=Seelos |first9=Kimberly D. |last10=Murchie |first10=Scott L. |last11=Cantor |first11=Bruce A. |last12=Arvidson |first12=Raymond E. |last13=Edgett |first13=Kenneth S. |last14=Reufer |first14=Andreas |last15=Thomas |first15=Nicolas |date=2009-09-25 |title=Distribution of Mid-Latitude Ground Ice on Mars from New Impact Craters |url=https://www.science.org/doi/10.1126/science.1175307 |journal=Science |language=en |volume=325 |issue=5948 |pages=1674–1676 |doi=10.1126/science.1175307 |pmid=19779195 |bibcode=2009Sci...325.1674B |s2cid=10657508 |issn=0036-8075 |access-date=July 14, 2023 |archive-date=July 14, 2023 |archive-url=https://web.archive.org/web/20230714044836/https://www.science.org/doi/10.1126/science.1175307 |url-status=live }}</ref><ref>{{Cite journal |last1=Dundas |first1=Colin M. |last2=Bramson |first2=Ali M. |last3=Ojha |first3=Lujendra |last4=Wray |first4=James J. |last5=Mellon |first5=Michael T. |last6=Byrne |first6=Shane |last7=McEwen |first7=Alfred S. |last8=Putzig |first8=Nathaniel E. |last9=Viola |first9=Donna |last10=Sutton |first10=Sarah |last11=Clark |first11=Erin |last12=Holt |first12=John W. |date=2018-01-12 |title=Exposed subsurface ice sheets in the Martian mid-latitudes |journal=Science |language=en |volume=359 |issue=6372 |pages=199–201 |doi=10.1126/science.aao1619 |pmid=29326269 |bibcode=2018Sci...359..199D |s2cid=206662378 |issn=0036-8075|doi-access=free }}</ref><ref>{{Cite journal |last1=Khuller |first1=Aditya |last2=Christensen |first2=Philip |date=2021-01-18 |title=Evidence of Exposed Dusty Water Ice within Martian Gullies |url=https://onlinelibrary.wiley.com/doi/10.1029/2020JE006539 |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=2 |doi=10.1029/2020JE006539 |bibcode=2021JGRE..12606539R |s2cid=234174382 |issn=2169-9097 |access-date=July 27, 2022 |archive-date=June 11, 2024 |archive-url=https://web.archive.org/web/20240611060735/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JE006539 |url-status=live }}</ref> Additionally, water ice is also visible at the surface at the [[Martian polar ice caps#North polar cap|north polar ice cap]].<ref name=":3">{{cite book |last=Carr |first=M. H. |date=1996 |title=Water on Mars |publisher=Oxford University Press |location=New York |page=197}}</ref> Abundant water ice is also present beneath the permanent [[dry ice|carbon dioxide]] ice cap at the Martian south pole. More than 5&nbsp;million&nbsp;km<sup>3</sup> of ice have been detected at or near the surface of Mars, enough to cover the whole planet to a depth of {{convert|35|meters}}.<ref name="ChristensenIceBudget" /> Even more ice might be locked away in the deep subsurface.<ref name=":6">Carr, 2006, p. 173.</ref><ref>
Chryse Planitia  
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Some liquid water may occur transiently on the Martian surface today, but limited to traces of dissolved moisture from the atmosphere and thin films, which are challenging environments for known life.<ref name="Ojhaetal2015"/><ref name="NASA-20131210">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA Mars Spacecraft Reveals a More Dynamic Red Planet |url=http://www.jpl.nasa.gov/news/news.php?release=2013-361&1#1 |date=December 10, 2013 |publisher=[[NASA]] |access-date=December 11, 2013 |archive-date=December 14, 2013 |archive-url=https://web.archive.org/web/20131214013848/http://www.jpl.nasa.gov/news/news.php?release=2013-361&1#1 |url-status=live }}</ref><ref name="Salt and water 2014">{{cite news |url=http://astrobiology.nasa.gov/articles/2014/7/3/liquid-water-from-ice-and-salt-on-mars/ |title=Liquid Water From Ice and Salt on Mars |work=Geophysical Research Letters |publisher=NASA Astrobiology |date=July 3, 2014 |access-date=August 13, 2014 |archive-url=https://web.archive.org/web/20140814092915/http://astrobiology.nasa.gov/articles/2014/7/3/liquid-water-from-ice-and-salt-on-mars/ |archive-date=August 14, 2014 |url-status=dead }}</ref> No evidence of present-day liquid water has been discovered on the planet's surface because under typical Martian conditions (water vapor pressure <1 Pa <ref>{{Cite journal |last1=Fischer |first1=E. |last2=Martínez |first2=G. M. |last3=Rennó |first3=N. O. |last4=Tamppari |first4=L. K. |last5=Zent |first5=A. P. |date=November 2019 |title=Relative Humidity on Mars: New Results From the Phoenix TECP Sensor |url=http://dx.doi.org/10.1029/2019je006080 |journal=Journal of Geophysical Research: Planets |volume=124 |issue=11 |pages=2780–2792 |doi=10.1029/2019je006080 |pmid=32025455 |issn=2169-9097 |pmc=6988475 |bibcode=2019JGRE..124.2780F |access-date=April 11, 2024 |archive-date=June 11, 2024 |archive-url=https://web.archive.org/web/20240611060735/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019JE006080 |url-status=live }}</ref> and ambient atmospheric pressure ~700 Pa <ref>{{Cite journal |last1=Hess |first1=Seymour L. |last2=Henry |first2=Robert M. |last3=Tillman |first3=James E. |date=1979-06-10 |title=The seasonal variation of atmospheric pressure on Mars as affected by the south polar cap |url=http://dx.doi.org/10.1029/jb084ib06p02923 |journal=Journal of Geophysical Research: Solid Earth |volume=84 |issue=B6 |pages=2923–2927 |doi=10.1029/jb084ib06p02923 |bibcode=1979JGR....84.2923H |issn=0148-0227 |access-date=April 11, 2024 |archive-date=June 11, 2024 |archive-url=https://web.archive.org/web/20240611060736/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/JB084iB06p02923 |url-status=live }}</ref>), warming water ice on the Martian surface would [[sublimation (phase transition)|sublime]] at rates of up to 4 meters per year.<ref>{{Cite journal |last1=Khuller |first1=Aditya R. |last2=Clow |first2=Gary D. |date=April 2024 |title=Turbulent Fluxes and Evaporation/Sublimation Rates on Earth, Mars, Titan, and Exoplanets |journal=Journal of Geophysical Research: Planets |language=en |volume=129 |issue=4 |doi=10.1029/2023JE008114 |issn=2169-9097 |doi-access=free |bibcode=2024JGRE..12908114K }}</ref> Before about [[Noachian|3.8&nbsp;billion years ago]], Mars may have had a denser [[atmosphere of Mars|atmosphere]] and higher surface temperatures,<ref name=":7">{{cite journal |last=Pollack |first=J. B. |date=1979 |title=Climatic Change on the Terrestrial Planets |journal=Icarus |volume=37 |issue=3 |pages=479–553 |doi=10.1016/0019-1035(79)90012-5 |bibcode=1979Icar...37..479P}}</ref><ref>{{cite journal |last1=Pollack |first1=J. B. |last2=Kasting |first2=J. F. |last3=Richardson |first3=S. M. |last4=Poliakoff |first4=K. |date=1987 |title=The Case for a Wet, Warm Climate on Early Mars |journal=Icarus |volume=71 |issue=2 |pages=203–224 |doi=10.1016/0019-1035(87)90147-3 |pmid=11539035 |bibcode=1987Icar...71..203P|hdl=2060/19870013977 |hdl-access=free }}</ref><ref>{{cite journal | last = Fairén | first = A. G. | year = 2010 | title = A cold and wet Mars Mars | journal = Icarus | volume = 208 | issue = 1| pages = 165–175 | doi=10.1016/j.icarus.2010.01.006 | bibcode = 2010Icar..208..165F }}</ref><ref>{{cite journal | last = Fairén | first = A. G. | display-authors = etal | year = 2009 | title = Stability against freezing of aqueous solutions on early Mars | url = https://zenodo.org/record/1233311 | journal = Nature | volume = 459 | issue = 7245 | pages = 401–404 | doi = 10.1038/nature07978 | pmid = 19458717 | bibcode = 2009Natur.459..401F | s2cid = 205216655 | access-date = August 29, 2020 | archive-date = August 3, 2020 | archive-url = https://web.archive.org/web/20200803152150/https://zenodo.org/record/1233311 | url-status = live }}</ref> potentially allowing greater amounts of liquid water on the surface,<ref name="sciencedaily">{{cite web|url=https://www.sciencedaily.com/releases/2015/03/150305140447.htm|title=releases/2015/03/150305140447|publisher=sciencedaily.com|access-date=May 25, 2015|archive-date=December 12, 2023|archive-url=https://web.archive.org/web/20231212070055/https://www.sciencedaily.com/releases/2015/03/150305140447.htm|url-status=live}}</ref><ref>{{cite journal | last1 = Villanueva | first1 = G. | last2 = Mumma | first2 = M. | last3 = Novak | first3 = R. | last4 = Käufl | first4 = H. | last5 = Hartogh | first5 = P. | last6 = Encrenaz | first6 = T. | author6-link = Thérèse Encrenaz | last7 = Tokunaga | first7 = A. | last8 = Khayat | first8 = A. | last9 = Smith | first9 = M. | year = 2015 | title = Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs | url = https://zenodo.org/record/1231265 | journal = Science | volume = 348 | issue = 6231 | pages = 218–221 | doi = 10.1126/science.aaa3630 | bibcode = 2015Sci...348..218V | pmid = 25745065 | s2cid = 206633960 | access-date = July 23, 2019 | archive-date = November 1, 2021 | archive-url = https://web.archive.org/web/20211101201308/https://zenodo.org/record/1231265 | url-status = live }}</ref><ref name="Baker">{{cite journal |last1=Baker |first1=V. R. |last2=Strom |first2=R. G. |last3=Gulick |first3=V. C. |last4=Kargel |first4=J. S. |last5=Komatsu |first5=G. |last6=Kale |first6=V. S. |date=1991 |title=Ancient oceans, ice sheets and the hydrological cycle on Mars |journal=Nature |volume=352 |issue=6348 |pages=589–594 |doi=10.1038/352589a0 |bibcode=1991Natur.352..589B|s2cid=4321529 }}</ref><ref>{{cite journal | last1 = Salese | first1 = F. | last2 = Ansan | first2 = V. | last3 = Mangold | first3 = N. | last4 = Carter | first4 = J. | last5 = Anouck | first5 = O. | last6 = Poulet | first6 = F. | last7 = Ori | first7 = G. G. | year = 2016 | title = A sedimentary origin for intercrater plains north of the Hellas basin: Implications for climate conditions and erosion rates on early Mars | journal = Journal of Geophysical Research: Planets | volume = 121 | issue = 11 | pages = 2239–2267 | doi = 10.1002/2016JE005039 | bibcode = 2016JGRE..121.2239S | s2cid = 132873898 | url = https://hal.archives-ouvertes.fr/hal-02305998/file/Salese2016_jgre20597.pdf | access-date = November 22, 2019 | archive-date = March 10, 2020 | archive-url = https://web.archive.org/web/20200310235340/https://hal.archives-ouvertes.fr/hal-02305998/file/Salese2016_jgre20597.pdf | url-status = live }}</ref> possibly including [[Mars ocean hypothesis|a large ocean]]<ref>{{cite journal |last1=Parker |first1=T. J. |last2=Saunders |first2=R. S. |last3=Schneeberger |first3=D. M. |date=1989 |title=Transitional Morphology in West Deuteronilus Mensae, Mars: Implications for Modification of the Lowland/Upland Boundary |journal=Icarus |volume=82 |issue=1 |pages=111–145 |doi=10.1016/0019-1035(89)90027-4 |bibcode=1989Icar...82..111P|s2cid=120460110 }}</ref><ref>{{cite journal |last1=Dohm |first1=J. M. |last2=Baker |first2=Victor R. |date=2009 |title=GRS Evidence and the Possibility of Paleooceans on Mars |journal=Planetary and Space Science |volume=57 |issue=5–6 |pages=664–684 |doi=10.1016/j.pss.2008.10.008 |bibcode=2009P&SS...57..664D |last3=Boynton |first3=William V. |last4=Fairén |first4=Alberto G. |last5=Ferris |first5=Justin C. |last6=Finch |first6=Michael |last7=Furfaro |first7=Roberto |last8=Hare |first8=Trent M. |last9=Janes |first9=Daniel M. |last10=Kargel |first10=Jeffrey S. |last11=Karunatillake |first11=Suniti |last12=Keller |first12=John |last13=Kerry |first13=Kris |last14=Kim |first14=Kyeong J. |last15=Komatsu |first15=Goro |last16=Mahaney |first16=William C. |last17=Schulze-Makuch |first17=Dirk |last18=Marinangeli |first18=Lucia |last19=Ori |first19=Gian G. |last20=Ruiz |first20=Javier |last21=Wheelock |first21=Shawn J. |url=http://eprints.ucm.es/10512/2/25-Marte_9_P%C3%A1gina_01.pdf |access-date=July 23, 2019 |archive-date=September 22, 2017 |archive-url=https://web.archive.org/web/20170922010358/http://eprints.ucm.es/10512/2/25-Marte_9_P%C3%A1gina_01.pdf |url-status=dead }}</ref><ref name="Sea">{{cite web |url=http://www.psrd.hawaii.edu/July03/MartianSea.html |title=PSRD: Ancient Floodwaters and Seas on Mars |work=Planetary Science Research Discoveries |publisher=University of Hawaii |date=July 16, 2003 |access-date=December 18, 2009 |archive-date=January 4, 2011 |archive-url=https://web.archive.org/web/20110104093144/http://www.psrd.hawaii.edu/July03/MartianSea.html |url-status=live }}</ref><ref>{{cite web |url=http://www.spaceref.com/news/viewpr.html?pid=26947 |title=Gamma-Ray Evidence Suggests Ancient Mars Had Oceans |work=SpaceRef |date=November 17, 2008}}</ref> that may have covered one-third of the planet.<ref>{{cite journal |last1=Clifford |first1=S. M. |last2=Parker |first2=T. J. |date=2001 |title=The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains |journal=Icarus |volume=154 |issue=1 |pages=40–79 |doi=10.1006/icar.2001.6671 |bibcode=2001Icar..154...40C|s2cid=13694518 }}</ref><ref name="ReferenceA" /><ref name="third">{{cite web |url=https://www.sciencedaily.com/releases/2010/06/100613181245.htm |title=Ancient ocean may have covered third of Mars |publisher=Science Daily |date=June 14, 2010 |access-date=February 28, 2018 |archive-date=October 9, 2021 |archive-url=https://web.archive.org/web/20211009114213/https://www.sciencedaily.com/releases/2010/06/100613181245.htm |url-status=live }}</ref> Water has also apparently flowed across the surface for short periods at various intervals more recently in Mars' history.<ref>Carr, 2006, pp. 144–147.</ref><ref>{{cite journal |last1=Fassett |first1=C. I. |last2=Dickson |first2=James L. |date=2010 |title=Supraglacial and Proglacial Valleys on Amazonian Mars |journal=Icarus |volume=208 |issue=1 |pages=86–100 |doi=10.1016/j.icarus.2010.02.021 |bibcode=2010Icar..208...86F|last3=Head |first3=James W. |last4=Levy |first4=Joseph S. |last5=Marchant |first5=David R.}}</ref><ref name="flashback">{{cite web |url=http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |title=Flashback: Water on Mars Announced 10 Years Ago |publisher=Space.com |date=June 22, 2000 |access-date=June 23, 2010 |archive-date=December 22, 2010 |archive-url=https://web.archive.org/web/20101222210332/http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |url-status=live }}</ref> [[Aeolis Palus]] in [[Gale (crater)|Gale Crater]], explored by the [[Curiosity (rover)|''Curiosity'' rover]], is the geological remains of an ancient [[freshwater lake]] that could have been a hospitable environment for [[microbial life]].<ref name="NYT-20131209">{{cite news |last=Chang |first=Kenneth |title=On Mars, an Ancient Lake and Perhaps Life |url=https://www.nytimes.com/2013/12/10/science/space/on-mars-an-ancient-lake-and-perhaps-life.html |date=December 9, 2013 |work=[[The New York Times]] |access-date=February 26, 2017 |archive-date=December 9, 2013 |archive-url=https://web.archive.org/web/20131209202521/http://www.nytimes.com/2013/12/10/science/space/on-mars-an-ancient-lake-and-perhaps-life.html |url-status=live }}</ref><ref name="SCI-20131209">{{cite journal |author=Various |title=Science – Special Collection – Curiosity Rover on Mars |url=https://www.science.org/action/doSearch?AllField=Curiosity+Mars |date=December 9, 2013 |journal=[[Science (journal)|Science]] |access-date=June 30, 2022 |archive-date=January 28, 2014 |archive-url=https://web.archive.org/web/20140128102653/http://www.sciencemag.org/site/extra/curiosity/ |url-status=live }}</ref><ref name="lpi.usra.edu">{{cite journal |last1=Parker |first1=T. |date=2000 |title=Argyre Planitia and the Mars Global Hydrologic Cycle |volume=XXXI |bibcode=2000LPI....31.2033P |journal=Lunar and Planetary Science |url=http://www.lpi.usra.edu/meetings/lpsc2000/pdf/2033.pdf |last2=Clifford |first2=S. M. |last3=Banerdt |first3=W. B. |page=2033 |access-date=December 19, 2010 |archive-date=July 6, 2021 |archive-url=https://web.archive.org/web/20210706110818/https://www.lpi.usra.edu/meetings/lpsc2000/pdf/2033.pdf |url-status=live }}</ref><ref name="Heisinger2002">{{cite journal |doi=10.1016/S0032-0633(02)00054-5 |last1=Heisinger |first1=H. |last2=Head |first2=J. |year=2002 |title=Topography and morphology of the Argyre basin, Mars: implications for its geologic and hydrologic history |journal=Planetary and Space Science |volume=50 |pages=939–981 |bibcode=2002P&SS...50..939H |issue=10–11}}</ref> The present-day inventory of '''water on Mars''' can be estimated from spacecraft images, [[remote sensing]] techniques ([[Spectroscopy|spectroscopic]] measurements,<ref>{{cite book |last=Soderblom |first=L. A. |date=1992 |title=The composition and mineralogy of the Martian surface from spectroscopic observations – 0.3 micron to 50 microns |editor-first=H. H. |editor-last=Kieffer |display-editors=etal |publisher=University of Arizona Press |location=Tucson, Arizona |pages=[https://archive.org/details/mars0000unse/page/557 557–593] |isbn=978-0-8165-1257-7 |url=https://archive.org/details/mars0000unse/page/557 }}</ref><ref>{{cite journal |last1=Glotch |first1=T. |first2=P. |last2=Christensen |date=2005 |title=Geologic and mineralogical mapping of Aram Chaos: Evidence for water-rich history |journal=Journal of Geophysical Research |volume=110 |issue=E9 |pages=E09006 |doi=10.1029/2004JE002389 |bibcode=2005JGRE..110.9006G|s2cid=53489327 |doi-access=free }}</ref> [[radar]],<ref name="Holt, J. 2008">{{cite journal |bibcode=2008LPI....39.2441H |year=2008 |title=Radar Sounding Evidence for Ice within Lobate Debris Aprons near Hellas Basin, Mid-Southern Latitudes of Mars |journal=Lunar and Planetary Science |volume=XXXIX |issue=1391 |url=http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2441.pdf |last1=Holt |first1=J. W. |last2=Safaeinili |first2=A. |last3=Plaut |first3=J. J. |last4=Young |first4=D. A. |last5=Head |first5=J. W. |last6=Phillips |first6=R. J. |last7=Campbell |first7=B. A. |last8=Carter |first8=L. M. |last9=Gim |first9=Y. |last10=Seu |first10=R. |first11=Sharad |last11=Team |page=2441 |access-date=December 19, 2010 |archive-date=June 11, 2016 |archive-url=https://web.archive.org/web/20160611110443/http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2441.pdf |url-status=live }}</ref> etc.), and surface investigations from landers and rovers.<ref name="Amos June 2013">{{cite news |first=Jonathan |last=Amos |title=Old Opportunity Mars rover makes rock discovery |date=June 10, 2013 |work=BBC News |url=https://www.bbc.co.uk/news/science-environment-22832673 |access-date=June 22, 2018 |archive-date=October 9, 2021 |archive-url=https://web.archive.org/web/20211009114213/https://www.bbc.co.uk/news/science-environment-22832673 |url-status=live }}</ref><ref name="Clay clues">{{cite news |title=Mars Rover Opportunity Examines Clay Clues in Rock |date=May 17, 2013 |publisher=Jet Propulsion Laboratory, NASA |url=http://www.jpl.nasa.gov/news/news.php?release=2013-167 |access-date=June 16, 2013 |archive-date=June 11, 2013 |archive-url=https://web.archive.org/web/20130611181547/http://www.jpl.nasa.gov/news/news.php?release=2013-167 |url-status=live }}</ref> Geologic evidence of past water includes enormous [[outflow channels]] carved by floods,<ref name="Floods 2015">{{cite news |url=http://spaceref.com/mars/regional-not-global-processes-led-to-huge-martian-floods.html |archive-url=https://archive.today/20150929035120/http://spaceref.com/mars/regional-not-global-processes-led-to-huge-martian-floods.html |url-status=dead |archive-date=September 29, 2015 |title=Regional, Not Global, Processes Led to Huge Martian Floods |work=Planetary Science Institute |via=SpaceRef |date=September 11, 2015 |access-date=September 12, 2015 }}</ref> ancient river [[Valley networks (Mars)|valley networks]],<ref name="Harrison 2005">{{cite journal |last1=Harrison |first1=K |last2=Grimm |first2=R. |date=2005 |title=Groundwater-controlled valley networks and the decline of surface runoff on early Mars |journal=Journal of Geophysical Research |volume=110 |issue=E12 |pages=E12S16 |doi=10.1029/2005JE002455 |bibcode=2005JGRE..11012S16H|s2cid=7755332 |doi-access=free }}</ref><ref name="Howard, A. 2005">{{cite journal |last1=Howard |first1=A. |last2=Moore |first2=Jeffrey M. |last3=Irwin |first3=Rossman P. |date=2005 |title=An intense terminal epoch of widespread fluvial activity on early Mars: 1. Valley network incision and associated deposits |journal=Journal of Geophysical Research |volume=110 |issue=E12 |pages=E12S14 |doi=10.1029/2005JE002459 |bibcode=2005JGRE..11012S14H|s2cid=14890033 |doi-access=free }}</ref> [[River delta|deltas]],<ref>{{cite journal | last1 = Salese | first1 = F. | last2 = Di Achille | first2 = G. | last3 = Neesemann | first3 = A. | last4 = Ori | first4 = G. G. | last5 = Hauber | first5 = E. | year = 2016 | title = Hydrological and sedimentary analyses of well-preserved paleofluvial-paleolacustrine systems at Moa Valles, Mars | journal = Journal of Geophysical Research: Planets | volume = 121 | issue = 2| pages = 194–232 | doi = 10.1002/2015JE004891 | bibcode = 2016JGRE..121..194S | s2cid = 130651090 | doi-access = free }}</ref> and [[dry lake|lakebeds]];<ref name="Irwin III 2005" /><ref name="Fassett2008">{{cite journal |doi=10.1016/j.icarus.2008.06.016 |last1=Fassett |first1=C. |last2=Head |first2=III |year=2008 |title=Valley network-fed, open-basin lakes on Mars: Distribution and implications for Noachian surface and subsurface hydrology |journal=Icarus |volume=198 |issue=1 |pages=37–56 |bibcode=2008Icar..198...37F}}</ref><ref name="Moore2001">{{cite journal |doi=10.1006/icar.2001.6736 |last1=Moore |first1=J. |last2=Wilhelms |first2=D. |date=2001 |title=Hellas as a possible site of ancient ice-covered lakes on Mars |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020050249_2002081883.pdf |journal=Icarus |volume=154 |pages=258–276 |bibcode=2001Icar..154..258M |issue=2 |hdl=2060/20020050249 |s2cid=122991710 |access-date=July 7, 2017 |archive-date=October 9, 2021 |archive-url=https://web.archive.org/web/20211009114217/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020050249_2002081883.pdf |url-status=live }}</ref><ref name="http">{{cite journal |last1=Weitz |first1=C. |first2=T. |last2=Parker |date=2000 |title=New evidence that the Valles Marineris interior deposits formed in standing bodies of water |journal=Lunar and Planetary Science |volume=XXXI |bibcode=2000LPI....31.1693W |url=http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1693.pdf |page=1693 |access-date=December 19, 2010 |archive-date=July 6, 2021 |archive-url=https://web.archive.org/web/20210706112057/http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1693.pdf |url-status=live }}</ref> and the detection of rocks and minerals on the surface that could only have formed in liquid water.<ref>{{cite news |url=http://www.space.com/6033-signs-ancient-mars-wet.html |title=New Signs That Ancient Mars Was Wet |work=Space.com |date=October 28, 2008 |access-date=October 3, 2011 |archive-date=November 10, 2021 |archive-url=https://web.archive.org/web/20211110082153/https://www.space.com/6033-signs-ancient-mars-wet.html |url-status=live }}</ref> Numerous [[Geomorphology|geomorphic]] features suggest the presence of ground ice ([[permafrost]])<ref>{{cite book |last=Squyres |first=S. W. |date=1992 |chapter=Ice in the Martian Regolith |title=Mars |editor-first=H. H. |editor-last=Kieffer |display-authors=etal |publisher=University of Arizona Press |location=Tucson, Arizona |pages=[https://archive.org/details/mars0000unse/page/523 523–554] |isbn=978-0-8165-1257-7 |chapter-url=https://archive.org/details/mars0000unse/page/523 }}</ref> and the movement of ice in [[glacier]]s, both in the recent past<ref name="HeadMarchant2006">{{cite journal |last1=Head |first1=J. |first2=D. |last2=Marchant |date=2006 |title=Modifications of the walls of a Noachian crater in Northern Arabia Terra (24 E, 39 N) during northern mid-latitude Amazonian glacial epochs on Mars: Nature and evolution of Lobate Debris Aprons and their relationships to lineated valley fill and glacial systems (abstract) |journal=Lunar and Planetary Science |volume=37 |page=1128}}</ref><ref>{{cite journal |last=Head |first=J. |display-authors=etal |date=2006 |title=Modification if the dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation |journal=Geophysical Research Letters |page=33 <!-- check this -->|doi=10.1029/2005gl024360 |bibcode=2006GeoRL..33.8S03H |volume=33|issue=8 |s2cid=9653193 |doi-access=free }}</ref><ref>{{cite journal |last1=Head |first1=J. |first2=D. |last2=Marchant |date=2006 |title=Evidence for global-scale northern mid-latitude glaciation in the Amazonian period of Mars: Debris-covered glacial and valley glacial deposits in the 30–50 N latitude band |journal=Lunar and Planetary Science |volume=37 |page=1127}}</ref><ref name="Richard Lewis">{{cite web |first=Richard |last=Lewis |url=http://news.brown.edu/pressreleases/2008/04/martian-glaciers |title=Glaciers Reveal Martian Climate Has Been Recently Active |publisher=Brown University |date=April 23, 2008 |access-date=October 12, 2009 |archive-date=October 12, 2013 |archive-url=https://web.archive.org/web/20131012034101/http://news.brown.edu/pressreleases/2008/04/martian-glaciers |url-status=live }}</ref> and present.<ref name="Plaut, J. 2008">{{cite journal |last1=Plaut |first1=Jeffrey J. |last2=Safaeinili |first2=Ali |last3=Holt |first3=John W. |last4=Phillips |first4=Roger J. |last5=Head |first5=James W. |last6=Seu |first6=Roberto |last7=Putzig |first7=Nathaniel E. |last8=Frigeri |first8=Alessandro |title=Radar Evidence for Ice in Lobate Debris Aprons in the Mid-Northern Latitudes of Mars |doi=10.1029/2008GL036379 |date=2009 |volume=36 |journal=Geophysical Research Letters |url=http://www.planetary.brown.edu/pdfs/3733.pdf |issue=2 |pages=n/a |bibcode=2009GeoRL..36.2203P |s2cid=17530607 |access-date=April 4, 2010 |archive-date=January 23, 2021 |archive-url=https://web.archive.org/web/20210123201616/http://www.planetary.brown.edu/pdfs/3733.pdf |url-status=dead }}</ref> [[Gully (Mars)|Gullies]] and [[Seasonal flows on warm Martian slopes|slope lineae]] along cliffs and crater walls suggest that flowing water continues to shape the surface of Mars, although to a far lesser degree than in the ancient past.

Although the surface of Mars was periodically wet and could have been hospitable to microbial life billions of years ago,<ref name="Wall">{{cite news |last=Wall |first=Mike |title=Q & A with Mars Life-Seeker Chris Carr |date=March 25, 2011 |url=http://www.space.com/11232-mars-life-evolution-carr-interview.html |work=Space.com |access-date=June 16, 2013 |archive-date=June 3, 2013 |archive-url=https://web.archive.org/web/20130603003111/http://www.space.com/11232-mars-life-evolution-carr-interview.html |url-status=live }}</ref> the current environment at the surface is dry and subfreezing, probably presenting an insurmountable obstacle for living organisms. In addition, Mars lacks a thick atmosphere, [[ozone layer]], and [[Earth's magnetic field|magnetic field]], allowing solar and [[cosmic radiation]] to strike the surface unimpeded. The damaging effects of ionizing radiation on cellular structure is another one of the prime limiting factors on the survival of life on the surface.<ref name="Dartnell-1">{{cite journal |title=Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology |journal=Geophysical Research Letters |date=January 30, 2007 |first1=L. R. |last1=Dartnell |last2=Desorgher |last3=Ward |last4=Coates |volume=34 |issue=2 |pages=L02207 |doi=10.1029/2006GL027494 |quote=The damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats. |bibcode=2007GeoRL..34.2207D |s2cid=59046908 |url=http://discovery.ucl.ac.uk/134609/ |doi-access=free |access-date=July 23, 2019 |archive-date=October 7, 2019 |archive-url=https://web.archive.org/web/20191007215744/http://discovery.ucl.ac.uk/134609/ |url-status=live }}</ref><ref name="ionising radiation">{{cite journal |title=Martian sub-surface ionising radiation: biosignatures and geology |journal=Biogeosciences |date=2007 |first1=L. R. |last1=Dartnell |first2=L. |last2=Desorgher |first3=J. M. |last3=Ward |first4=A. J. |last4=Coates |volume=4 |issue=4 |pages=545–558 |quote=This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation. ... Even at a depth of 2 meters beneath the surface, any microbes would likely be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs. |bibcode=2007BGeo....4..545D |doi=10.5194/bg-4-545-2007 |url=http://hal.archives-ouvertes.fr/docs/00/29/76/31/PDF/bg-4-545-2007.pdf |doi-access=free |access-date=September 1, 2019 |archive-date=July 9, 2014 |archive-url=https://web.archive.org/web/20140709003524/http://hal.archives-ouvertes.fr/docs/00/29/76/31/PDF/bg-4-545-2007.pdf |url-status=live }}</ref> Therefore, the best potential locations for discovering [[life on Mars]] may be in subsurface environments.<ref name="subsurface habitability model">{{cite journal |title=A Possible Biochemical Model for Mars |journal=43rd Lunar and Planetary Science Conference |date=2012 |first=A. |last=de Morais |url=http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2943.pdf |access-date=June 5, 2013 |quote=The extensive volcanism at that time much possibly created subsurface cracks and caves within different strata, and the liquid water could have been stored in these subterraneous places, forming large aquifers with deposits of saline liquid water, minerals organic molecules, and geothermal heat – ingredients for life as we know on Earth. |archive-date=July 6, 2021 |archive-url=https://web.archive.org/web/20210706110831/https://www.lpi.usra.edu/meetings/lpsc2012/pdf/2943.pdf |url-status=live }}</ref><ref name="Parnell">{{cite news |first=JohnThomas |last=Didymus |title=Scientists find evidence Mars subsurface could hold life |date=January 21, 2013 |url=http://digitaljournal.com/article/341801 |work=Digital Journal – Science |quote=There can be no life on the surface of Mars, because it is bathed in radiation and it's completely frozen. Life in the subsurface would be protected from that. – Prof. Parnell. |access-date=June 16, 2013 |archive-date=December 13, 2013 |archive-url=https://web.archive.org/web/20131213201137/http://digitaljournal.com/article/341801 |url-status=live }}</ref><ref name=Steigerwald>{{cite news |first=Bill |last=Steigerwald |title=Martian Methane Reveals the Red Planet is not a Dead Planet |date=January 15, 2009 |publisher=NASA |url=http://www.nasa.gov/mission_pages/mars/news/marsmethane.html |work=NASA's Goddard Space Flight Center |quote=If microscopic Martian life is producing the methane, it likely resides far below the surface, where it's still warm enough for liquid water to exist |access-date=June 16, 2013 |archive-date=January 17, 2009 |archive-url=https://web.archive.org/web/20090117141425/http://www.nasa.gov/mission_pages/mars/news/marsmethane.html |url-status=dead }}</ref> Large amounts of underground ice have been found on Mars; the volume of water detected is equivalent to the volume of water in [[Lake Superior]].<ref name="NASA-20161122">{{cite web |author=Staff |title=Scalloped Terrain Led to Finding of Buried Ice on Mars |url=http://photojournal.jpl.nasa.gov/catalog/PIA21136 |date=November 22, 2016 |work=[[NASA]] |access-date=November 23, 2016 |archive-date=November 24, 2016 |archive-url=https://web.archive.org/web/20161124094205/http://photojournal.jpl.nasa.gov/catalog/PIA21136 |url-status=live }}</ref><ref name="Register-2016">{{cite web |url=https://www.theregister.co.uk/2016/11/22/nasa_finds_ice_under_martian_surface/ |title=Lake of frozen water the size of New Mexico found on Mars – NASA |work=The Register |date=November 22, 2016 |access-date=November 23, 2016 |archive-date=November 23, 2016 |archive-url=https://web.archive.org/web/20161123120850/http://www.theregister.co.uk/2016/11/22/nasa_finds_ice_under_martian_surface/ |url-status=live }}</ref><ref name="NASA-20161122jpl">{{cite web |url=http://www.jpl.nasa.gov/news/news.php?release=2016-299 |title=Mars Ice Deposit Holds as Much Water as Lake Superior |publisher=NASA |date=November 22, 2016 |access-date=November 23, 2016 |archive-date=November 23, 2016 |archive-url=https://web.archive.org/web/20161123145052/http://www.jpl.nasa.gov/news/news.php?release=2016-299 |url-status=live }}</ref> In 2018, scientists reported the discovery of [[#Subglacial liquid water|a subglacial lake on Mars]], {{convert|1.5|km|mi|abbr=on}} below the [[Planum Australe|southern polar ice cap]], with a horizontal extent of about {{convert|20|km|mi|abbr=on}}, the first known stable body of liquid water on the planet,<ref name="SCI-20180725"/><ref>{{cite news |last1=Halton |first1=Mary |title=Liquid water 'lake' revealed on Mars |url=https://www.bbc.co.uk/news/science-environment-44952710 |access-date=July 26, 2018 |work=BBC News |date=July 25, 2018 |archive-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725141308/https://www.bbc.co.uk/news/science-environment-44952710 |url-status=live }}</ref> but subsequent work has questioned this detection.<ref>{{cite journal |last1=Grima |first1=Cyril |last2=Mouginot |first2=Jeremie |last3=Kofman |first3=Wlodek |last4=Herique |first4=A. |last5=Beck |first5=P. |title=The Basal Detectability of an Ice-Covered Mars by MARSIS |journal=Geophysical Research Letters |volume=49 |date=January 2022 |issue=2 |doi=10.1029/2021GL096518 |bibcode=2022GeoRL..4996518G |s2cid=246327935 |url=https://hal-insu.archives-ouvertes.fr/insu-03705391/file/Geophysical%20Research%20Letters%20-%202022%20-%20Grima%20-%20The%20Basal%20Detectability%20of%20an%20Ice%25E2%2580%2590Covered%20Mars%20by%20MARSIS.pdf |access-date=August 29, 2022 |archive-date=June 11, 2024 |archive-url=https://web.archive.org/web/20240611060733/https://hal-insu.archives-ouvertes.fr/insu-03705391/file/Geophysical%20Research%20Letters%20-%202022%20-%20Grima%20-%20The%20Basal%20Detectability%20of%20an%20Ice%25E2%2580%2590Covered%20Mars%20by%20MARSIS.pdf |url-status=live }}</ref><ref>{{cite news |url=https://www.space.com/mars-underground-lake-volcanic-rock |last=Howell |first=Elizabeth |title=Mars' suspected underground lake could be just volcanic rock, new study finds |work=[[Space.com]] |date=January 25, 2022 |access-date=April 4, 2022 |archive-date=April 4, 2022 |archive-url=https://web.archive.org/web/20220404083531/https://www.space.com/mars-underground-lake-volcanic-rock |url-status=live }}</ref>

Understanding the extent and situation of water on Mars is vital to assess the planet's potential for harboring life and for providing usable [[in situ resource utilisation|resources for future human exploration]]. For this reason, "Follow the Water" was the science theme of [[NASA]]'s [[Mars Exploration Program]] (MEP) in the first decade of the 21st century. NASA and [[European Space Agency|ESA]] missions including [[2001 Mars Odyssey]], [[Mars Express]], [[Mars Exploration Rover]]s (MERs), [[Mars Reconnaissance Orbiter]] (MRO), and Mars [[Phoenix Lander|''Phoenix'' lander]] have provided information about water's abundance and distribution on Mars.<ref>{{Cite web |last1=Wilson |first1=Jim |last2=Dunbar |first2=Brian |date=August 3, 2017 |title=Mars Overview |url=http://www.nasa.gov/mission_pages/mars/overview/index.html |url-status=dead |archive-url=https://web.archive.org/web/20211209015359/http://www.nasa.gov/mission_pages/mars/overview/index.html |archive-date=December 9, 2021 |website=[[NASA|NASA.gov]]}}</ref> Mars Odyssey, Mars Express, MRO, and [[Curiosity (rover)|Mars Science Lander ''Curiosity'' rover]] are still operating, and discoveries continue to be made. In September 2020, scientists confirmed the existence of several large [[Salt lake|saltwater lakes]] under ice in the [[Planum Australe|south polar region]] of the planet [[Mars]]. According to one of the researchers, "We identified the same body of water [as suggested earlier in a preliminary initial detection], but we also found three other bodies of water around the main one ... It's a complex system."<ref name="NAT-20200928">{{cite journal |last=Lauro |first=Sebastian Emanuel |display-authors=etal |title=Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data |url=https://www.nature.com/articles/s41550-020-1200-6 |date=28 September 2020 |journal=[[Nature Astronomy]] |volume=5 |pages=63–70 |doi=10.1038/s41550-020-1200-6 |arxiv=2010.00870 |bibcode=2021NatAs...5...63L |s2cid=222125007 |access-date=29 September 2020 |archive-date=December 31, 2021 |archive-url=https://web.archive.org/web/20211231032305/https://www.nature.com/articles/s41550-020-1200-6 |url-status=live }}</ref><ref name="NNEWS-20200928">{{cite journal |last=O'Callaghan |first=Jonathan |title=Water on Mars: discovery of three buried lakes intrigues scientists |url=https://www.nature.com/articles/d41586-020-02751-1 |date=28 September 2020 |journal=[[Nature (journal)|Nature]] |doi=10.1038/d41586-020-02751-1 |pmid=32989309 |s2cid=222155190 |access-date=29 September 2020 |archive-date=January 11, 2022 |archive-url=https://web.archive.org/web/20220111055610/https://www.nature.com/articles/d41586-020-02751-1 |url-status=live }}</ref> In March 2021, researchers reported that a considerable amount of water on ancient Mars has remained but that, for the most part, has likely been sequestered into the rocks and crust of the planet over the years.<ref name="NASA-20210316">{{cite news |last1=Hautaluoma |first1=Grey |last2=Johnson |first2=Alana |last3=Good |first3=Andrew |title=New Study Challenges Long-Held Theory of Fate of Mars' Water |url=https://www.jpl.nasa.gov/news/new-study-challenges-long-held-theory-of-fate-of-mars-water |date=16 March 2021 |work=[[NASA]] |access-date=16 March 2021 |archive-date=October 11, 2021 |archive-url=https://web.archive.org/web/20211011081355/https://www.jpl.nasa.gov/news/new-study-challenges-long-held-theory-of-fate-of-mars-water |url-status=live }}</ref><ref name="CNET-20210316">{{cite news |last=Mack |first=Eric |title=Mars hides an ancient ocean beneath its surface |url=https://www.cnet.com/news/mars-hides-an-ancient-ocean-beneath-its-surface/ |date=16 March 2021 |work=[[CNET]] |access-date=16 March 2021 |archive-date=March 17, 2021 |archive-url=https://web.archive.org/web/20210317155333/https://www.cnet.com/news/mars-hides-an-ancient-ocean-beneath-its-surface/ |url-status=live }}</ref><ref name="SCI-20210316">{{cite journal |last=Scheller |first=E. L. |display-authors=et al. |title=Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust |date=16 March 2021 |journal=[[Science (journal)|Science]] |volume=372 |issue=6537 |pages=56–62 |doi=10.1126/science.abc7717|pmc=8370096  |pmid=33727251 |bibcode=2021Sci...372...56S |doi-access=free }}</ref><ref name="NYT-20210319">{{cite news |last=Chang |first=Kenneth |title=The Water on Mars Vanished. This Might Be Where It Went |url=https://www.nytimes.com/2021/03/19/science/mars-water-missing.html |date=19 March 2021 |work=[[The New York Times]] |access-date=19 March 2021 |archive-date=November 24, 2021 |archive-url=https://web.archive.org/web/20211124184254/https://www.nytimes.com/2021/03/19/science/mars-water-missing.html |url-status=live }}</ref> In August 2024, further analysis of data from NASA's [[InSight|InSight Mars Lander]] enabled researchers to discover a reservoir of liquid water at depths of {{convert|10–20|km}} under the Martian crust.<ref>{{Cite journal |last1=Wright |first1=Vashan |last2=Morzfeld |first2=Matthias |last3=Manga |first3=Michael |editor=David Kohlstedt |title=Liquid water in the Martian mid-crust |journal=[[Proceedings of the National Academy of Sciences of the United States of America|PNAS]] |volume=121 |issue=35 |date=August 12, 2024 |pages=e2409983121 |doi=10.1073/pnas.2409983121 |pmid=39133865 |doi-access=free |pmc=11363344 |bibcode=2024PNAS..12109983W }}</ref>

== Historical background ==
{{Main|History of Mars observation}}
The notion of water on Mars preceded the [[space age]] by hundreds of years. Early [[Optical telescope|telescopic]] observers correctly assumed that the white polar caps and clouds were indications of water's presence. These observations, coupled with the fact that Mars has a 24-hour day, led astronomer [[William Herschel]] to declare in 1784 that Mars probably offered its inhabitants "a situation in many respects similar to ours."<ref>Sheehan, 1996, p. 35.</ref>
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| caption1 = Historical map of Mars drawn by [[Giovanni Schiaparelli]] during the planet's "Great Opposition" of 1877.
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}}

By the start of the 20th century, most astronomers recognized that Mars was far colder and drier than Earth. The presence of oceans was no longer accepted, so the paradigm changed to an image of Mars as a "dying" planet with only a meager amount of water. The dark areas, which could be seen to change seasonally, were then thought to be tracts of vegetation.<ref>{{cite book |last1=Kieffer |first1=H.H. |last2=Jakosky |first2=B.M |last3=Snyder |first3=C. |date=1992 |chapter=The Planet Mars: From Antiquity to the Present |title=Mars |editor-first=H.H. |editor-last=Kieffer |display-editors=etal |publisher=University of Arizona Press |location=Tucson, AZ |pages=1–33}}</ref> The person most responsible for popularizing this view of Mars was [[Percival Lowell]] (1855–1916), who imagined a race of Martians constructing a network of [[Martian canals|canals]] to bring water from the poles to the inhabitants at the equator. Although generating tremendous public enthusiasm, Lowell's ideas were rejected by most astronomers. The majority view of the scientific establishment at the time is probably best summarized by English astronomer [[Edward Walter Maunder]] (1851–1928) who compared the climate of Mars to conditions atop a {{convert|20,000|foot|m|adj=on|spell=in}} peak on an arctic island<ref>hartmann, 2003, p. 20.</ref> where only [[lichen]] might be expected to survive.

In the meantime, many astronomers were refining the tool of planetary [[spectroscopy]] in hope of determining the composition of the [[Martian atmosphere]]. Between 1925 and 1943, [[Walter Sydney Adams|Walter Adams]] and [[Theodore Dunham, Jr.|Theodore Dunham]] at the [[Mount Wilson Observatory]] tried to identify oxygen and water vapor in the Martian atmosphere, with generally negative results. The only component of the Martian atmosphere known for certain was carbon dioxide (CO<sub>2</sub>) identified spectroscopically by [[Gerard Kuiper]] in 1947.<ref>Sheehan, 1996, p. 150.</ref> Water vapor was not unequivocally detected on Mars until 1963.<ref>{{cite journal |last1=Spinrad |first1=H. |last2=Münch |first2=G. |last3=Kaplan |first3=L. D. |date=1963 |title=Letter to the Editor: the Detection of Water Vapor on Mars |journal=Astrophysical Journal |volume=137 |page=1319 |doi=10.1086/147613 |bibcode=1963ApJ...137.1319S}}</ref>
[[File:Mariner 4 craters.gif|thumb|left|[[Mariner 4]] acquired this image showing a barren planet (1965).]]

The composition of the [[Martian polar ice caps|polar caps]], assumed to be water ice since the time of [[Giovanni Domenico Cassini|Cassini]] (1666), was questioned by a few scientists in the late 1800s who favored CO<sub>2</sub> ice, because of the planet's overall low temperature and apparent lack of appreciable water. This hypothesis was confirmed theoretically by [[Robert B. Leighton|Robert Leighton]] and [[Bruce C. Murray|Bruce Murray]] in 1966.<ref>{{cite journal |last1=Leighton |first1=R.B. |last2=Murray |first2=B.C. |date=1966 |title=Behavior of Carbon Dioxide and Other Volatiles on Mars |journal=Science |volume=153 |issue=3732 |pages=136–144 |doi=10.1126/science.153.3732.136 |pmid=17831495|bibcode=1966Sci...153..136L |s2cid=28087958 }}</ref> Today it is known that the winter caps at both poles are primarily composed of CO<sub>2</sub> ice, but that a permanent (or perennial) cap of water ice remains during the summer at the northern pole. At the southern pole, a small cap of CO<sub>2</sub> ice remains during summer, but this cap too is underlain by water ice.

The final piece of the Martian climate puzzle was provided by [[Mariner 4]] in 1965. Grainy television pictures from the spacecraft showed a surface dominated by [[impact crater]]s, which implied that the surface was very old and had not experienced the level of erosion and tectonic activity seen on Earth. Little erosion meant that liquid water had probably not played a large role in the planet's [[geomorphology]] for billions of years.<ref>{{cite journal |last=Leighton |first=R.B. |author2=Murray, B.C. |author3=Sharp, R.P. |author4=Allen, J.D. |author5=Sloan, R.K. |date=1965 |title=Mariner IV Photography of Mars: Initial Results |journal=Science |volume=149 |issue=3684 |pages=627–630 |doi=10.1126/science.149.3684.627 |pmid=17747569|bibcode=1965Sci...149..627L |s2cid=43407530 }}</ref> Furthermore, the variations in the radio signal from the spacecraft as it passed behind the planet allowed scientists to calculate the density of the atmosphere. The results showed an atmospheric pressure less than 1% of Earth's at sea level, effectively precluding the existence of liquid water, which would rapidly boil or freeze at such low pressures.<ref>{{cite journal |last1=Kliore |first1=A. |display-authors=etal |date=1965 |title=Occultation Experiment: Results of the First Direct Measurement of Mars's Atmosphere and Ionosphere |journal=Science |volume=149 |issue=3689 |pages=1243–1248 |doi=10.1126/science.149.3689.1243 |pmid=17747455|bibcode=1965Sci...149.1243K |s2cid=34369864 }}</ref> Thus, a vision of Mars was born of a world much like the Moon, but with just a wisp of an atmosphere to blow the dust around. This view of Mars would last nearly another decade until [[Mariner 9]] showed a much more dynamic Mars with hints that the planet's past environment was more clement than the present one.

On January 24, 2014, NASA reported that [[Timeline of Mars Science Laboratory#Current status|current studies]] on Mars by the [[Curiosity (rover)|''Curiosity'']] and [[Opportunity (rover)|''Opportunity'']] [[Mars rover|rovers]] will be searching for evidence of ancient life, including a [[biosphere]] based on [[autotroph]]ic, [[chemotroph]]ic and/or [[Chemolithotrophs|chemo-litho-autotrophic]] [[microorganism]]s, as well as ancient water, including [[Lacustrine plain|fluvio-lacustrine environments]] ([[plain]]s related to ancient rivers or lakes) that may have been [[Planetary habitability|habitable]].<ref name="SCI-20140124a">{{cite journal |last=Grotzinger |first=John P. |title=Introduction to Special Issue – Habitability, Taphonomy, and the Search for Organic Carbon on Mars |journal=[[Science (journal)|Science]] |date=January 24, 2014 |volume=343 |number=6169 |pages=386–387 |doi=10.1126/science.1249944 |bibcode=2014Sci...343..386G |pmid=24458635|doi-access=free }}</ref><ref name="SCI-20140124special">{{cite journal |author=Various |title=Special Issue – Table of Contents – Exploring Martian Habitability |url=https://www.science.org/toc/science/343/6169 |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |number=6169 |pages=345–452 |access-date=June 30, 2022 |archive-date=January 29, 2014 |archive-url=https://web.archive.org/web/20140129042127/http://www.sciencemag.org/content/343/6169.toc |url-status=live }}</ref><ref name="SCI-20140124c">{{cite journal |author=Grotzinger, J.P. |display-authors=etal |title=A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |number=6169 |doi=10.1126/science.1242777 |pages=1242777 |pmid=24324272|bibcode=2014Sci...343A.386G |citeseerx=10.1.1.455.3973 |s2cid=52836398 }}</ref>

For many years it was thought that the observed remains of floods were caused by the release of water from a global water table, but research published in 2015 reveals regional deposits of sediment and ice emplaced 450 million years earlier to be the source.<ref name="Rodriguez 2015">{{cite journal |title=Martian outflow channels: How did their source aquifers form, and why did they drain so rapidly? |journal=Scientific Reports |date=September 8, 2015 |last1=Rodriguez |first1=J. Alexis P. |last2=Kargel |first2= Jeffrey S. |last3=Baker |first3=Victor R. |last4=Gulick |first4=Virginia C. |volume=5 |doi=10.1038/srep13404 |pmid=26346067 |pmc=4562069 |display-authors=etal |pages=13404|bibcode=2015NatSR...513404R }}</ref> "Deposition of sediment from rivers and glacial melt filled giant canyons beneath primordial ocean contained within the planet's northern lowlands. It was the water preserved in these canyon sediments that was later released as great floods, the effects of which can be seen today."<ref name="Floods 2015"/><ref name="Rodriguez 2015"/>

== Evidence from rocks and minerals ==
{{Main|Composition of Mars}}
It is widely accepted that Mars had abundant water very early in its history,<ref name="Space-20120702">{{cite news |author=<!-- Staff writer(s); no by-line --> |title=Ancient Mars Water Existed Deep Underground |url=http://www.space.com/16335-mars-underground-water-impact-craters.html |date=July 2, 2012 |work=[[Space.com]] |access-date=July 13, 2012 |archive-date=May 9, 2021 |archive-url=https://web.archive.org/web/20210509184104/https://www.space.com/16335-mars-underground-water-impact-craters.html |url-status=live }}</ref><ref>{{cite journal |last1=Craddock |first1=R. |last2=Howard |first2=A. |date=2002 |title=The case for rainfall on a warm, wet early Mars |journal=Journal of Geophysical Research |volume=107 |issue=E11 |page=E11 |doi=10.1029/2001je001505 |bibcode=2002JGRE..107.5111C|doi-access=free }}</ref> but all large areas of liquid water have since disappeared. A fraction of this water is retained on modern Mars as both ice and locked into the structure of abundant water-rich materials, including [[clay mineral]]s ([[phyllosilicates]]) and [[sulfate mineral|sulfates]].<ref>{{cite journal |last=Head |first=J. |display-authors=etal |date=2006 |title=Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for the late Amazonian obliquity-driven climate change |journal=Earth and Planetary Science Letters |volume=241 |issue=3–4 |pages=663–671 |bibcode=2006E&PSL.241..663H |doi=10.1016/j.epsl.2005.11.016}}</ref><ref name="wwwspaceref.com">{{cite web |author=Staff |publisher=NASA |date=October 28, 2008 |url=http://www.spaceref.com/news/viewpr.html?pid=26817 |archive-url=https://archive.today/20130202173747/http://www.spaceref.com/news/viewpr.html?pid=26817 |url-status=dead |archive-date=February 2, 2013 |title=NASA Mars Reconnaissance Orbiter Reveals Details of a Wetter Mars |website=SpaceRef }}</ref> Studies of hydrogen isotopic ratios indicate that asteroids and comets from beyond 2.5 [[astronomical unit]]s (AU) provide the source of Mars' water,<ref name="Lunine 2003">{{cite journal |title=The Origin of Water on Mars |journal=Icarus |date=September 2003 |first1=Jonathan I. |last1=Lunine |first2=John |last2=Chambers |display-authors=etal |volume=165 |issue=1 |doi=10.1016/S0019-1035(03)00172-6 |bibcode=2003Icar..165....1L |pages=1–8}}</ref> that currently totals 6% to 27% of the Earth's present ocean.<ref name="Lunine 2003" />
[[File:History of Water on Mars.jpg|left|thumb|History of water on Mars. Numbers represent how many billions of years ago.]]

=== Water in weathering products (aqueous minerals) ===
The primary rock type on the surface of Mars is [[basalt]], a fine-grained [[igneous]] rock made up mostly of the [[mafic]] silicate minerals [[olivine]], [[pyroxene]], and [[plagioclase feldspar]].<ref>{{cite book |last1=Soderblom |first1=L. A. |last2=Bell |first2=J. F. |date=2008 |chapter=Exploration of the Martian Surface: 1992–2007 |title=The Martian Surface: Composition, Mineralogy, and Physical Properties |url=https://archive.org/details/martiansurfaceco00bell |url-access=limited |editor-first=J. F. |editor-last=Bell |publisher=Cambridge University Press |pages=[https://archive.org/details/martiansurfaceco00bell/page/n22 3]–19|bibcode=2008mscm.book.....B |isbn=9780521866989 }}</ref> When exposed to water and atmospheric gases, these minerals [[chemical weathering|chemically weather]] into new (secondary) minerals, some of which may incorporate water into their crystalline structures, either as H<sub>2</sub>O or as [[hydroxyl]] (OH). Examples of [[Mineral hydration|hydrated]] (or hydroxylated) minerals include the iron hydroxide [[goethite]] (a common component of terrestrial [[soils]]); the [[evaporite]] minerals [[gypsum]] and [[kieserite]]; [[opal|opa]]line silica; and [[phyllosilicate]]s (also called [[clay minerals]]), such as [[kaolinite]] and [[montmorillonite]]. All of these minerals have been detected on Mars.<ref>{{cite book |last1=Ming |first1=D. W. |last2=Morris |first2=R. V. |last3=Clark |first3=R. C. |date=2008 |chapter=Aqueous Alteration on Mars |title=The Martian Surface: Composition, Mineralogy, and Physical Properties |url=https://archive.org/details/martiansurfaceco00bell |url-access=limited |editor-first=J. F. |editor-last=Bell |publisher=Cambridge University Press |pages=[https://archive.org/details/martiansurfaceco00bell/page/n570 519]–540|bibcode=2008mscm.book.....B |isbn=9780521866989 }}</ref>

One direct effect of chemical weathering is to consume water and other reactive chemical species, taking them from mobile reservoirs like the [[atmosphere]] and [[hydrosphere]] and sequestering them in rocks and minerals.<ref>{{cite book |last=Lewis |first=J. S. |date=1997 |title=Physics and Chemistry of the Solar System |edition=revised |publisher=Academic Press |location=San Diego, California |isbn=978-0-12-446742-2}}</ref> The amount of water in the Martian crust stored as [[Mineral hydration|hydrated minerals]] is currently unknown, but may be quite large.<ref>{{cite journal |last=Lasue |first=J. |display-authors=etal |date=2013 |title=Quantitative Assessments of the Martian Hydrosphere |journal=Space Science Reviews |volume=174 |issue=1–4 |pages=155–212 |doi=10.1007/s11214-012-9946-5|bibcode=2013SSRv..174..155L |s2cid=122747118 }}</ref> For example, mineralogical models of the rock outcroppings examined by instruments on the [[Opportunity (rover)|''Opportunity'' rover]] at [[Meridiani Planum]] suggest that the [[sulfate mineral|sulfate]] deposits there could contain up to 22% water by weight.<ref>{{cite journal |last=Clark |first=B. C. |display-authors=etal |date=2005 |title=Chemistry and Mineralogy of Outcrops at Meridiani Planum |journal=Earth and Planetary Science Letters |volume=240 |issue=1 |pages=73–94 |doi=10.1016/j.epsl.2005.09.040 |bibcode=2005E&PSL.240...73C}}</ref>

On Earth, all chemical weathering reactions involve water to some degree.<ref>{{cite book |last=Bloom |first=A. L. |date=1978 |title=Geomorphology: A Systematic Analysis of Late Cenozoic Landforms |url=https://archive.org/details/geomorphologysys0000bloo |url-access=registration |publisher=Prentice-Hall |location=Englewood Cliffs, New Jersey <!-- Please check |ISBN=0-13-353080-68 --> |page=[https://archive.org/details/geomorphologysys0000bloo/page/114 114]|isbn=9780133530865 }}</ref> Thus, many secondary minerals do not actually incorporate water, but still require water to form. Some examples of anhydrous secondary minerals include many [[carbonate minerals|carbonates]], some [[sulfate minerals|sulfates]] (e.g., [[anhydrite]]), and metallic oxides such as the iron oxide mineral [[hematite]]. On Mars, a few of these weathering products may theoretically form without water or with scant amounts present as ice or in thin molecular-scale films ([[monolayers]]).<ref>{{cite journal |last=Boynton |first=W. V. |display-authors=etal |date=2009 |title=Evidence for Calcium Carbonate at the Mars Phoenix Landing Site |journal=Science |volume=325 |pages=61–4 |doi=10.1126/science.1172768 |pmid=19574384 |issue=5936|bibcode=2009Sci...325...61B |s2cid=26740165 }}</ref><ref>{{cite book |last1=Gooding |first1=J. L. |last2=Arvidson |first2=R. E. |last3=Zolotov |first3=M. Yu. |date=1992 |chapter=Physical and Chemical Weathering |title=Mars |editor-first=H. H. |editor-last=Kieffer |display-editors=etal |publisher=University of Arizona Press |location=Tucson, Arizona |pages=[https://archive.org/details/mars0000unse/page/626 626–651] |isbn=978-0-8165-1257-7 |chapter-url=https://archive.org/details/mars0000unse/page/626 }}</ref> The extent to which such exotic weathering processes operate on Mars is still uncertain. Minerals that incorporate water or form in the presence of water are generally termed "aqueous minerals".

Aqueous minerals are sensitive indicators of the type of environment that existed when the minerals formed. The ease with which aqueous reactions occur (see [[Gibbs free energy]]) depends on the pressure, temperature, and on the concentrations of the gaseous and soluble species involved.<ref>{{cite book |last=Melosh |first=H. J. |date=2011 |title=Planetary Surface Processes |url=https://archive.org/details/planetarysurface00melo |url-access=limited |publisher=Cambridge University Press |isbn=978-0-521-51418-7 |page=[https://archive.org/details/planetarysurface00melo/page/n316 296]}}</ref> Two important properties are [[pH]] and [[reduction potential|oxidation-reduction potential (E<sub>h</sub>)]]. For example, the sulfate mineral [[jarosite]] forms only in low pH (highly acidic) water. Phyllosilicates usually form in water of neutral to high pH (alkaline). E<sub>h</sub> is a measure of the [[oxidation state]] of an aqueous system. Together E<sub>h</sub> and pH indicate the types of minerals that are thermodynamically most stable and therefore most likely to form from a given set of aqueous components. Thus, past environmental conditions on Mars, including those conducive to life, can be inferred from the types of minerals present in the rocks.

=== Hydrothermal alteration ===
Aqueous minerals can also form in the subsurface by [[hydrothermal]] fluids migrating through pores and fissures. The heat source driving a hydrothermal system may be nearby [[magma]] bodies or residual heat from large [[impact crater|impacts]].<ref>{{cite journal |last1=Abramov |first1=O. |last2=Kring |first2=D. A. |date=2005 |title=Impact-Induced Hydrothermal Activity on Early Mars |journal=Journal of Geophysical Research |volume=110 |issue=E12 |page=E12S09 |doi=10.1029/2005JE002453 |bibcode=2005JGRE..11012S09A|s2cid=20787765 |doi-access=free }}</ref> One important type of hydrothermal alteration in the Earth's oceanic crust is [[serpentinite|serpentinization]], which occurs when seawater migrates through [[ultramafic]] and basaltic rocks. The water-rock reactions result in the oxidation of ferrous iron in olivine and pyroxene to produce ferric iron (as the mineral [[magnetite]]) yielding molecular [[hydrogen]] (H<sub>2</sub>) as a byproduct. The process creates a highly alkaline and reducing (low Eh) environment favoring the formation of certain phyllosilicates (serpentine minerals) and various carbonate minerals, which together form a rock called [[serpentinite]].<ref>{{cite journal |last1=Schrenk |first1=M. O. |last2=Brazelton |first2=W. J. |last3=Lang |first3=S. Q. |date=2013 |title=Serpentinization, Carbon, and Deep Life |journal=Reviews in Mineralogy & Geochemistry |volume=75 |issue=1 |pages=575–606 |doi=10.2138/rmg.2013.75.18|bibcode=2013RvMG...75..575S |s2cid=8600635 }}</ref> The hydrogen gas produced can be an important energy source for [[chemosynthesis|chemosynthtetic]] organisms or it can react with CO<sub>2</sub> to produce [[methane]] gas, a process that has been considered as a non-biological source for the trace amounts of methane reported in the Martian atmosphere.<ref>{{cite journal |last=Baucom |first=Martin |title=Life on Mars? |journal=American Scientist |date=March–April 2006 |volume=94 |issue=2 |pages=119 |doi=10.1511/2006.58.119 |url=http://www.americanscientist.org/issues/pub/life-on-mars |access-date=October 23, 2013 |archive-date=June 15, 2017 |archive-url=https://web.archive.org/web/20170615115316/http://www.americanscientist.org/issues/pub/life-on-mars |url-status=dead }}</ref> Serpentine minerals can also store a lot of water (as hydroxyl) in their crystal structure. A recent study has argued that hypothetical serpentinites in the ancient highland crust of Mars could hold as much as a {{convert|500|m}}-thick global equivalent layer (GEL) of water.<ref>{{citation |last1=Chassefière |first1=E |last2=Langlais |first2=B. |last3=Quesnel |first3=Y. |last4=Leblanc |first4=F. |date=2013 |title=The Fate of Early Mars' Lost Water: The Role of Serpentinization |work=EPSC Abstracts |volume=8 |page=EPSC2013-188 |url=http://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-188.pdf |access-date=October 23, 2013 |archive-date=July 6, 2021 |archive-url=https://web.archive.org/web/20210706110825/https://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-188.pdf |url-status=live }}</ref> Although some serpentine minerals have been detected on Mars, no widespread outcroppings are evident from remote sensing data.<ref>{{cite journal |last1=Ehlmann |first1=B. L. |author2-link=John F. Mustard |last2=Mustard |first2=J. F. |last3=Murchie |first3=S. L. |date=2010 |title=Geologic Setting of Serpentine Deposits on Mars |journal=Geophysical Research Letters |volume=37 |issue=6 |page=L06201 |doi=10.1029/2010GL042596 |bibcode=2010GeoRL..37.6201E |s2cid=10738206 |url=https://authors.library.caltech.edu/34912/1/2010GL042596.pdf |access-date=July 23, 2019 |archive-date=September 18, 2021 |archive-url=https://web.archive.org/web/20210918070155/https://authors.library.caltech.edu/34912/1/2010GL042596.pdf |url-status=live }}</ref> This fact does not preclude the presence of large amounts of serpentinite hidden at depth in the Martian crust.

=== Weathering rates ===
The rates at which primary minerals convert to secondary aqueous minerals vary. Primary silicate minerals crystallize from magma under pressures and temperatures vastly higher than conditions at the surface of a planet. When exposed to a surface environment these minerals are out of [[chemical equilibrium|equilibrium]] and will tend to interact with available chemical components to form more stable mineral phases. In general, the silicate minerals that crystallize at the highest temperatures (solidify first in a cooling magma) weather the most rapidly.<ref>{{cite book |last=Bloom |first=A. L. |date=1978 |title=Geomorphology: A Systematic Analysis of Late Cenozoic Landforms |url=https://archive.org/details/geomorphologysys0000bloo |url-access=registration |publisher=Prentice-Hall |location=Englewood Cliffs, New Jersey <!-- Please check |ISBN=0-13-353080-68 -->|isbn=9780133530865 }}., p. 120</ref><ref>Melosh, H.J., 2011. Planetary surface processes. Cambridge Univ. Press., pp. 500</ref> On Earth and Mars, the most common mineral to meet this criterion is [[olivine]], which readily weathers to [[clay minerals]] in the presence of water. Olivine is widespread on Mars,<ref>{{cite journal |last=Ody |first=A. |display-authors=etal |date=2013 |title=Global Investigation of Olivine on Mars: Insights into Crust and Mantle Compositions |journal=Journal of Geophysical Research |volume=118 |issue=2 |pages=234–262 |doi=10.1029/2012JE004149 |bibcode=2013JGRE..118..234O|doi-access=free }}</ref> suggesting that Mars' surface has not been pervasively altered by water; abundant geological evidence suggests otherwise.<ref>{{cite journal |title=Noble Gases in Iddingsite from the Lafayette meteorite: Evidence for Liquid water on Mars in the last few hundred million years |journal=Meteoritics and Planetary Science |volume=35 |issue=1 |pages=107–115 |date=2000 |doi=10.1111/j.1945-5100.2000.tb01978.x |last1=Swindle |first1=T. D. |last2=Treiman |first2=A. H. |last3=Lindstrom |first3=D. J. |last4=Burkland |first4=M. K. |last5=Cohen |first5=B. A. |last6=Grier |first6=J. A.|author6-link=JA Grier |last7=Li |first7=B. |last8=Olson |first8=E. K. |bibcode=2000M&PS...35..107S |doi-access=free }}</ref><ref>{{cite journal |last1=Head |first1=J. |last2=Kreslavsky |first2=M. A. |last3=Ivanov |first3=M. A. |last4=Hiesinger |first4=H. |last5=Fuller |first5=E. R. |last6=Pratt |first6=S. |date=2001 |title=Water in Middle Mars History: New Insights From MOLA Data |journal= AGU Spring Meeting Abstracts|volume=2001 |pages=P31A–02 INVITED |bibcode=2001AGUSM...P31A02H }}</ref><ref>{{cite journal |last=Head |first=J. |display-authors=etal |date=2001 |title=Exploration for standing Bodies of Water on Mars: When Were They There, Where did They go, and What are the Implications for Astrobiology? |bibcode=2001AGUFM.P21C..03H |journal= AGU Fall Meeting Abstracts|volume=21 |pages=P21C–03 }}</ref>

=== Martian meteorites ===
[[File:ALH84001.jpg|thumb|Mars meteorite [[ALH84001]].]]
Over 60 meteorites have been found that came from Mars.<ref>Meyer, C. (2012) The Martian Meteorite Compendium; National Aronautics and Space Administration. http://curator.jsc.nasa.gov/antmet/mmc/ {{Webarchive|url=https://web.archive.org/web/20210507123018/http://curator.jsc.nasa.gov/antmet/mmc/ |date=May 7, 2021 }}.</ref> Some of them contain evidence that they were exposed to water when on Mars. Some [[Martian meteorite]]s called [[basalt]]ic shergottites, appear (from the presence of hydrated [[carbonate]]s and [[sulfate]]s) to have been exposed to liquid water prior to ejection into space.<ref>{{cite web |url=http://www2.jpl.nasa.gov/snc/shergotty.html |title=Shergotty Meteorite – JPL, NASA |publisher=NASA |access-date=December 19, 2010 |archive-date=January 18, 2011 |archive-url=https://web.archive.org/web/20110118011546/http://www2.jpl.nasa.gov/snc/shergotty.html |url-status=live }}</ref><ref>{{cite journal |last1=Hamiliton |first1=W. |last2=Christensen |first2=Philip R. |last3=McSween |first3=Harry Y. |date=1997 |title=Determination of Martian meteorite lithologies and mineralogies using vibrational spectroscopy |journal=Journal of Geophysical Research |volume=102 |issue=E11 |pages=25593–25603 |doi=10.1029/97JE01874 |bibcode=1997JGR...10225593H}}</ref> It has been shown that another class of meteorites, the [[nakhlite]]s, were suffused with liquid water around 620&nbsp;million years ago and that they were ejected from Mars around 10.75&nbsp;million years ago by an asteroid impact. They fell to Earth within the last 10,000 years.<ref name=Nakhlites>{{cite journal |url=http://www.lpi.usra.edu/science/treiman/nakhlite_rev.pdf |last=Treiman |first=A. |title=The nakhlite meteorites: Augite-rich igneous rocks from Mars |access-date=September 8, 2006 |journal=Chemie der Erde – Geochemistry |volume=65 |pages=203–270 |date=2005 |doi=10.1016/j.chemer.2005.01.004 |bibcode=2005ChEG...65..203T |issue=3 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327135357/http://www.lpi.usra.edu/science/treiman/nakhlite_rev.pdf |url-status=live }}</ref> Martian meteorite [[Northwest Africa 7034|NWA 7034]] has one order of magnitude more water than most other Martian meteorites. It is similar to the basalts studied by rover missions, and it was formed in the early [[Amazonian (Mars)|Amazonian epoch]].<ref>{{cite journal|title=Unique Meteorite from Early Amazonian Mars: Water-Rich Basaltic Breccia Northwest Africa 7034|first1=Carl B.|last1=Agee|first2=Nicole V.|last2=Wilson|first3=Francis M.|last3=McCubbin|first4=Karen|last4=Ziegler|first5=Victor J.|last5=Polyak|first6=Zachary D.|last6=Sharp|first7=Yemane|last7=Asmerom|first8=Morgan H.|last8=Nunn|first9=Robina|last9=Shaheen|first10=Mark H.|last10=Thiemens|first11=Andrew|last11=Steele|first12=Marilyn L.|last12=Fogel|first13=Roxane|last13=Bowden|first14=Mihaela|last14=Glamoclija|first15=Zhisheng|last15=Zhang|first16=Stephen M.|last16=Elardo|date=February 15, 2013|journal=Science|volume=339|issue=6121|pages=780–785|doi=10.1126/science.1228858|pmid=23287721|bibcode=2013Sci...339..780A|s2cid=206544554|doi-access=free}}</ref><ref>{{cite journal |last=Agree |first=C. |display-authors=etal |year=2013 |title=Unique Meteorite from Early Amazonian Mars: Water-Rich Basaltic Breccia Northwest Africa 7034 |journal=Science |volume=339 |issue=6121 |pages=780–785|doi=10.1126/science.1228858 |pmid=23287721 |bibcode=2013Sci...339..780A |s2cid=206544554 |doi-access=free }}</ref>

In 1996, a group of scientists reported the possible presence of microfossils in the [[Allan Hills 84001]], a meteorite from Mars.<ref>{{cite journal |doi=10.1126/science.273.5277.924 |last1=McKay |first1=D. Jr. |last2=Gibson |first2=E. K. |last3=Thomas-Keprta |first3=K. L. |last4=Vali |first4=H. |last5=Romanek |first5=C. S. |last6=Clemett |first6=S. J. |last7=Chillier |first7=X. D. |last8=Maechling |first8=C. R. |last9=Zare |first9=R. N. |date=1996 |title=Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite AL84001 |journal=Science |volume=273 |issue=5277 |pages=924–930 |pmid=8688069 |bibcode=1996Sci...273..924M|s2cid=40690489 }}</ref> Many studies disputed the validity of their interpretation mainly based on the shape of these presumed fossils.<ref>{{cite news |last1=Gibbs |first1=W. |first2=C. |last2=Powell |title=Bugs in the Data? |date=August 19, 1996 |work=Scientific American |url=http://www.scientificamerican.com/article.cfm?id=bugs-in-the-data |access-date=December 19, 2010 |archive-date=October 17, 2012 |archive-url=https://web.archive.org/web/20121017033400/http://www.scientificamerican.com/article.cfm?id=bugs-in-the-data |url-status=live }}</ref><ref>{{cite web |url=http://www.space.com/scienceastronomy/solarsystem/mars_meteorite_020320.html |title=Controversy Continues: Mars Meteorite Clings to Life – Or Does It? |publisher=Space.com |date=March 20, 2002 |access-date=November 27, 2009 |archive-date=April 4, 2002 |archive-url=https://web.archive.org/web/20020404034759/http://www.space.com/scienceastronomy/solarsystem/mars_meteorite_020320.html |url-status=dead }}</ref> It was found that most of the [[organic matter]] in the meteorite was of terrestrial origin.<ref>{{cite journal |doi=10.1126/science.279.5349.362 |last1=Bada |first1=J. |last2=Glavin |first2=D. P. |last3=McDonald |first3=G. D. |last4=Becker |first4=L. |date=1998 |title=A Search for Endogenous Amino Acids in Martian Meteorite AL84001 |journal=Science |volume=279 |issue=5349 |pages=362–365 |pmid=9430583 |bibcode=1998Sci...279..362B|s2cid=32301715 }}</ref> In addition, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection".<ref name=morphology>{{cite book | title = Instruments, Methods, and Missions for Astrobiology II | journal = SPIE Proceedings | date = December 30, 1999 | first1 = Juan-Manuel Garcia-Ruiz | volume = 3755 | pages = 74–82 | doi = 10.1117/12.375088 | quote = It is concluded that 'morphology cannot be used unambiguously as a tool for primitive life detection'.| last1 = Garcia-Ruiz | editor-first1 = Richard B. | editor-last1 = Hoover | chapter = Morphological behavior of inorganic precipitation systems | s2cid = 84764520 }}</ref><ref>{{cite news|last1=Agresti|last2=House|last3=Jögi|last4=Kudryavstev|last5=McKeegan|last6=Runnegar|last7=Schopf|last8=Wdowiak|title=Detection and geochemical characterization of Earth's earliest life|date=December 3, 2008|publisher=NASA|url=http://astrobiology.ucla.edu/pages/res3e.html|work=NASA Astrobiology Institute|access-date=January 15, 2013|url-status=dead|archive-url=https://web.archive.org/web/20130123132429/http://astrobiology.ucla.edu/pages/res3e.html|archive-date=January 23, 2013}}</ref><ref>{{cite journal | title = Evidence of Archean life: Stromatolites and microfossils | journal = Precambrian Research | date = April 28, 2007 | first1 = J. William | last1 = Schopf | first2 = Anatoliy B. | last2 = Kudryavtsev | first3 = Andrew D. | last3 = Czaja | first4 = Abhishek B. | last4 = Tripathi | volume = 158 | issue = 3–4 | pages = 141–155 | url = http://www.cornellcollege.edu/geology/courses/greenstein/paleo/schopf_07.pdf | access-date = January 15, 2013 | doi = 10.1016/j.precamres.2007.04.009 | bibcode = 2007PreR..158..141S | archive-url = https://web.archive.org/web/20121224202951/http://www.cornellcollege.edu/geology/courses/greenstein/paleo/schopf_07.pdf | archive-date = December 24, 2012 | url-status = dead }}</ref> Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.<ref name=morphology/>

== Geomorphic evidence ==
=== Lakes and river valleys ===
{{see also|Lakes on Mars}}
The 1971 [[Mariner 9]] spacecraft caused a revolution in our ideas about water on Mars. Huge river valleys were found in many areas. Images showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers.<ref name="Floods 2015"/> Areas of branched streams, in the southern hemisphere, suggested that rain once fell.<ref name="Raeburn" /><ref name="Moore" /> The numbers of recognised valleys has increased through time. Research published in June 2010 mapped 40,000 river valleys on Mars, roughly quadrupling the number of river valleys that had previously been identified.<ref name=third /> Martian water-worn features can be classified into two distinct classes: 1) dendritic (branched), terrestrial-scale, widely distributed, [[Noachian]]-age [[Valley networks (Mars)|valley networks]] and 2) exceptionally large, long, single-thread, isolated, [[Hesperian]]-age [[outflow channels]]. Recent work suggests that there may also be a class of currently enigmatic, smaller, younger ([[Hesperian]] to [[Amazonian (Mars)|Amazonian]]) channels in the mid-latitudes, perhaps associated with the occasional local melting of ice deposits.<ref name="Berman">{{cite journal |last1=Berman |first1=Daniel C. |last2=Crown |first2=David A. |last3=Bleamaster |first3=Leslie F. |date=2009 |pages=77–95 |volume=200 |issue=1 |journal=Icarus |title=Degradation of mid-latitude craters on Mars |doi=10.1016/j.icarus.2008.10.026 |bibcode=2009Icar..200...77B}}</ref><ref name="Fassett">{{cite journal |last1=Fassett |first1=Caleb I. |last2=Head |first2=James W. |date=2008 |pages=61–89 |volume=195 |issue=1 |journal=Icarus |title=The timing of martian valley network activity: Constraints from buffered crater counting |doi=10.1016/j.icarus.2007.12.009 |bibcode=2008Icar..195...61F}}</ref>

[[File:Kasei Valles topo.jpg|thumb|Kasei Valles—a major outflow channel—seen in [[Mars Orbiter Laser Altimeter|MOLA]] elevation data. Flow was from bottom left to right. Image is approx. 1600&nbsp;km across. The channel system extends another 1200&nbsp;km south of this image to [[Echus Chasma]].]]

Some parts of Mars show [[inverted relief]]. This occurs when sediments are deposited on the floor of a stream and then become resistant to erosion, perhaps by cementation. Later the area may be buried. Eventually, erosion removes the covering layer and the former streams become visible since they are resistant to erosion.<ref>{{cite web | url=https://www.uahirise.org/hipod/PSP_002424_1765 | title=HiRISE &#124; HiPOD: 29 Jul 2023 | access-date=July 31, 2023 | archive-date=July 31, 2023 | archive-url=https://web.archive.org/web/20230731120813/https://www.uahirise.org/hipod/PSP_002424_1765 | url-status=live }}</ref>  Mars Global Surveyor found several examples of this process.<ref>{{cite journal |last=Malin |first=Michael C. |date=2010 |pages=1–60 |volume=5 |journal=The Mars Journal |title=An overview of the 1985–2006 Mars Orbiter Camera science investigation |doi=10.1555/mars.2010.0001 |bibcode=2010IJMSE...5....1M|s2cid=128873687 }}</ref><ref>{{cite web |url=http://hiroc.lpl.arizona.edu/images/PSP/diafotizo.php?ID=PSP_002279_1735 |title=Sinuous Ridges Near Aeolis Mensae |publisher=University of Arizona |date=January 31, 2007 |access-date=October 8, 2009 |archive-url=https://web.archive.org/web/20160305025124/http://hiroc.lpl.arizona.edu/images/PSP/diafotizo.php?ID=PSP_002279_1735 |archive-date=March 5, 2016 |url-status=dead }}</ref> Many inverted streams have been discovered in various regions of Mars, especially in the [[Medusae Fossae Formation]],<ref>{{cite journal |doi=10.1016/j.icarus.2009.04.003 |last1=Zimbelman |first1=J. |last2=Griffin |first2=L. |date=2010 |title=HiRISE images of yardangs and sinuous ridges in the lower member of the Medusae Fossae Formation, Mars |journal=Icarus |volume=205 |issue=1 |pages=198–210 |bibcode=2010Icar..205..198Z}}</ref> [[Miyamoto Crater]],<ref name="ReferenceB">{{cite journal |doi=10.1016/j.icarus.2009.03.030 |last1=Newsom |first1=H. |last2=Lanza |first2=Nina L. |last3=Ollila |first3=Ann M. |last4=Wiseman |first4=Sandra M. |last5=Roush |first5=Ted L. |last6=Marzo |first6=Giuseppe A. |last7=Tornabene |first7=Livio L. |last8=Okubo |first8=Chris H. |last9=Osterloo |first9=Mikki M. |last10=Hamilton |first10=Victoria E. |last11=Crumpler |first11=Larry S. |date=2010 |title=Inverted channel deposits on the floor of Miyamoto crater, Mars |journal=Icarus |volume=205 |issue=1 |pages=64–72 |bibcode=2010Icar..205...64N}}</ref> [[Saheki Crater]],<ref>{{cite journal |doi=10.1016/j.icarus.2013.11.007 |last1=Morgan |first1=A. M. |last2=Howard |first2=A. D. |last3=Hobley |first3=D. E. J. |last4=Moore |first4=J. M. |last5=Dietrich |first5=W. E. |last6=Williams |first6=R. M. E. |last7=Burr |first7=D. M. |last8=Grant |first8=J. A. |last9=Wilson |first9=S. A. |last10=Matsubara |first10=Y. |date=2014 |title=Sedimentology and climatic environment of alluvial fans in the martian Saheki crater and a comparison with terrestrial fans in the Atacama Desert |journal=Icarus |volume=229 |pages=131–156 |bibcode=2014Icar..229..131M |url=https://repository.si.edu/bitstream/handle/10088/21823/nasm_201440.pdf |access-date=July 23, 2019 |archive-date=July 20, 2018 |archive-url=https://web.archive.org/web/20180720144834/https://repository.si.edu/bitstream/handle/10088/21823/nasm_201440.pdf |url-status=live }}</ref> and the Juventae Plateau.<ref name=Weitz /><ref name="Icarus Vol 210" />

[[File:Antoniadi Crater Stream Channels.JPG|left|thumb|Inverted stream channels in [[Antoniadi (Martian crater)|Antoniadi Crater]]. Location is [[Syrtis Major quadrangle]].]]
A variety of lake basins have been discovered on Mars.<ref name="Cabrol, N 2010" /> Some are comparable in size to the largest lakes on Earth, such as the [[Caspian Sea]], [[Black Sea]], and [[Lake Baikal]]. Lakes that were fed by valley networks are found in the southern highlands. There are places that are closed depressions with river valleys leading into them. These areas are thought to have once contained lakes; one is in [[Terra Sirenum]] that had its overflow move through [[Ma'adim Vallis]] into [[Gusev Crater]], explored by the [[Mars Exploration Rover]] [[Spirit Rover|Spirit]]. Another is near [[Parana Valles]] and Loire Vallis.<ref>{{cite journal |doi=10.1006/icar.2000.6465 |last1=Goldspiel |first1=J. |last2=Squires |first2=S. |date=2000 |title=Groundwater sapping and valley formation on Mars |journal=Icarus |volume=148 |issue=1 |pages=176–192 |bibcode=2000Icar..148..176G}}</ref> Some lakes are thought to have formed by precipitation, while others were formed from groundwater.<ref name="Irwin III 2005" /><ref name="Fassett2008" /> Lakes are estimated to have existed in the Argyre basin,<ref name="lpi.usra.edu"/><ref name="Heisinger2002" /> the Hellas basin,<ref name="Moore2001" /> and maybe in [[Valles Marineris]].<ref name="http" /><ref name="Carr">{{cite book |title=The Surface of Mars |series=Cambridge Planetary Science |publisher=Cambridge University Press |number=6 |isbn=978-0-511-26688-1 |first=Michael H. |last=Carr <!-- United States Geological Survey, Menlo Park -->}}</ref><ref>{{cite journal |doi=10.1016/0019-1035(87)90086-8 |last1=Nedell |first1=S. |last2=Squyres |first2=Steven W. |last3=Andersen |first3=David W. |date=1987 |title=Origin and evolution of the layered deposits in the Valles Marineris, Mars |journal=Icarus |volume=70 |pages=409–441 |bibcode=1987Icar...70..409N |issue=3}}</ref> It is likely that at times in the Noachian, many craters hosted lakes. These lakes are consistent with a cold, dry (by Earth standards) hydrological environment somewhat like that of the [[Great Basin]] of the western USA during the [[Last Glacial Maximum]].<ref>{{cite journal |last1=Matsubara |first1=Yo |first2=Alan D. |last2=Howard |first3=Sarah A. |last3=Drummond |title=Hydrology of early Mars: Lake basins |journal=Journal of Geophysical Research: Planets |issue=116.E4 |year=2011|volume=116 |doi=10.1029/2010JE003739 |bibcode=2011JGRE..116.4001M }}</ref>

Research from 2010 suggests that Mars also had lakes along parts of the equator. Although earlier research had showed that Mars had a warm and wet early history that has long since dried up, these lakes existed in the [[Hesperian]] Epoch, a much later period. Using detailed images from NASA's [[Mars Reconnaissance Orbiter]], the researchers speculate that there may have been increased volcanic activity, meteorite impacts or shifts in Mars' orbit during this period to warm Mars' atmosphere enough to melt the abundant ice present in the ground. Volcanoes would have released gases that thickened the atmosphere for a temporary period, trapping more sunlight and making it warm enough for liquid water to exist. In this study, channels were discovered that connected lake basins near [[Ares Vallis]]. When one lake filled up, its waters overflowed the banks and carved the channels to a lower area where another lake would form.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2012/01/100104092452.htm |title=Spectacular Mars images reveal evidence of ancient lakes |publisher=Sciencedaily.com |date=January 4, 2010 |access-date=February 28, 2018 |archive-url=https://web.archive.org/web/20160823210537/https://www.sciencedaily.com/releases/2012/01/100104092452.htm |archive-date=August 23, 2016 |url-status=dead }}</ref><ref>{{cite journal |doi=10.1130/G30579.1 |last1=Gupta |first1=Sanjeev |last2=Warner |first2=Nicholas |last3=Kim |first3=Rack |last4=Lin |first4=Yuan |last5=Muller |first5=Jan |last6=-1#Jung- |first6=Shih- |date=2010 |title=Hesperian equatorial thermokarst lakes in Ares Vallis as evidence for transient warm conditions on Mars |journal=Geology |volume=38 |issue=1 |pages=71–74|bibcode=2010Geo....38...71W }}</ref> These dry lakes would be targets to look for evidence ([[biosignature]]s) of past life.

On September 27, 2012, NASA scientists announced that the [[Curiosity rover|''Curiosity'' rover]] found direct evidence for an ancient [[streambed]] in [[Gale Crater]], suggesting an ancient "vigorous flow" of water on Mars.<ref name="NASA-20120927" /><ref name="NASA-20120927a" /><ref name="AP-20120927" /><ref name="ancient life">{{cite news |title=NASA Rover Finds Conditions Once Suited for Ancient Life on Mars |date=March 12, 2013 |url=http://www.nasa.gov/mission_pages/msl/news/msl20130312.html |publisher=NASA |access-date=June 16, 2013 |archive-date=July 3, 2013 |archive-url=https://web.archive.org/web/20130703035324/http://www.nasa.gov/mission_pages/msl/news/msl20130312.html |url-status=dead }}</ref> In particular, analysis of the now dry streambed indicated that the water ran at {{convert|3.3|km/h|m/s|abbr=on}},<ref name="NASA-20120927" /> possibly at hip-depth. Proof of running water came in the form of rounded pebbles and gravel fragments that could have only been weathered by strong liquid currents. Their shape and orientation suggests long-distance transport from above the rim of the crater, where a channel named [[Peace Vallis]] feeds into the [[alluvial fan]].

[[Eridania Lake]] is a theorized ancient lake with a surface area of roughly 1.1&nbsp;million square kilometers.<ref>{{Cite journal |last1=Parker |first1=Timothy J. |last2=Currey |first2=Donald R. |date=April 2001 |title=Extraterrestrial coastal geomorphology |url=https://doi.org/10.1016/S0169-555X(00)00089-1 |journal=Geomorphology |volume=37 |issue=3–4 |pages=303–328 |doi=10.1016/s0169-555x(00)00089-1 |bibcode=2001Geomo..37..303P |issn=0169-555X}}</ref><ref>{{Cite journal |last1=de Pablo |first1=M.A. |last2=Druet |first2=M. |title=Description, Origin and Evolution of a Basin in Sirenum Terrae, Mars, Including Atlantis Chaos: a Preliminary Study. |url=https://www.lpi.usra.edu/meetings/lpsc2002/pdf/1032.pdf |journal=Lunar and Planetary Science Conference XXXIII 11-15 March, 2002 |date=2002 |page=1032 |bibcode=2002LPI....33.1032D |id=abstract no.1032}}</ref><ref>{{Cite journal |last=de Pablo |first=M.A. |title=Mola Topographic Data Analysis of the Atlantis Paleolake Basin, Sirenum Terrae, Mars |url=https://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3037.pdf |journal=Sixth International Conference on Mars. 20–25 July, 2003 |date=2003 |page=3037 |location=Pasadena, California |bibcode=2003mars.conf.3037D |id=abstract #3037}}</ref> Its maximum depth is 2,400 meters and its volume is 562,000&nbsp;km<sup>3</sup>. It was larger than the largest landlocked sea on Earth, the [[Caspian Sea]], and contained more water than all the other Martian lakes together. The Eridania sea held more than nine times as much water as all of North America's [[Great Lakes]].<ref>{{cite magazine|url=https://www.astrobio.net/also-in-news/mars-study-yields-clues-possible-cradle-life/|title=Mars Study Yields Clues to Possible Cradle of Life |magazine=Astrobiology Magazine|date=October 8, 2017 |archive-url=https://web.archive.org/web/20171011233107/https://www.astrobio.net/also-in-news/mars-study-yields-clues-possible-cradle-life/ |archive-date=2017-10-11 |url-status=usurped}}</ref><ref>{{cite web|url=http://www.sci-news.com/space/mars-eridania-basin-vast-sea-05301.html|author=<!-- Staff writer(s); no by-line -->|title=Mars' Eridania Basin Once Held Vast Sea|website=Sci-News.com|date=October 9, 2017|access-date=6 June 2022|archive-date=April 22, 2023|archive-url=https://web.archive.org/web/20230422115652/http://www.sci-news.com/space/mars-eridania-basin-vast-sea-05301.html|url-status=live}}</ref><ref name="ReferenceC">{{cite journal | last1 = Michalski | first1 = J. |display-authors=etal | year = 2017 | title = Ancient hydrothermal seafloor deposits in Eridania basin on Mars | journal = Nature Communications | volume = 8 | page = 15978 | bibcode = 2017NatCo...815978M | doi = 10.1038/ncomms15978 | pmid = 28691699 | pmc = 5508135 }}</ref> The upper surface of the lake was assumed to be at the elevation of valley networks that surround the lake; they all end at the same elevation, suggesting that they emptied into a lake.<ref>{{cite journal | last1 = Irwin | first1 = R. |display-authors=etal | year = 2004 | title = Geomorphology of Ma'adim Vallis, Mars, and associated paleolake basins | journal = Journal of Geophysical Research: Planets | volume = 109 | issue = E12| page = E12009 | doi=10.1029/2004je002287 | bibcode=2004JGRE..10912009I| s2cid = 12637702 | doi-access = free }}</ref><ref>{{cite journal | last1 = Hynek | first1 = B. |display-authors=etal | year = 2010 | title = Updated global map of Martian valley networks and implications for climate and hydrologic processes | journal = Journal of Geophysical Research | volume = 115 | issue = E9| page = E09008 | doi=10.1029/2009je003548 | bibcode=2010JGRE..115.9008H| doi-access = free }}</ref> Research on this basin with CRISM found thick deposits, greater than 400 meters thick, that contained the minerals [[saponite]], talc-saponite, Fe-rich [[mica]] (for example, [[glauconite]]-[[nontronite]]), Fe- and Mg-serpentine, Mg-Fe-Ca-[[carbonate]] and probable Fe-[[sulfide]]. The Fe-sulfide probably formed in deep water from water heated by [[volcano]]es. Such a process, classified as [[hydrothermal]] may have been a place where life on Earth began.<ref name="ReferenceC"/>

<gallery class="center" widths="190px" heights="180px">
PIA22059 fig1eridaniadepths.jpg|Map showing estimated water depth in different parts of Eridania Sea.<br>This map is about 530 miles across.
PIA22058 hireseridanaregion.jpg|Deep-basin deposits from the floor of Eridania Sea. The mesas on the floor are there because they were protected against intense erosion by deep water/ice cover. [[CRISM]] measurements show minerals may be from seafloor hydrothermal deposits.
PIA22060 hireseridania.jpg|Diagram showing how volcanic activity may have caused deposition of minerals on floor of Eridania Sea. Chlorides were deposited along the shoreline by evaporation. 
</gallery>

=== Lake deltas ===
[[File:Distributary fan-delta.jpg|thumb|Delta in [[Eberswalde (crater)|Eberswalde crater]].]]
Researchers have found a number of examples of [[River delta|delta]]s that formed in Martian lakes.<ref name="ReferenceA">{{cite journal |last1=Di Achille |first1=Gaetano |last2=Hynek |first2=Brian M. |title=Ancient ocean on Mars supported by global distribution of deltas and valleys |journal=Nature Geoscience |volume=3 |pages=459–463 |date=2010 |doi=10.1038/ngeo891 |bibcode=2010NatGe...3..459D |issue=7}}</ref> Finding deltas is a major sign that Mars once had a lot of liquid water. Deltas usually require deep water over a long period of time to form. Also, the water level needs to be stable to keep [[sediment]] from washing away. Deltas have been found over a wide geographical range,<ref name="Irwin III 2005" /> though there is some indication that deltas may be concentrated around the edges of the putative former [[#Mars ocean hypothesis|northern ocean of Mars]].<ref>{{cite journal | last1 = Di Achille | first1 = Gaetano | last2 = Hynek | first2 = Brian M. | year = 2010 | title = Ancient ocean on Mars supported by global distribution of deltas and valleys | journal = Nature Geoscience | volume = 3 | issue = 7| pages = 459–463 | doi=10.1038/ngeo891 | bibcode=2010NatGe...3..459D}}</ref>

=== Groundwater ===
<!--This section should probably also contain info on chemical evidence for past water both from rovers and satellite wiggle-matching, e.g., "blueberries"-->
{{Main|Groundwater on Mars}}
[[File:Groundwaterseries8final.jpg|thumb|left|Layers may be formed by [[Groundwater on Mars|groundwater]] rising up gradually.]]

By 1979 it was thought that [[outflow channels]] formed in single, catastrophic ruptures of subsurface water reservoirs, possibly sealed by ice, discharging colossal quantities of water across an otherwise arid Mars surface.<ref>{{cite journal |last=Carr |first=M. H. |date=1979 |title=Formation of Martian flood features by release of water from confined aquifers |url=http://www.es.ucsc.edu/~rcoe/eart206/Carr_MarsFloodFeatures_JGR79.pdf |journal=Journal of Geophysical Research |volume=84 |pages=2995–3007 |bibcode=1979JGR....84.2995C |doi=10.1029/JB084iB06p02995 |access-date=June 16, 2013 |archive-date=September 24, 2015 |archive-url=https://web.archive.org/web/20150924002006/http://www.es.ucsc.edu/~rcoe/eart206/Carr_MarsFloodFeatures_JGR79.pdf |url-status=dead }}</ref><ref>{{cite journal |doi=10.1016/0019-1035(74)90101-8 |last1=Baker |first1=V. |last2=Milton |first2=D. |date=1974 |title=Erosion by Catastrophic Floods on Mars and Earth |journal=Icarus |volume=23 |issue=1 |pages=27–41 |bibcode=1974Icar...23...27B}}</ref> In addition, evidence in favor of heavy or even catastrophic flooding is found in the [[Giant current ripples|giant ripples]] in the [[Athabasca Vallis]].<ref>{{cite web |url=http://www.msss.com/mars_images/moc/2004/09/27/ |title=Mars Global Surveyor MOC2-862 Release |publisher=Malin Space Science Systems |access-date=January 16, 2012 |archive-url=https://web.archive.org/web/20090412041936/http://www.msss.com/mars_images/moc/2004/09/27/ |archive-date=April 12, 2009 |url-status=dead }}</ref><ref name="DOInature">{{cite journal |doi=10.1038/nature05594 |title=Meridiani Planum and the global hydrology of Mars |date=2007 |last1=Andrews-Hanna |first1=Jeffrey C. |last2=Phillips |first2=Roger J. |last3=Zuber |first3=Maria T. |journal=Nature |volume=446 |issue=7132 |pages=163–136 |pmid=17344848 |bibcode=2007Natur.446..163A |s2cid=4428510 }}</ref> Many outflow channels begin at [[Martian chaos terrain|Chaos]] or [[Chasma]] features, providing evidence for the rupture that could have breached a subsurface ice seal.<ref name="Carr" />

The branching [[Valley networks (Mars)|valley networks]] of Mars are not consistent with formation by sudden catastrophic release of groundwater, both in terms of their dendritic shapes that do not come from a single outflow point, and in terms of the discharges that apparently flowed along them.<ref>{{cite journal |last1=Irwin |last2=Rossman |first2=P. |first3=Robert A. |last3=Craddock |first4=Alan D. |last4=Howard |title=Interior channels in Martian valley networks: Discharge and runoff production |journal=Geology |volume=33 |issue=6 |date=2005 |pages=489–492 |doi=10.1130/g21333.1|bibcode=2005Geo....33..489I |s2cid=5663347 }}</ref> Instead, some authors have argued that they were formed by slow seepage of groundwater from the subsurface essentially as springs.<ref name="Jakosky1999">{{cite journal |last=Jakosky |first=Bruce M. |date=1999 |title=Water, Climate, and Life |journal=Science |volume=283 |issue=5402 |pages=648–649 |doi=10.1126/science.283.5402.648 |pmid=9988657|s2cid=128560172 }}</ref> In support of this interpretation, the upstream ends of many valleys in such networks begin with [[box canyon]] or "amphitheater" heads, which on Earth are typically associated with groundwater seepage. There is also little evidence of finer scale channels or valleys at the tips of the channels, which some authors have interpreted as showing the flow appeared suddenly from the subsurface with appreciable discharge, rather than accumulating gradually across the surface.<ref name="Carr" /> Others have disputed the link between amphitheater heads of valleys and formation by groundwater for terrestrial examples,<ref>{{cite journal |last=Lamb |first=Michael P. |display-authors=etal |title=Can springs cut canyons into rock? |journal=Journal of Geophysical Research: Planets |issue=111.E7 |year=2006 |volume=111 |doi=10.1029/2005JE002663 |bibcode=2006JGRE..111.7002L |url=https://authors.library.caltech.edu/15925/ |access-date=June 23, 2022 |archive-date=April 22, 2023 |archive-url=https://web.archive.org/web/20230422115651/https://authors.library.caltech.edu/15925/ |url-status=dead }}</ref> and have argued that the lack of fine scale heads to valley networks is due to their removal by [[weathering]] or [[impact gardening]].<ref name="Carr" /> Most authors accept that most valley networks were at least partly influenced and shaped by groundwater seep processes.

[[File:Burns cliff.jpg|thumb|right|The preservation and cementation of aeolian dune [[stratigraphy]] in Burns Cliff in [[Endurance (crater)|Endurance Crater]] are thought to have been controlled by flow of shallow groundwater.<ref name="BurnsCliff">{{cite journal |last1=Grotzinger |first1=J. P. |first2=R. E. |last2=Arvidson |first3=J. F. |last3=Bell III |first4=W. |last4=Calvin |first5=B. C. |last5=Clark |first6=D. A. |last6=Fike |first7=M. |last7=Golombek |first8=R. |last8=Greeley |first9=A. |last9=Haldemann |first10=K. E. |last10=Herkenhoff |first11=B. L. |last11=Jolliff |first12=A. H. |last12=Knoll |first13=M. |last13=Malin |first14=S. M. |last14=McLennan |first15=T. |last15=Parker |first16=L. |last16=Soderblom |first17=J. N. |last17=Sohl-Dickstein |first18=S. W. |last18=Squyres |first19=N. J. |last19=Tosca |first20=W. A. |last20=Watters |title=Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum |journal=Earth and Planetary Science Letters |volume=240 |issue=1 |date=November 25, 2005 |pages=11–72 |issn=0012-821X |doi=10.1016/j.epsl.2005.09.039 |bibcode=2005E&PSL.240...11G}}</ref>]]

[[Groundwater]] also played a vital role in controlling broad scale sedimentation patterns and processes on Mars.<ref name="Michalski">{{cite journal |title=Groundwater activity on Mars and implications for a deep biosphere |journal=Nature Geoscience |date=January 20, 2013 |first1=Joseph R. |last1=Michalski |first2=Paul B. |last2=Niles |first3=Javier |last3=Cuadros |first4=John |last4=Parnell |first5=A. Deanne |last5=Rogers |first6=Shawn P. |last6=Wright |volume=6 |pages=133–138 |doi=10.1038/ngeo1706 |quote=Here we present a conceptual model of subsurface habitability of Mars and evaluate evidence for groundwater upwelling in deep basins. |bibcode=2013NatGe...6..133M |issue=2}}</ref> According to this hypothesis, groundwater with dissolved minerals came to the surface, in and around craters, and helped to form layers by adding minerals—especially sulfate—and [[Clastic sediment#Cementation|cementing sediments]].<ref name="BurnsCliff" /><ref name=Zuber /><ref>{{cite journal |last1=Andrews-Hanna |first1=J. C. |first2=M. T. |last2=Zuber |first3=R. E. |last3=Arvidson |first4=S. M. |last4=Wiseman |date=2010 |title=Early Mars hydrology: Meridiani playa deposits and the sedimentary record of Arabia Terra |journal=Journal of Geophysical Research |volume=115 |issue=E6 |page=E06002 |doi=10.1029/2009JE003485 |bibcode=2010JGRE..115.6002A|doi-access=free |hdl=1721.1/74246 |hdl-access=free }}</ref><ref>{{cite journal |last=McLennan |first=S. M. |display-authors=etal |date=2005 |title=Provenance and diagenesis of the evaporitebearing Burns formation, Meridiani Planum, Mars |journal=Earth and Planetary Science Letters |volume=240 |issue=1 |pages=95–121 |doi=10.1016/j.epsl.2005.09.041 |bibcode=2005E&PSL.240...95M}}</ref><ref>{{cite journal |last1=Squyres |first1=S. W. |first2=A. H. |last2=Knoll |date=2005 |title=Sedimentary rocks at Meridiani Planum: Origin, diagenesis, and implications for life on Mars |journal=Earth and Planetary Science Letters |volume=240 |issue=1 |pages=1–10 |doi=10.1016/j.epsl.2005.09.038 |bibcode=2005E&PSL.240....1S}}.</ref><ref>{{cite journal |last=Squyres |first=S. W. |display-authors=etal |date=2006 |title=Two years at Meridiani Planum: Results from the Opportunity rover |journal=Science |volume=313 |issue=5792 |pages=1403–1407 |doi=10.1126/science.1130890 |url=https://eprints.utas.edu.au/2614/1/Science2007.pdf |bibcode=2006Sci...313.1403S |pmid=16959999 |s2cid=17643218 |access-date=March 16, 2019 |archive-date=August 31, 2021 |archive-url=https://web.archive.org/web/20210831003806/https://eprints.utas.edu.au/2614/1/Science2007.pdf |url-status=live }}.</ref> In other words, some layers may have been formed by groundwater rising up depositing minerals and cementing existing, loose, [[Aeolian processes|aeolian]] sediments. The hardened layers are consequently more protected from [[erosion]]. A study published in 2011 using data from the [[Mars Reconnaissance Orbiter]], show that the same kinds of sediments exist in a large area that includes [[Arabia Terra]].<ref>{{cite conference |first1=M. |last1=Wiseman |first2=J. C. |last2=Andrews-Hanna |first3=R. E. |last3=Arvidson |first4=J. F. |last4=Mustard |first5=K. J. |last5=Zabrusky |title=Distribution of Hydrated Sulfates Across Arabia Terra Using CRISM Data: Implications for Martian Hydrology |url=https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2133.pdf |conference=42nd Lunar and Planetary Science Conference |date=2011 |access-date=October 3, 2018 |archive-date=September 18, 2021 |archive-url=https://web.archive.org/web/20210918073811/https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2133.pdf |url-status=live }}</ref> It has been argued that areas that are rich in sedimentary rocks are also those areas that most likely experienced groundwater upwelling on a regional scale.<ref>{{cite journal |last1=Andrews-Hanna |first1=Jeffrey C. |first2=Kevin W. |last2=Lewis |title=Early Mars hydrology: 2. Hydrological evolution in the Noachian and Hesperian epochs |journal=Journal of Geophysical Research: Planets |volume=116 |issue=E2 |page=E2 |date=2011 |doi=10.1029/2010je003709 |bibcode=2011JGRE..116.2007A|s2cid=17293290 |doi-access=free }}</ref>

In February 2019, European scientists published geological evidence of an ancient planet-wide groundwater system that was, arguably, connected to a putative vast ocean.<ref name="ESA-20190228">{{cite news |author=ESA Staff |title=First Evidence of 'Planet-Wide Groundwater System' on Mars Found |url=https://www.esa.int/Our_Activities/Space_Science/Mars_Express/First_evidence_of_planet-wide_groundwater_system_on_Mars |date=28 February 2019 |work=[[European Space Agency]] |access-date=28 February 2019 |archive-date=September 15, 2019 |archive-url=https://web.archive.org/web/20190915103854/http://www.esa.int/Our_Activities/Space_Science/Mars_Express/First_evidence_of_planet-wide_groundwater_system_on_Mars |url-status=live }}</ref><ref name="FTR-20190228">{{cite news |last=Houser |first=Kristin |title=First Evidence of 'Planet-Wide Groundwater System' on Mars Found |url=https://futurism.com/the-byte/mars-groundwater-system-planet-wide |date=28 February 2019 |work=Futurism.com |access-date=28 February 2019 |archive-date=January 19, 2021 |archive-url=https://web.archive.org/web/20210119124513/https://futurism.com/the-byte/mars-groundwater-system-planet-wide |url-status=live }}</ref><ref>{{Cite journal | doi=10.1029/2018JE005802| pmid=31007995| pmc=6472477| title=Geological Evidence of Planet-Wide Groundwater System on Mars| journal=Journal of Geophysical Research: Planets| volume=124| issue=2| pages=374–395| year=2019| last1=Salese| first1=Francesco| last2=Pondrelli| first2=Monica| last3=Neeseman| first3=Alicia| last4=Schmidt| first4=Gene| last5=Ori| first5=Gian Gabriele| bibcode=2019JGRE..124..374S}}</ref><ref>{{Cite web|url = https://www.leonarddavid.com/planet%E2%80%90wide-groundwater-system-on-mars-new-geological-evidence/|title = Mars: Planet-Wide Groundwater System – New Geological Evidence|date = February 19, 2019|access-date = March 2, 2019|archive-date = August 18, 2020|archive-url = https://web.archive.org/web/20200818025504/https://www.leonarddavid.com/planet%E2%80%90wide-groundwater-system-on-mars-new-geological-evidence/|url-status = live}}</ref> In September 2019, researchers reported that the ''[[InSight]]'' lander uncovered unexplained [[Magnetic anomaly|magnetic pulses]], and [[Mirnov oscillations|magnetic oscillations]] consistent with a planet-wide reservoir of liquid water deep underground.<ref name="NG-20190920">{{cite news |last=Andrews |first=Robin George |title=Mysterious magnetic pulses discovered on Mars |url=https://www.nationalgeographic.com/science/2019/09/mars-insight-feels-mysterious-magnetic-pulsations-at-midnight/ |archive-url=https://web.archive.org/web/20190920141718/https://www.nationalgeographic.com/science/2019/09/mars-insight-feels-mysterious-magnetic-pulsations-at-midnight/ |url-status=dead |archive-date=September 20, 2019 |date=20 September 2019 |work=[[National Geographic Society]] |access-date=20 September 2019 }}</ref>

=== Mars ocean hypothesis ===
{{Main|Mars ocean hypothesis}}
[[Image:MarsTopoMap-PIA02031 modest.jpg|upright=1.6|thumb|The blue region of low topography in the Martian northern hemisphere is hypothesized to be the site of a primordial ocean of liquid water.<ref name="Brandenburg 1987">{{citation |first=John E. |last=Brandenburg |contribution=The Paleo-Ocean of Mars |title=MECA Symposium on Mars: Evolution of its Climate and Atmosphere |publisher=Lunar and Planetary Institute |pages=20–22 |date=1987 |bibcode=1987meca.symp...20B }}</ref>]]
The Mars ocean hypothesis proposes that the [[Vastitas Borealis]] basin was the site of an ocean of liquid water at least once,<ref name="Baker" /> and presents evidence that nearly a third of the [[Geography of Mars|surface]] of Mars was covered by a liquid ocean early in the planet's [[Geology of Mars|geologic history]].<ref name="Cabrol, N 2010" /><ref name="Clifford">{{cite journal |doi=10.1006/icar.2001.6671 |last1=Clifford |first1=S. M. |last2=Parker |first2=T. J. |date=2001 |title=The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains |journal=Icarus |volume=154 |issue=1 |pages=40–79 |bibcode=2001Icar..154...40C|s2cid=13694518 }}</ref> This ocean, dubbed '''Oceanus Borealis''',<ref name="Baker" /> would have filled the Vastitas Borealis basin in the northern hemisphere, a region that lies {{convert|4-5|km}} below the mean planetary elevation. Two major putative shorelines have been suggested: a higher one, dating to a time period of approximately 3.8&nbsp;billion years ago and concurrent with the formation of the [[valley networks (Mars)|valley networks]] in the Highlands, and a lower one, perhaps correlated with the younger [[outflow channels]]. The higher one, the 'Arabia shoreline', can be traced all around Mars except through the Tharsis volcanic region. The lower, the 'Deuteronilus', follows the [[Vastitas Borealis]] formation.<ref name="Carr" />

A study in June 2010 concluded that the more ancient ocean would have covered 36% of Mars.<ref name="ReferenceA" /><ref name="third" /> Data from the Mars Orbiter Laser Altimeter (MOLA), which measures the altitude of all terrain on Mars, was used in 1999 to determine that the [[Drainage basin|watershed]] for such an ocean would have covered about 75% of the planet.<ref name="Smith">{{cite journal |last=Smith |first=D. |display-authors=etal |date=1999 |title=The Gravity Field of Mars: Results from Mars Global Surveyor |journal=Science |volume=286 |issue=5437 |pages=94–97 |doi=10.1126/science.286.5437.94 |bibcode=1999Sci...286...94S |url=http://seismo.berkeley.edu/~rallen/eps122/reading/Smithetal1999.pdf |pmid=10506567 |access-date=December 19, 2010 |archive-date=March 5, 2016 |archive-url=https://web.archive.org/web/20160305005726/http://seismo.berkeley.edu/~rallen/eps122/reading/Smithetal1999.pdf |url-status=dead }}</ref> Early Mars would have required a warmer climate and denser atmosphere to allow liquid water to exist at the surface.<ref name="ReadandLewis">{{cite book |isbn=978-3-540-40743-0 |last1=Read |first1=Peter L. |first2=S. R. |last2=Lewis |title=The Martian Climate Revisited: Atmosphere and Environment of a Desert Planet |publisher=Praxis |location=Chichester, UK |date=2004 |url=http://www.praxis-publishing.co.uk/9783540407430.htm |format=Paperback |access-date=December 19, 2010 |archive-date=July 24, 2011 |archive-url=https://web.archive.org/web/20110724101309/http://www.praxis-publishing.co.uk/9783540407430.htm |url-status=dead }}</ref><ref>{{cite web |url=http://www.astrobio.net/pressrelease/3322/martian-north-once-covered-by-ocean |title=Martian North Once Covered by Ocean |work=Astrobiology Magazine |access-date=December 19, 2010|date=November 26, 2009 |archive-url=https://web.archive.org/web/20110604121418/http://www.astrobio.net/pressrelease/3322/martian-north-once-covered-by-ocean |archive-date=2011-06-04 |url-status=usurped}}</ref> In addition, the large number of valley networks strongly supports the possibility of a [[hydrology|hydrological cycle]] on the planet in the past.<ref name="Zuber">{{cite journal |doi=10.1038/447785a |last=Zuber |first=Maria T. |date=2007 |title=Planetary Science: Mars at the tipping point |journal=Nature |volume=447 |issue=7146 |pages=785–786 |pmid=17568733 |bibcode=2007Natur.447..785Z |s2cid=4427572 }}</ref><ref>{{cite web |url=http://www.space.com/scienceastronomy/091123-mars-ocean.html |title=New Map Bolsters Case for Ancient Ocean on Mars |publisher=SPACE.com |date=November 23, 2009 |access-date=November 24, 2009 |archive-date=March 15, 2010 |archive-url=https://web.archive.org/web/20100315193249/http://www.space.com/scienceastronomy/091123-mars-ocean.html |url-status=live }}</ref>

The existence of a primordial Martian ocean remains controversial among scientists, and the interpretations of some features as 'ancient shorelines' has been challenged.<ref name="2003JGRE..108.5042C">{{cite journal |last1=Carr |first1=M. |last2=Head |first2=J. |date=2003 |title=Oceans on Mars: An assessment of the observational evidence and possible fate |journal=Journal of Geophysical Research |volume=108 |issue=E5 |page=5042 |bibcode=2003JGRE..108.5042C |doi=10.1029/2002JE001963 |s2cid=16367611 |doi-access=free }}</ref><ref>{{cite web |url=http://astrobiology.nasa.gov/articles/mars-ocean-hypothesis-hits-the-shore/ |title=Mars Ocean Hypothesis Hits the Shore |work=NASA Astrobiology |publisher=NASA |date=January 26, 2001 |url-status=dead |archive-url=https://web.archive.org/web/20120220081803/http://astrobiology.nasa.gov/articles/mars-ocean-hypothesis-hits-the-shore/ |archive-date=February 20, 2012  }}</ref> One problem with the conjectured 2-billion-year-old (2&nbsp;[[Gigaannum#SI prefix multipliers|Ga]]) shoreline is that it is not flat—i.e., does not follow a line of constant gravitational potential. This could be due to a change in distribution in Mars' mass, perhaps due to volcanic eruption or meteor impact;<ref>{{cite journal |last1=Perron |first2=J. |last2=Taylor |display-authors=etal |title=Evidence for an ancient Martian ocean in the topography of deformed shorelines |journal=Nature |volume=447 |issue=7146 |date=2007 |pages=840–843 |doi=10.1038/nature05873 |pmid=17568743|bibcode=2007Natur.447..840P |s2cid=4332594 }}</ref> the Elysium volcanic province or the massive Utopia basin that is buried beneath the northern plains have been put forward as the most likely causes.<ref name="Zuber" />

In March 2015, scientists stated that evidence exists for an ancient Martian ocean, likely in the planet's northern hemisphere and about the size of Earth's [[Arctic Ocean]], or approximately 19% of the Martian surface. This finding was derived from the ratio of water and [[deuterium]] in the modern Martian atmosphere compared to the ratio found on Earth. Eight times as much deuterium was found at Mars than exists on Earth, suggesting that ancient Mars had significantly higher levels of water. Results from the [[Curiosity (rover)|''Curiosity'']] rover had previously found a high ratio of deuterium in [[Gale Crater]], though not significantly high enough to suggest the presence of an ocean. Other scientists caution that this new study has not been confirmed, and point out that Martian climate models have not yet shown that the planet was warm enough in the past to support bodies of liquid water.<ref name="NYT-20150305">{{cite news |url=https://www.nytimes.com/2015/03/06/science/mars-had-an-ocean-scientists-say-pointing-to-new-data.html |title=Mars Had an Ocean, Scientists Say, Pointing to New Data |work=[[The New York Times]] |last=Kaufman |first=Marc |date=March 5, 2015 |access-date=March 5, 2015 |archive-date=March 7, 2020 |archive-url=https://web.archive.org/web/20200307000937/https://www.nytimes.com/2015/03/06/science/mars-had-an-ocean-scientists-say-pointing-to-new-data.html |url-status=live }}</ref>

Additional evidence for a northern ocean was published in May 2016, describing how some of the surface in Ismenius Lacus quadrangle was altered by two [[tsunami]]s. The tsunamis were caused by asteroids striking the ocean.  Both were thought to have been strong enough to create 30&nbsp;km diameter craters. The first tsunami picked up and carried boulders the size of cars or small houses. The backwash from the wave formed channels by rearranging the boulders. The second came in when the ocean was 300&nbsp;m lower. The second carried a great deal of ice which was dropped in valleys. Calculations show that the average height of the waves would have been 50&nbsp;m, but the heights would vary from 10&nbsp;m to 120&nbsp;m. Numerical simulations show that in this particular part of the ocean two impact craters of the size of 30&nbsp;km in diameter would form every 30 million years. The implication here is that a great northern ocean may have existed for millions of years. One argument against an ocean has been the lack of shoreline features. These features may have been washed away by these tsunami events. The parts of Mars studied in this research are [[Chryse Planitia]] and northwestern [[Arabia Terra]]. These tsunamis affected some surfaces in the Ismenius Lacus quadrangle and in the [[Mare Acidalium quadrangle]].<ref>{{cite press release|url=http://astrobiology.com/2016/05/ancient-tsunami-evidence-on-mars-reveals-life-potential.html|title=Ancient Tsunami Evidence on Mars Reveals Life Potential|website=Astrobiology|agency=Cornell University|date=May 20, 2016|access-date=May 30, 2016|archive-date=June 11, 2024|archive-url=https://web.archive.org/web/20240611060734/https://astrobiology.com/2016/05/ancient-tsunami-evidence-on-mars-reveals-life-potential.html|url-status=live}}</ref><ref>{{cite journal|title=Tsunami waves extensively resurfaced the shorelines of an early Martian ocean|first1=J. Alexis P.|last1=Rodriguez|first2=Alberto G.|last2=Fairén|first3=Kenneth L.|last3=Tanaka|first4=Mario|last4=Zarroca|first5=Rogelio|last5=Linares|first6=Thomas|last6=Platz|first7=Goro|last7=Komatsu|first8=Hideaki|last8=Miyamoto|first9=Jeffrey S.|last9=Kargel|first10=Jianguo|last10=Yan|first11=Virginia|last11=Gulick|first12=Kana|last12=Higuchi|first13=Victor R.|last13=Baker|first14=Natalie|last14=Glines|date=May 19, 2016|journal=Scientific Reports|volume=6|issue=1|pages=25106|doi=10.1038/srep25106|pmid=27196957|pmc=4872529|bibcode=2016NatSR...625106R}}</ref><ref>{{cite press release |agency=Cornell University |title=Ancient tsunami evidence on Mars reveals life potential |work=ScienceDaily |date=May 19, 2016 |url=https://www.sciencedaily.com/releases/2016/05/160519101756.htm |access-date=February 28, 2018 |archive-date=October 9, 2021 |archive-url=https://web.archive.org/web/20211009114213/https://www.sciencedaily.com/releases/2016/05/160519101756.htm |url-status=live }}</ref>

In July 2019, support was reported for an [[Mars ocean hypothesis|ancient ocean]] on Mars that may have been formed by a possible [[Megatsunami|mega-tsunami]] source resulting from a [[meteorite impact]] creating [[Lomonosov (Martian crater)|Lomonosov crater]].<ref name="NYT-20190730">{{cite news |last=Andrews |first=Robin George |title=When a Mega-Tsunami Drowned Mars, This Spot May Have Been Ground Zero |url=https://www.nytimes.com/2019/07/30/science/mars-tsunami-crater.html |date=July 30, 2019 |work=[[The New York Times]] |access-date=July 31, 2019 |archive-date=December 14, 2021 |archive-url=https://web.archive.org/web/20211214174217/https://www.nytimes.com/2019/07/30/science/mars-tsunami-crater.html |url-status=live }}</ref><ref name="JGRP-20190626">{{cite journal |last=Costard |first=F. |display-authors=et al. |title=The Lomonosov Crater Impact Event: A Possible Mega-Tsunami Source on Mars |date=June 26, 2019 |journal=[[Journal of Geophysical Research: Planets]] |volume=124 |issue=7 |pages=1840–1851 |doi=10.1029/2019JE006008 |bibcode=2019JGRE..124.1840C |hdl=20.500.11937/76439 |s2cid=198401957 |hdl-access=free }}</ref>

In January 2022, a study about the climate 3 Gy ago on Mars shows that an ocean is stable with a water cycle that is closed.<ref>{{cite journal |last1=Schmidt |first1=Frédéric |last2=Way |first2=Michael|display-authors=et al.|title=Circumpolar ocean stability on Mars 3 Gy ago |journal=Proceedings of the National Academy of Sciences |date=2022 |volume=119 |issue=4 |doi=10.1073/pnas.2112930118 |pmid=35042794 |pmc=8795497 |bibcode=2022PNAS..11912930S |doi-access=free |arxiv=2310.00461 }}</ref> They estimate a return water flow, in form of ice in glacier, from the icy highlands to the ocean is in magnitude less than the Earth at the last glacial maximum. This simulation includes for the first time a circulatin of the ocean. They demonstrate that the ocean's circulation prevent the ocean to freeze. These also shows that simulations are in agreement with observed geomorphological features identified as ancient glacial valleys.

=== Evidence for recent flows ===
{{Main|Seasonal flows on warm Martian slopes}}
{{See also|Gully (Mars)}}
[[File:Warm Season Flows on Slope in Newton Crater (animated).gif|thumb|left|Warm-season flows on slope in [[Newton (Martian crater)|Newton Crater]].<ref>Stillman, D., et al.  2017.  Characteristics of the numerous and widespread recurring slope lineae (RSL) in Valles Marineris, Mars.  Icarus.  Volume 285.  Pages 195-210</ref>]]
[[File:Branched gullies.jpg|thumb|Branched gullies.]]
[[File:Deep Gullies.jpg|thumb|Group of deep gullies.]]

Pure liquid water cannot exist in a stable form on the surface of Mars with its present low atmospheric pressure and low temperature because it would boil, except at the lowest elevations for a few hours.<ref name="Kostama" /><ref name="flows" /> So, a geological mystery commenced in 2006 when observations from NASA's ''[[Mars Reconnaissance Orbiter]]'' revealed [[Gully (Mars)|gully]] deposits that were not there ten years prior, possibly caused by flowing liquid [[brine]] during the warmest months on Mars.<ref name="Impact Cratering Rate">{{cite journal |title=Present-Day Impact Cratering Rate and Contemporary Gully Activity on Mars |journal=Science |date=December 8, 2006 |first1=Michael C. |last1=Malin |first2=Kenneth S. |last2=Edgett |first3=Liliya V. |last3=Posiolova |first4=Shawn M. |last4=McColley |first5=Eldar Z. Noe |last5=Dobrea |volume=314 |issue=5805 |pages=1573–1577 |doi=10.1126/science.1135156 |pmid=17158321 |bibcode=2006Sci...314.1573M|s2cid=39225477 }}</ref><ref name="Head 2008 PNAS">{{cite journal |pmid=18725636 |date=2008 |last1=Head |first1=J. W. |last2=Marchant |first2=D. R |last3=Kreslavsky |first3=M. A. |title=Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin |volume=105 |issue=36 |pages=13258–13263 |doi=10.1073/pnas.0803760105 |pmc=2734344 |journal=[[Proceedings of the National Academy of Sciences of the United States of America|PNAS]] |bibcode=2008PNAS..10513258H|doi-access=free }}</ref> The images were of two craters in [[Terra Sirenum]] and [[Centauri Montes]] that appear to show the presence of flows (wet or dry) on Mars at some point between 1999 and 2001.<ref name="Impact Cratering Rate" /><ref>{{cite news |url=https://www.thetimes.co.uk/article/water-has-been-flowing-on-mars-within-past-five-years-nasa-says-srm2hk5hxd0 |title=Water has been flowing on Mars within past five years, Nasa says |work=[[The Times]] |location=London |first=Mark |last=Henderson |date=December 7, 2006 |access-date=June 6, 2022 |archive-date=April 22, 2023 |archive-url=https://web.archive.org/web/20230422115651/https://www.thetimes.co.uk/article/water-has-been-flowing-on-mars-within-past-five-years-nasa-says-srm2hk5hxd0 |url-status=live }}</ref><ref>{{cite journal |last1=Malin |first1=Michael C. |last2=Edgett |first2=Kenneth S. |date=2000 |title=Evidence for Recent Groundwater Seepage and Surface Runoff on Mars |journal=Science |volume=288 |issue=5475 |pages=2330–2335 |doi=10.1126/science.288.5475.2330 |pmid=10875910 |bibcode=2000Sci...288.2330M|s2cid=14232446 }}</ref>

There is disagreement in the scientific community as to whether or not gullies are formed by liquid water. While some scientists believe that most gullies are formed by liquid water formed from snow or ice melting,<ref>{{Cite journal |last=Christensen |first=Philip R. |date=2003-02-19 |title=Formation of recent martian gullies through melting of extensive water-rich snow deposits |url=http://dx.doi.org/10.1038/nature01436 |journal=Nature |volume=422 |issue=6927 |pages=45–48 |doi=10.1038/nature01436 |pmid=12594459 |bibcode=2003Natur.422...45C |issn=0028-0836 |access-date=April 11, 2024 |archive-date=June 11, 2024 |archive-url=https://web.archive.org/web/20240611060735/https://www.nature.com/articles/nature01436 |url-status=live }}</ref><ref>{{Cite journal |last1=Rai Khuller |first1=Aditya |last2=Russel Christensen |first2=Philip |date=February 2021 |title=Evidence of Exposed Dusty Water Ice within Martian Gullies |url=http://dx.doi.org/10.1029/2020je006539 |journal=Journal of Geophysical Research: Planets |volume=126 |issue=2 |doi=10.1029/2020je006539 |bibcode=2021JGRE..12606539R |issn=2169-9097 |access-date=April 11, 2024 |archive-date=June 11, 2024 |archive-url=https://web.archive.org/web/20240611060736/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JE006539 |url-status=live }}</ref><ref>{{cite journal |last1=Dickson |first1=J. L. |last2=Palumbo |first2=A. M. |last3=Head |first3=J. W. |last4=Kerber |first4=L. |last5=Fassett |first5=C. I. |last6=Kreslavsky |first6=M. A. |year=2023 |title=Gullies on Mars could have formed by melting of water ice during periods of high obliquity |journal=Science |volume=380 |issue=6652 |pages=1363–1367 |bibcode=2023Sci...380.1363D |doi=10.1126/science.abk2464 |pmid=37384686 |s2cid=259287608 |doi-access=free}}</ref> other scientists believe that gullies are formed by dry flows possibly lubricated by sublimating carbon dioxide that forms from freezing of the martian atmosphere.<ref>{{Cite journal |last1=Dundas |first1=Colin M. |last2=McEwen |first2=Alfred S. |last3=Diniega |first3=Serina |last4=Hansen |first4=Candice J. |last5=Byrne |first5=Shane |last6=McElwaine |first6=Jim N. |date=2017-11-27 |title=The formation of gullies on Mars today |url=http://dx.doi.org/10.1144/sp467.5 |journal=Geological Society, London, Special Publications |volume=467 |issue=1 |pages=67–94 |doi=10.1144/sp467.5 |issn=0305-8719|hdl=10150/633371 |hdl-access=free }}</ref><ref name="ICRS-20170928"/><ref>{{cite journal |doi=10.1016/j.icarus.2009.09.009 |last1=Kolb |first1=K. |last2=Pelletier |date=2010 |first2=Jon D. |last3=McEwen |first3=Alfred S. |title=Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water |journal=Icarus |volume=205 |issue=1 |pages=113–137 |bibcode=2010Icar..205..113K}}</ref>

Some studies attest that gullies forming in the southern highlands could not be formed by water due to improper conditions. The low pressure, non-geothermal, colder regions would not give way to liquid water at any point in the year but would be ideal for solid carbon dioxide. The carbon dioxide melting in the warmer summer would yield liquid carbon dioxide which would then form the gullies.<ref>{{cite journal |last=Hoffman |first=Nick |title=Active polar gullies on Mars and the role of carbon dioxide |journal=Astrobiology |volume=2 |issue=3 |date=2002 |pages=313–323 |doi=10.1089/153110702762027899 |pmid=12530241|bibcode=2002AsBio...2..313H }}</ref><ref>{{cite journal |last1=Musselwhite |first1=Donald S. |first2=Timothy D. |last2=Swindle |first3=Jonathan I. |last3=Lunine |title=Liquid CO2 breakout and the formation of recent small gullies on Mars |journal=Geophysical Research Letters |volume=28 |issue=7 |date=2001 |pages=1283–1285 |doi=10.1029/2000gl012496 |bibcode=2001GeoRL..28.1283M|doi-access=free }}</ref> Even if gullies are carved by flowing water at the surface, the exact source of the water and the mechanisms behind its motion are not understood.<ref name="hirise2">{{cite journal |last1=McEwen |first1=Alfred. S. |last2=Ojha |first2=Lujendra |last3=Dundas |first3=Colin M. |date=June 17, 2011 |title=Seasonal Flows on Warm Martian Slopes |journal=Science |volume=333 |issue=6043 |pages=740–743 |publisher=American Association for the Advancement of Science |doi=10.1126/science.1204816 |issn=0036-8075 |bibcode=2011Sci...333..740M |pmid=21817049|s2cid=10460581 }}</ref>

In August 2011, NASA announced the discovery of current seasonal changes on steep slopes below rocky outcrops near crater rims in the Southern hemisphere. These dark streaks, now called [[recurrent slope lineae]] (RSL), were seen to grow downslope during the warmest part of the Martian Summer, then to gradually fade through the rest of the year, recurring cyclically between years.<ref name="NASA-20131210" /> The researchers suggested these marks were consistent with salty water ([[brine]]s) flowing downslope and then evaporating, possibly leaving some sort of residue.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/MRO/news/mro20110804.html |title=NASA Spacecraft Data Suggest Water Flowing on Mars |publisher=[[NASA]] |date=August 4, 2011 |access-date=August 4, 2011 |archive-date=March 4, 2016 |archive-url=https://web.archive.org/web/20160304075750/http://www.nasa.gov/mission_pages/MRO/news/mro20110804.html |url-status=dead }}</ref><ref name="SeasonalFlowsScience">{{cite journal| last1=McEwen| first1=Alfred| last2=Lujendra| first2=Ojha| last3=Dundas| first3=Colin| last4=Mattson| first4=Sarah| last5=Bryne| first5=S| last6=Wray| first6=J.| last7=Cull| first7=Selby| last8=Murchie| first8=Scott| last9=Thomas| first9=Nicholas| last10=Gulick| first10=Virginia| title=Seasonal Flows On Warm Martian Slopes| journal=Science| date=August 5, 2011| volume=333| issue=6043| pages=743| doi=10.1126/science.1204816| pmid=21817049| df=mdy-all| bibcode=2011Sci...333..740M| s2cid=10460581}}</ref> The CRISM spectroscopic instrument has since made direct observations of hydrous salts appearing at the same time that these recurrent slope lineae form, confirming in 2015 that these lineae are produced by the flow of liquid brines through shallow soils. The lineae contain hydrated chlorate and [[perchlorate]] salts ({{chem|Cl|O|4}}<sup>−</sup>), which contain liquid water molecules.<ref>{{Cite web| title = NASA Finds 'Definitive' Liquid Water on Mars|url = http://news.nationalgeographic.com/2015/09/150928-mars-liquid-water-confirmed-surface-streaks-space-astronomy/|archive-url = https://web.archive.org/web/20150930194303/http://news.nationalgeographic.com/2015/09/150928-mars-liquid-water-confirmed-surface-streaks-space-astronomy/|url-status = dead|archive-date = September 30, 2015|website = National Geographic News|access-date = September 30, 2015|first1 = Nadia|last1 = Drake|author1-link = Nadia Drake |date = September 28, 2015}}</ref> The lineae flow downhill in Martian summer, when the temperature is above {{convert|-23|C|F K}}.<ref>{{Cite web|title = Water Flows on Mars Today, NASA Announces|url = http://www.scientificamerican.com/article/water-flows-on-mars-today-nasa-announces/|access-date = September 30, 2015|first = Clara|last = Moskowitz|website = [[Scientific American]]|archive-date = May 15, 2021|archive-url = https://web.archive.org/web/20210515071020/https://www.scientificamerican.com/article/water-flows-on-mars-today-nasa-announces/|url-status = live}}</ref> However, the source of the water remains unknown.<ref name="Ojhaetal2015" /><ref>{{Cite web|url = https://www.youtube.com/watch?v=bDv4FRHI3J8|title = NASA News Conference: Evidence of Liquid Water on Today's Mars|date = September 28, 2015|publisher = NASA|access-date = September 30, 2015|archive-date = October 1, 2015|archive-url = https://web.archive.org/web/20151001113935/https://www.youtube.com/watch?v=bDv4FRHI3J8|url-status = live}}</ref><ref>{{Cite web|title = NASA Confirms Evidence That Liquid Water Flows on Today's Mars|url = http://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars/|access-date = September 30, 2015|date = September 28, 2015|archive-date = January 4, 2022|archive-url = https://web.archive.org/web/20220104123015/https://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars/|url-status = live}}</ref> However, neutron spectrometer data by the ''[[Mars Odyssey]]'' orbiter obtained over one decade, was published in December 2017, and shows no evidence of water (hydrogenated regolith) at the active sites, so its authors also support the hypotheses of either short-lived atmospheric water vapour deliquescence, or dry granular flows.<ref name="ICRS-20170928" /> They conclude that liquid water on today's Mars may be limited to traces of dissolved moisture from the atmosphere and thin films, which are challenging environments for life as it is currently known.<ref name="jpl.nasa.gov">{{cite web |url=https://www.jpl.nasa.gov/news/news.php?release=2017-299 |title=Recurring Martian Streaks: Flowing Sand, Not Water? |work=JPL NASA News |publisher=Jet Propulsion Laboratory, NASA |date=November 20, 2017 |access-date=December 18, 2017 |archive-date=November 9, 2020 |archive-url=https://web.archive.org/web/20201109033737/https://www.jpl.nasa.gov/news/news.php?release=2017-299 |url-status=live }}</ref>

An alternative scenario is a Knudsen pump effect, from photophoretic when shadows occurs in a granular material.<ref name='Scmhidt Knudsen pump 2017'>{{cite journal | doi = 10.1038/ngeo2917 | volume=10 | title=Formation of recurring slope lineae on Mars by rarefied gas-triggered granular flows | year=2017 | journal=Nature Geoscience | pages=270–273 | last1 = Schmidt | first1 = Frédéric | last2 = Andrieu | first2 = François | last3 = Costard | first3 = François | last4 = Kocifaj | first4 = Miroslav | last5 = Meresescu | first5 = Alina G.| issue=4 | arxiv = 1802.05018 | bibcode=2017NatGe..10..270S | s2cid=55016186 }}</ref> The authors demonstrated that the RSLs stopped at an angle of 28° in Garni crater, in agreement with dry granular avalanche. In addition, the authors pointed out several limitations of the wet hypothesis, such as the fact that the detection of water was only indirect (salt detection but not water).

== Present water ==
{{multiple image
 |align=right
 |direction=vertical
 |width=200
 |image1=Water equivalent hydrogen abundance in the lower latitudes of Mars 01.jpg
 |image2=Water equivalent hydrogen abundance in the high latitudes of Mars.jpg
 |caption2=Proportion of water ice present in the upper meter of the Martian surface for lower (top) and higher (bottom) latitudes. The percentages are derived through stoichiometric calculations based on epithermal neutron fluxes. These fluxes were detected by the Neutron Spectrometer aboard the 2001 Mars Odyssey spacecraft.}}
{{See also|Groundwater on Mars}}
A significant amount of surface [[hydrogen]] has been observed globally by the [[Mars Odyssey]] neutron spectrometer and [[Gamma Ray Spectrometer (2001 Mars Odyssey)|gamma ray spectrometer]]<ref name="Boynton2007">{{cite journal |last=Boynton |first=W. V. |display-authors=etal |date=2007 |title=Concentration of H, Si, Cl, K, Fe, and Th in the low and mid latitude regions of Mars |journal=[[Journal of Geophysical Research: Planets]] |volume=112 |issue=E12 |pages=E12S99 |doi=10.1029/2007JE002887 |bibcode=2007JGRE..11212S99B|doi-access=free }}</ref> and the [[Mars Express]] High Resolution Stereo Camera (HRSC).<ref>{{Cite web|title=Mars Express|url=https://www.esa.int/Science_Exploration/Space_Science/Mars_Express|access-date=2022-01-21|website=www.esa.int|language=en|archive-date=January 21, 2022|archive-url=https://web.archive.org/web/20220121174039/https://www.esa.int/Science_Exploration/Space_Science/Mars_Express|url-status=live}}</ref> This hydrogen is thought to be incorporated into the molecular structure of ice, and through [[stoichiometry|stoichiometric]] calculations the observed fluxes have been converted into concentrations of water ice in the upper meter of the Martian surface. This process has revealed that ice is both widespread and abundant on the present surface. Below 60 degrees of latitude, ice is concentrated in several regions, particularly around the [[Elysium Mons|Elysium]] volcanoes, [[Terra Sabaea]], and northwest of [[Terra Sirenum]], and exists in concentrations up to 18% ice in the subsurface. Above 60 degrees latitude, ice is highly abundant. Polewards on 70 degrees of latitude, ice concentrations exceed 25% almost everywhere, and approach 100% at the poles.<ref>{{cite journal |last1=Feldman |first1=W. C. |last2=Prettyman |first2=T. H. |last3=Maurice |first3=S. |last4=Plaut |first4=J. J. |last5=Bish |first5=D. L. |last6=Vaniman |first6=D. T. |last7=Tokar |first7=R. L. |date=2004 |title=Global distribution of near-surface hydrogen on Mars |journal=Journal of Geophysical Research |volume=109 |issue=E9 |page=E9 |id=E09006 |doi=10.1029/2003JE002160 |bibcode=2004JGRE..109.9006F|doi-access=free }}</ref> The [[SHARAD]] and [[MARSIS]] radar sounding instruments have also confirmed that individual surface features are ice rich. Due to the known instability of ice at current Martian surface conditions, it is thought that almost all of this ice is covered by a thin layer of rocky or dusty material.

The Mars Odyssey neutron spectrometer observations indicate that if all the ice in the top meter of the Martian surface were spread evenly, it would give a Water Equivalent Global layer (WEG) of at least ≈{{convert|14|cm}}—in other words, the globally averaged Martian surface is approximately 14% water.<ref name=Feldman2004>{{cite journal |last=Feldman |first=W. C. |display-authors=etal |date=2004 |title=Global distribution of near-surface hydrogen on Mars|journal= Journal of Geophysical Research|doi=10.1029/2003JE002160 |bibcode=2004JGRE..109.9006F |volume=109 |issue=E9|pages=E09006 |doi-access=free }}</ref> The water ice currently locked in both Martian poles corresponds to a WEG of {{convert|30|m}}, and geomorphic evidence favors significantly larger quantities of [[surface water]] over geologic history, with WEG as deep as {{convert|500|m}}.<ref name="ChristensenIceBudget">{{cite journal |last=Christensen |first=P. R. |date=2006 |title=Water at the Poles and in Permafrost Regions of Mars |journal=Elements |issue=2 |volume=3 |pages=151–155|doi=10.2113/gselements.2.3.151 |bibcode=2006Eleme...2..151C }}</ref><ref name="Feldman2004" /> It is thought that part of this past water has been lost to the deep subsurface, and part to space, although the detailed mass balance of these processes remains poorly understood.<ref name="Carr" /> The current atmospheric reservoir of water is important as a conduit allowing gradual migration of ice from one part of the surface to another on both seasonal and longer timescales, but it is insignificant in volume, with a WEG of no more than {{convert|10|μm}}.<ref name="Feldman2004" />

It is possible that liquid water could also exist on the surface of Mars through the formation of [[brine]]s suggested by the abundance of hydrated salts.<ref>{{Cite journal |last1=Chevrier |first1=Vincent F. |last2=Rivera-Valentin |first2=Edgard G. |date=November 2012 |title=Formation of recurring slope lineae by liquid brines on present-day Mars: LIQUID BRINES ON MARS |journal=Geophysical Research Letters |language=en |volume=39 |issue=21 |pages=n/a |doi=10.1029/2012GL054119|s2cid=1077206 |doi-access=free }}</ref><ref>{{Cite journal |last1=Gough |first1=R.V. |last2=Primm |first2=K.M. |last3=Rivera-Valentín |first3=E.G. |last4=Martínez |first4=G.M. |last5=Tolbert |first5=M.A. |date=March 2019 |title=Solid-solid hydration and dehydration of Mars-relevant chlorine salts: Implications for Gale Crater and RSL locations |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103518304469 |journal=Icarus |language=en |volume=321 |pages=1–13 |doi=10.1016/j.icarus.2018.10.034 |bibcode=2019Icar..321....1G |s2cid=106323485 |access-date=May 13, 2022 |archive-date=July 7, 2022 |archive-url=https://web.archive.org/web/20220707162400/https://linkinghub.elsevier.com/retrieve/pii/S0019103518304469 |url-status=live }}</ref> Brines are significant on Mars because they can stabilize liquid water at lower temperatures than pure water on its own.<ref name=":0">{{Cite journal |last1=Chevrier |first1=Vincent F. |last2=Altheide |first2=Travis S. |date=2008-11-18 |title=Low temperature aqueous ferric sulfate solutions on the surface of Mars |journal=Geophysical Research Letters |language=en |volume=35 |issue=22 |pages=L22101 |doi=10.1029/2008GL035489 |bibcode=2008GeoRL..3522101C |s2cid=97468338 |issn=0094-8276|doi-access=free }}</ref><ref>{{Cite journal |last1=Chevrier |first1=Vincent F. |last2=Hanley |first2=Jennifer |last3=Altheide |first3=Travis S. |date=2009-05-20 |title=Stability of perchlorate hydrates and their liquid solutions at the Phoenix landing site, Mars |journal=Geophysical Research Letters |language=en |volume=36 |issue=10 |pages=L10202 |doi=10.1029/2009GL037497 |bibcode=2009GeoRL..3610202C |s2cid=42150205 |issn=0094-8276|doi-access=free }}</ref> Pure liquid water is unstable on the surface of the planet, as it is subjected to freezing, evaporation, and boiling.<ref name=":0" /> Similar to how salt is applied to roads on Earth to prevent them from icing over, briny mixtures of water and salt on Mars may have low enough freezing points to lead to stable liquid at the surface. Given the complex nature of the Martian [[regolith]], mixtures of salts are known to change the stability of brines.<ref>{{Cite journal |last1=Gough |first1=R.V. |last2=Chevrier |first2=V.F. |last3=Tolbert |first3=M.A. |date=May 2014 |title=Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures |url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X14000752 |journal=Earth and Planetary Science Letters |language=en |volume=393 |pages=73–82 |doi=10.1016/j.epsl.2014.02.002 |bibcode=2014E&PSL.393...73G |access-date=May 13, 2022 |archive-date=July 7, 2022 |archive-url=https://web.archive.org/web/20220707055829/https://linkinghub.elsevier.com/retrieve/pii/S0012821X14000752 |url-status=live }}</ref> Modeling the [[deliquescence]] of salt mixtures can be used to test for brine stability and can help us determine if liquid brines are present on the surface of Mars. The composition of the Martian regolith, determined by the [[Phoenix (spacecraft)|Phoenix]] lander, can be used to constrain these models and give an accurate representation of how brines may actually form on the planet.<ref>{{Cite journal |last1=Hecht |first1=M. H. |last2=Kounaves |first2=S. P. |last3=Quinn |first3=R. C. |last4=West |first4=S. J. |last5=Young |first5=S. M. M. |last6=Ming |first6=D. W. |last7=Catling |first7=D. C. |last8=Clark |first8=B. C. |last9=Boynton |first9=W. V. |last10=Hoffman |first10=J. |last11=DeFlores |first11=L. P. |date=2009-07-03 |title=Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site |url=https://www.science.org/doi/10.1126/science.1172466 |journal=Science |language=en |volume=325 |issue=5936 |pages=64–67 |doi=10.1126/science.1172466 |pmid=19574385 |bibcode=2009Sci...325...64H |s2cid=24299495 |issn=0036-8075 |access-date=May 13, 2022 |archive-date=May 13, 2022 |archive-url=https://web.archive.org/web/20220513184052/https://www.science.org/doi/10.1126/science.1172466 |url-status=live }}</ref><ref>{{Cite journal |last1=Kounaves |first1=Samuel P. |last2=Hecht |first2=Michael H. |last3=Kapit |first3=Jason |last4=Quinn |first4=Richard C. |last5=Catling |first5=David C. |last6=Clark |first6=Benton C. |last7=Ming |first7=Douglas W. |last8=Gospodinova |first8=Kalina |last9=Hredzak |first9=Patricia |last10=McElhoney |first10=Kyle |last11=Shusterman |first11=Jennifer |date=May 2010 |title=Soluble sulfate in the martian soil at the Phoenix landing site: SULFATE AT THE PHOENIX LANDING SITE |url=http://doi.wiley.com/10.1029/2010GL042613 |journal=Geophysical Research Letters |language=en |volume=37 |issue=9 |pages=n/a |doi=10.1029/2010GL042613|bibcode=2010GeoRL..37.9201K |s2cid=12914422 }}</ref> Results of these models give [[water activity]] values for various salts at different temperatures, where the lower the water activity, the more stable the brine. At temperatures between 208 K and 253 K, [[chlorate]] salts exhibit the lowest water activity values, and below 208 K [[chloride]] salts exhibit the lowest values. Results of modeling show that the aforementioned complex mixtures of salts do not significantly increase the stability of brines, indicating that brines may not be a significant source of liquid water at the surface of Mars.<ref>{{Cite journal |last=Chevrier |first=Vincent |date=2022 |title=Limited stability of multi-component brines on the surface of Mars |journal=The Planetary Science Journal|volume=3 |issue=5 |page=125 |doi=10.3847/PSJ/ac6603 |bibcode=2022PSJ.....3..125C |s2cid=249227810 |doi-access=free }}</ref>

=== Polar ice caps ===
{{Main|Martian polar ice caps}}
[[File:Martian north polar cap.jpg|thumb|The [[Mars Global Surveyor]] acquired this image of the Martian north polar ice cap in early northern summer.]]

The existence of ice in the Martian northern ([[Planum Boreum]]) and southern ([[Planum Australe]]) polar caps has been known since the time of [[Mariner 9]] orbiter.<ref>{{Cite journal|last=Cutts|first=James A.|date=1973-07-10|title=Nature and origin of layered deposits of the Martian polar regions|journal=Journal of Geophysical Research|language=en|volume=78|issue=20|pages=4231–4249|doi=10.1029/JB078i020p04231|bibcode=1973JGR....78.4231C}}</ref> However, the amount and purity of this ice were not known until the early 2000s. In 2004, the [[MARSIS]] radar sounder on the European ''[[Mars Express]]'' satellite confirmed the existence of relatively clean ice in the south polar ice cap that extends to a depth of {{convert|3.7|km|mi}} below the surface.<ref name="NASAwater">{{cite web |publisher=NASA |date=March 15, 2007 |title=Mars' South Pole Ice Deep and Wide |work=NASA News & Media Resources |url=http://www.nasa.gov/mission_pages/mars/news/mars-20070315.html |access-date=March 18, 2013 |archive-date=December 8, 2021 |archive-url=https://web.archive.org/web/20211208131250/http://www.nasa.gov/mission_pages/mars/news/mars-20070315.html |url-status=dead }}</ref><ref name="Plaut2007">{{cite journal |last=Plaut |first=J. J. |display-authors=etal |title=Subsurface Radar Sounding of the South Polar Layered Deposits of Mars |journal=Science |date=March 15, 2007 |doi=10.1126/science.1139672 |volume=316 |issue=5821 |pages=92–95 |pmid=17363628|bibcode=2007Sci...316...92P |s2cid=23336149 |doi-access=free }}</ref> Similarly, the SHARAD radar sounder on board the ''[[Mars Reconnaissance Orbiter]]'' observed the base of the north polar cap 1.5 – 2&nbsp;km beneath the surface. Together, the volume of ice present in the Martian north and south polar ice caps is similar to that of the [[Greenland ice sheet]].<ref>{{Cite journal|last=Byrne|first=Shane|date=2009|title=The Polar Deposits of Mars|journal=Annual Review of Earth and Planetary Sciences|volume=37|issue=1|pages=535–560|doi=10.1146/annurev.earth.031208.100101|bibcode=2009AREPS..37..535B|s2cid=54874200}}</ref> [[File:PIA13164 North Polar Cap Cross Section, Annotated Version.jpg|thumb|left|Cross-section of a portion of the north polar ice cap of Mars, derived from satellite radar sounding.]]

An even larger ice sheet on south polar region sheet is suspected to have retreated in ancient times ([[Hesperian|Hesperian period]]), that may have contained 20 million km<sup>3</sup> of water ice, which is equivalent to a layer 137 m deep over the entire planet.<ref>Scanlon, K., et al.  2018.  The Dorsa Argentea Formation and the Noachian-Hesperian climate transition.  Icarus:  299, 339–363.</ref><ref>Head, J, S. Pratt.  2001.  Extensive Hesperian-aged south polar ice sheet on Mars: Evidence for massive melting and retreat, and lateral flow and pending of meltwater.  J. Geophys. Res.-Planet, 106 (E6), 12275-12299.</ref>

Both polar caps reveal abundant internal layers of ice and dust when examined with images of the spiral-shaped troughs that cut through their volume, and the subsurface radar measurements showed that these layers extend continuously across the ice sheets. This layering contains a record of past climates on Mars, just how Earth's ice sheets have a record for Earth's climate. Reading this record is not straightforward however,<ref>{{cite journal|last1=Fishbaugh|first1=KE|last2=Byrne|first2=Shane|last3=Herkenhoff|first3=Kenneth E.|last4=Kirk|first4=Randolph L.|last5=Fortezzo|first5=Corey|last6=Russell|first6=Patrick S.|last7=McEwen|first7=Alfred|date=2010|title=Evaluating the meaning of "layer" in the Martian north polar layered depsoits and the impact on the climate connection|url=http://www.lpl.arizona.edu/~shane/publications/fishbaugh_etal_icarus_2010.pdf|journal=Icarus|volume=205|issue=1|pages=269–282|bibcode=2010Icar..205..269F|doi=10.1016/j.icarus.2009.04.011|access-date=January 19, 2012|archive-date=July 6, 2021|archive-url=https://web.archive.org/web/20210706110813/https://www.lpl.arizona.edu/~shane/publications/fishbaugh_etal_icarus_2010.pdf|url-status=live}}</ref> so, many researchers have studied this layering not only to understand the structure, history, and flow properties of the caps,<ref name="Carr" /> but also to understand the evolution of climate on Mars.<ref>{{Cite web|url=https://eos.org/research-spotlights/how-mars-got-its-layered-north-polar-cap|title=How Mars Got Its Layered North Polar Cap|website=Eos|date=February 8, 2017|language=en-US|access-date=2019-09-26|archive-date=November 10, 2021|archive-url=https://web.archive.org/web/20211110122705/https://eos.org/research-spotlights/how-mars-got-its-layered-north-polar-cap|url-status=live}}</ref><ref>{{Cite web|url=https://eos.org/editor-highlights/peeling-back-the-layers-of-the-climate-of-mars|title=Peeling Back the Layers of the Climate of Mars|website=Eos|date=July 18, 2019|language=en-US|access-date=2019-09-26|archive-date=December 5, 2021|archive-url=https://web.archive.org/web/20211205213045/https://eos.org/editor-highlights/peeling-back-the-layers-of-the-climate-of-mars|url-status=live}}</ref>

Surrounding the polar caps are many smaller ice sheets inside craters, some of which lie under thick deposits of sand or martian dust.<ref>{{Cite journal|last1=Conway|first1=Susan J.|last2=Hovius|first2=Niels|last3=Barnie|first3=Talfan|last4=Besserer|first4=Jonathan|last5=Le Mouélic|first5=Stéphane|last6=Orosei|first6=Roberto|last7=Read|first7=Natalie Anne|date=2012-07-01|title=Climate-driven deposition of water ice and the formation of mounds in craters in Mars' north polar region|journal=Icarus|volume=220|issue=1|pages=174–193|doi=10.1016/j.icarus.2012.04.021|issn=0019-1035|bibcode=2012Icar..220..174C|s2cid=121435046|url=https://hal-insu.archives-ouvertes.fr/insu-02276816/file/HAL_Conway_icarus_2012.pdf|access-date=October 14, 2019|archive-date=September 18, 2021|archive-url=https://web.archive.org/web/20210918070138/https://hal-insu.archives-ouvertes.fr/insu-02276816/file/HAL_Conway_icarus_2012.pdf|url-status=live}}</ref><ref>{{Cite web|url=https://phys.org/news/2019-09-ice-islands-mars-pluto-reveal.html|title=Ice islands on Mars and Pluto could reveal past climate change|website=phys.org|language=en-us|access-date=2019-09-26|archive-date=October 9, 2021|archive-url=https://web.archive.org/web/20211009114210/https://phys.org/news/2019-09-ice-islands-mars-pluto-reveal.html|url-status=live}}</ref> Particularly, the {{convert|81.4|km|mi}} wide [[Korolev (Martian crater)|Korolev Crater]], is estimated to contain approximately {{convert|2200|km3|mi3}} of water ice exposed to the surface.<ref name="DLR">{{cite web |title=A winter wonderland in red and white – Korolev Crater on Mars |url=https://www.dlr.de/content/en/articles/news/2018/4/20181220_korolev-crater-on-mars.html |website=German Aerospace Center (DLR) |access-date=20 December 2018 |archive-date=October 17, 2020 |archive-url=https://web.archive.org/web/20201017062246/https://www.dlr.de/content/en/articles/news/2018/4/20181220_korolev-crater-on-mars.html |url-status=live }}</ref> Korolev's floor lies about {{convert|2|km|mi}} below the rim, and is covered by a {{convert|1.8|km|mi}} deep central mound of permanent water ice, up to {{convert|60|km|mi}} in diameter.<ref name="DLR"/><ref name="TG1218">{{cite news|url=https://www.theguardian.com/science/2018/dec/21/mars-express-beams-back-images-of-ice-filled-korolev-crater|newspaper=The Guardian|access-date=December 21, 2018|title=Mars Express beams back images of ice-filled Korolev crater|date=December 21, 2018|first1=Ian|last1=Sample|archive-date=February 8, 2020|archive-url=https://web.archive.org/web/20200208045902/https://www.theguardian.com/science/2018/dec/21/mars-express-beams-back-images-of-ice-filled-korolev-crater|url-status=live}}</ref>

===={{anchor|Subglacial liquid water}}Subglacial liquid water====
{{main|Subglacial lakes on Mars}}
[[File:Mars-SubglacialWater-SouthPoleRegion-20180725.jpg|thumb|Site of [[Planum Australe|south polar]] subglacial water body (reported July 2018).]]
The existence of subglacial lakes on Mars was hypothesised when modelling of [[Lake Vostok]] in [[Antarctica]] showed that this lake could have existed before the Antarctic glaciation, and that a similar scenario could potentially have occurred on Mars.<ref>{{cite journal |url=http://www.agu.org/journals/je/v106/iE01/2000JE001254/2000JE001254.pdf |last1=Duxbury |first1=N. S. |last2=Zotikov |first2=I. A. |last3=Nealson |first3=K. H. |last4=Romanovsky |first4=V. E. |last5=Carsey |first5=F. D. |title=A numerical model for an alternative origin of Lake Vostok and its exobiological implications for Mars |doi=10.1029/2000JE001254 |date=2001 |page=1453 |volume=106 |issue=E1 |journal=Journal of Geophysical Research |bibcode=2001JGR...106.1453D|doi-access=free }}</ref> In July 2018, scientists from the [[Italian Space Agency]] reported the detection of such a [[subglacial lake]] on Mars, {{convert|1.5|km|0}} below the [[Planum Australe|southern polar ice cap]], and spanning {{convert|20|km|-1}} horizontally, the first evidence for a stable body of liquid water on the planet.<ref name="SCI-20180725">{{cite journal |author=Orosei, R. |display-authors=etal |title=Radar evidence of subglacial liquid water on Mars |date=July 25, 2018 |journal=[[Science (journal)|Science]] |volume=361 |issue=6401 |pages=490–493 |doi=10.1126/science.aar7268 |pmid=30045881 |arxiv=2004.04587 |bibcode=2018Sci...361..490O |hdl=11573/1148029 |s2cid=206666385 |hdl-access=free }}</ref><ref name="NYT-20180725">{{cite news |last1=Chang |first1=Kenneth |last2=Overbye |first2=Dennis |author-link2=Dennis Overbye |title=A Watery Lake Is Detected on Mars, Raising the Potential for Alien Life – The discovery suggests that watery conditions beneath the icy southern polar cap may have provided one of the critical building blocks for life on the red planet. |url=https://www.nytimes.com/2018/07/25/science/mars-liquid-alien-life.html |date=July 25, 2018 |work=[[The New York Times]] |access-date=July 25, 2018 |archive-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725205154/https://www.nytimes.com/2018/07/25/science/mars-liquid-alien-life.html |url-status=live }}</ref><ref>{{cite web |title=Huge reservoir of liquid water detected under the surface of Mars |url=https://www.eurekalert.org/pub_releases/2018-07/aaft-hro072318.php |work=[[EurekAlert]] |date=July 25, 2018 |access-date=July 25, 2018 |archive-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725163215/https://www.eurekalert.org/pub_releases/2018-07/aaft-hro072318.php |url-status=live }}</ref><ref>{{cite news |title=Liquid water 'lake' revealed on Mars |url=https://www.bbc.co.uk/news/science-environment-44952710 |work=BBC News |date=July 25, 2018 |access-date=July 25, 2018 |archive-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725141308/https://www.bbc.co.uk/news/science-environment-44952710 |url-status=live }}</ref> The evidence for this [[lakes on Mars|Martian lake]] was deduced from a bright spot in the radar echo sounding data of the [[MARSIS]] radar on board the European ''[[Mars Express]]'' orbiter,<ref name="Suppl material">[https://www.science.org/doi/10.1126/science.aar7268 Supplementary Materials] {{Webarchive|url=https://web.archive.org/web/20220709092553/https://www.science.org/doi/10.1126/science.aar7268 |date=July 9, 2022 }} for: {{cite journal | doi = 10.1126/science.aar7268 | pmid=30045881 | volume=361 | title=Radar evidence of subglacial liquid water on Mars | year=2018 | journal=Science | pages=490–493 | last1 = Orosei | first1 = R | last2 = Lauro | first2 = SE | last3 = Pettinelli | first3 = E | last4 = Cicchetti | first4 = A | last5 = Coradini | first5 = M | last6 = Cosciotti | first6 = B | last7 = Di Paolo | first7 = F | last8 = Flamini | first8 = E | last9 = Mattei | first9 = E | last10 = Pajola | first10 = M | last11 = Soldovieri | first11 = F | last12 = Cartacci | first12 = M | last13 = Cassenti | first13 = F | last14 = Frigeri | first14 = A | last15 = Giuppi | first15 = S | last16 = Martufi | first16 = R | last17 = Masdea | first17 = A | last18 = Mitri | first18 = G | last19 = Nenna | first19 = C | last20 = Noschese | first20 = R | last21 = Restano | first21 = M | last22 = Seu | first22 = R | issue=6401 | arxiv=2004.04587 | bibcode = 2018Sci...361..490O| doi-access = free }}</ref> collected between May 2012 and December 2015.  The detected lake is centred at 193°E, 81°S, a flat area that does not exhibit any peculiar topographic characteristics but is surrounded by higher ground, except on its eastern side where there is a depression.<ref name="SCI-20180725"/> The [[SHARAD]] radar on board NASA's ''[[Mars Reconnaissance Orbiter]]'' has seen no sign of the lake. The operating frequencies of SHARAD are designed for higher resolution, but lower penetration depth, so if the overlying ice contains a significant amount of silicates, it is unlikely that SHARAD will be able to detect the putative lake.

On 28 September 2020, the MARSIS discovery was confirmed, using new data, and reanalysing all the data with a new technique. These new radar studies report three more subglacial lakes on Mars. All are {{convert|1.5|km|mi|abbr=on}} below the [[Planum Australe|southern polar ice cap]]. The size of the first lake found, and the largest, has been corrected to {{convert|30|km|mi|abbr=on}} wide. It is surrounded by 3 smaller lakes, each a few kilometres wide.<ref name="NatAstro">{{cite journal |last1=Lauro |first1=Sebastian Emanuel |last2=Pettinelli |first2=Elena |last3=Caprarelli |first3=Graziella |last4=Guallini |first4=Luca |last5=Rossi |first5=Angelo Pio |last6=Mattei |first6=Elisabetta |last7=Cosciotti |first7=Barbara |last8=Cicchetti |first8=Andrea |last9=Soldovieri |first9=Francesco |last10=Cartacci |first10=Marco |last11=Di Paolo |first11=Federico |last12=Noschese |first12=Raffaella |last13=Orosei |first13=Roberto |title=Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data |journal=Nature Astronomy |date=28 September 2020 |volume=5 |pages=63–70 |doi=10.1038/s41550-020-1200-6 |arxiv=2010.00870 |bibcode=2021NatAs...5...63L |s2cid=222125007 |language=en |issn=2397-3366}}</ref>
[[File:Frouin (Martian crater).jpg|thumb|Patch of water ice sitting on the floor of the Frouin Crater near the North Pole of Mars (70.5° North and 103° East)]]
Because the temperature at the base of the polar cap is estimated to be {{convert|205|K}}, scientists assume that the water may remain liquid through the antifreeze effect of magnesium and calcium [[perchlorate]]s.<ref name="SCI-20180725"/><ref>{{cite news |url=https://www.bbc.com/news/science-environment-44952710 |title=Liquid water 'lake' revealed on Mars |first=Mary |last=Halton |work=BBC News |date=July 25, 2018 |access-date=July 25, 2018 |archive-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725141646/https://www.bbc.com/news/science-environment-44952710 |url-status=live }}</ref> The {{convert|1.5|km|adj=on}} ice layer covering the lake is composed of water ice with 10 to 20% admixed dust, and seasonally covered by a {{convert|1|m|ftin|adj=mid|-thick}} layer of {{CO2}} ice.<ref name="SCI-20180725"/> Since the raw-data coverage of the south polar ice cap is limited, the discoverers stated that "there is no reason to conclude that the presence of subsurface water on Mars is limited to a single location."<ref name="SCI-20180725"/>

In 2019, a study was published that explored the physical conditions necessary for such a lake to exist.<ref>{{Cite journal|last1=Sori|first1=Michael M.|last2=Bramson|first2=Ali M.|date=2019|title=Water on Mars, With a Grain of Salt: Local Heat Anomalies Are Required for Basal Melting of Ice at the South Pole Today|journal=Geophysical Research Letters|language=en|volume=46|issue=3|pages=1222–1231|doi=10.1029/2018GL080985|issn=1944-8007|bibcode=2019GeoRL..46.1222S|hdl=10150/633584|s2cid=134166238 |hdl-access=free}}</ref> The study calculated the amount of geothermal heat necessary to reach temperatures under which a liquid water and perchlorate mix would be stable under the ice. The authors concluded that "even if there are local concentrations of large amounts of perchlorate salts at the base of the south polar ice, typical Martian conditions are too cold to melt the ice&nbsp;... a local heat source within the crust is needed to increase the temperatures, and a magma chamber within 10 km of the ice could provide such a heat source. This result suggests that if the liquid water interpretation of the observations is correct, magmatism on Mars may have been active extremely recently."

China's Zhurong rover that studied Utopia Planitia region of Mars found a shift in sand dunes at around the same time as layers in the North polar region changed. Researchers believe that the tilt of Mars changed at that time and produced changes in the winds at Zhurong's landing site and in the layers in the ice cap.<ref>Liu, J., et al. 2023. "Martian dunes indicative of wind regime shift in line with end of ice age". ''Nature''. {{doi|10.1038/s41586-023-06206-1}}</ref>

If a liquid lake does indeed exist, its salty water may also be mixed with soil to form a sludge.<ref name="RTE2018-07-25a">{{cite web|title=Giant liquid water lake found under Martian ice|url=https://www.rte.ie/news/2018/0725/981031-mars-lake/|date=July 25, 2018|access-date=July 26, 2018|website=[[RTÉ]]|archive-date=July 25, 2021|archive-url=https://web.archive.org/web/20210725160157/https://www.rte.ie/news/2018/0725/981031-mars-lake/|url-status=live}}</ref> The lake's high levels of salt would present difficulties for most lifeforms. On Earth, organisms called [[halophile]]s exist that thrive in extremely salty conditions, though not in dark, cold, concentrated perchlorate solutions.<ref name="RTE2018-07-25a" /> Nevertheless, halotolerant organisms might be able to cope with enhanced perchlorate concentrations by drawing on physiological adaptations similar to those observed in the yeast ''[[Debaryomyces hansenii]]'' exposed in lab experiments to increasing [[Sodium perchlorate|NaClO<sub>4</sub>]] concentrations.<ref>{{Cite journal |last1=Heinz |first1=Jacob |last2=Doellinger |first2=Joerg |last3=Maus |first3=Deborah |last4=Schneider |first4=Andy |last5=Lasch |first5=Peter |last6=Grossart |first6=Hans-Peter |last7=Schulze-Makuch |first7=Dirk |date=2022-08-10 |title=Perchlorate-specific proteomic stress responses of Debaryomyces hansenii could enable microbial survival in Martian brines |journal=Environmental Microbiology |volume=24 |issue=11 |language=en |pages=1462–2920.16152 |doi=10.1111/1462-2920.16152 |pmid=35920032 |issn=1462-2912|doi-access=free |bibcode=2022EnvMi..24.5051H }}</ref>

=== Ground ice and subsurface ice ===
{{See also|Groundwater on Mars}}
For many years, various scientists have suggested that some Martian surfaces look like [[periglacial]] regions on Earth.<ref name="Kieffer1992" /> By analogy with these terrestrial features, it has been argued for many years that these may be regions of [[permafrost]]. This would suggest that frozen water lies right beneath the surface.<ref name="ICRS-20170928">{{cite journal |author=Wilson, Jack T. |display-authors=etal |title=Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data |date=January 2018 |journal=[[Icarus (journal)|Icarus]] |doi=10.1016/j.icarus.2017.07.028 |bibcode=2018Icar..299..148W |volume=299 |pages=148–160|arxiv=1708.00518 |s2cid=59520156 }}</ref><ref name="SP-20171002">{{cite web |last=Howell |first=Elizabeth |title=Water Ice Mystery Found at Martian Equator |url=https://www.space.com/38330-water-ice-mystery-at-mars-equator.html |date=October 2, 2017 |work=[[Space.com]] |access-date=October 2, 2017 |archive-date=November 11, 2021 |archive-url=https://web.archive.org/web/20211111082320/https://www.space.com/38330-water-ice-mystery-at-mars-equator.html |url-status=live }}</ref> A common feature in the higher latitudes, [[patterned ground]], can occur in a number of shapes, including stripes and polygons. On the Earth, these shapes are caused by the freezing and thawing of soil.<ref>{{cite web |url=http://www.spaceref.com/news/viewnews.html?id=494 |title=Polygonal Patterned Ground: Surface Similarities Between Mars and Earth |publisher=SpaceRef |date=September 28, 2002}}</ref> There are other types of evidence for large amounts of frozen water under the surface of Mars, such as [[Terrain softening (Mars)|terrain softening]], which rounds sharp topographical features.<ref>{{cite journal |doi=10.1016/0019-1035(89)90078-X |last=Squyres |first=S. |date=1989 |title=Urey Prize Lecture: Water on Mars |journal=Icarus |volume=79 |pages=229–288 |bibcode=1989Icar...79..229S |issue=2}}</ref> Evidence from Mars Odyssey's [[Gamma Ray Spectrometer (2001 Mars Odyssey)|gamma ray spectrometer]] and direct measurements with the [[Phoenix (spacecraft)|Phoenix]] lander have corroborated that many of these features are intimately associated with the presence of ground ice.<ref name="Scalloped terrains in the Peneus an">{{cite journal |doi=10.1016/j.icarus.2009.06.005 |last1=Lefort |first1=A. |last2=Russell |date=2010 |first2=P.S. |last3=Thomas |first3=N. |title=Scalloped terrains in the Peneus and Amphitrites Paterae region of Mars as observed by HiRISE |journal=Icarus |volume=205 |issue=1 |pages=259–268 |bibcode=2010Icar..205..259L}}</ref>

[[File:Mars exposed subsurface ice.jpg|thumb|A cross-section of underground water ice is exposed at the steep slope that appears bright blue in this enhanced-color view from the [[Mars Reconnaissance Orbiter|MRO]].<ref name='exposed ice 2018'/> The scene is about 500 meters wide. The scarp drops about 128 meters from the level ground. The ice sheets extend from just below the surface to a depth of 100 meters or more.<ref>{{cite journal | year = 2018| title =  Exposed subsurface ice sheets in the Martian mid-latitudes| journal = Science | volume =  359| issue = 6372| pages =  199–201| doi = 10.1126/science.aao1619  | last1 = Dundas | first1 = Colin M. | last2 = Bramson | first2 = Ali M. | last3 = Ojha | first3 = Lujendra | last4 = Wray | first4 = James J. | last5 = Mellon | first5 = Michael T. | last6 = Byrne | first6 = Shane | last7 = McEwen | first7 = Alfred S. | last8 = Putzig | first8 = Nathaniel E. | last9 = Viola | first9 = Donna | last10 = Sutton | first10 = Sarah | last11 = Clark | first11 = Erin | last12 = Holt | first12 = John W. | pmid = 29326269 | bibcode = 2018Sci...359..199D| doi-access = free }}</ref>]]

In 2018, using the HiRISE camera on board the [[Mars Reconnaissance Orbiter]] (MRO), researchers found at least eight eroding slopes showing exposed water ice sheets as thick as 100 meters, covered by a layer of about 1 or 2 meters thick of [[Martian soil|soil]].<ref name='exposed ice 2018'>[https://www.jpl.nasa.gov/news/news.php?feature=7038 Steep Slopes on Mars Reveal Structure of Buried Ice] {{Webarchive|url=https://web.archive.org/web/20190617060621/https://www.jpl.nasa.gov/news/news.php?feature=7038 |date=June 17, 2019 }}. NASA Press Release. January 11, 2018.</ref><ref>[https://www.science.org/content/article/ice-cliffs-spotted-mars Ice cliffs spotted on Mars] {{Webarchive|url=https://web.archive.org/web/20180128062432/http://www.sciencemag.org/news/2018/01/ice-cliffs-spotted-mars |date=January 28, 2018 }}. ''Science News''. Paul Voosen. January 11, 2018.</ref> The sites are at latitudes from about 55 to 58 degrees, suggesting that there is shallow ground ice under roughly a third of the Martian surface.<ref name='exposed ice 2018'/> This image confirms what was previously detected with the spectrometer on [[2001 Mars Odyssey]], the ground-penetrating radars on MRO and on [[Mars Express]], and by the [[Phoenix (spacecraft)|''Phoenix'' lander]] ''in situ'' excavation.<ref name='exposed ice 2018'/> These ice layers hold easily accessible clues about Mars' climate history and make frozen water accessible to future robotic or human explorers.<ref name='exposed ice 2018'/> Some researchers suggested these deposits could be the remnants of glaciers that existed millions of years ago when the planet's spin axis and orbit were different. (See section [[#Ice ages|Mars' Ice ages]] below.) A more detailed study published in 2019 discovered that water ice exists at latitudes north of 35°N and south of 45°S, with some ice patches only a few centimeters from the surface covered by dust. Extraction of water ice at these conditions would not require complex equipment.<ref>{{cite journal |url=https://www.hou.usra.edu/meetings/ninthmars2019/pdf/6027.pdf |title=Widespread Shallow Water Ice on Mars at High and Mid Latitudes |journal=[[Geophysical Research Letters]] |first1=Sylvain |last1=Piqueux |first2=Jennifer |last2=Buz |first3=Christopher S. |last3=Edwards |first4=Joshua L. |last4=Bandfield |first5=Armin |last5=Kleinböhl |first6=David M. |last6=Kass |first7=Paul O. |last7=Hayne |doi=10.1029/2019GL083947 |date=December 10, 2019 |s2cid=212982895 |access-date=December 12, 2019 |archive-date=September 18, 2021 |archive-url=https://web.archive.org/web/20210918070202/https://www.hou.usra.edu/meetings/ninthmars2019/pdf/6027.pdf |url-status=live }}</ref><ref>{{cite web |url=https://www.jpl.nasa.gov/news/news.php?feature=7557 |title=NASA's Treasure Map for Water Ice on Mars |date=2019-12-10 |publisher=Jet Propulsion Laboratory |access-date=December 12, 2019 |archive-date=June 29, 2021 |archive-url=https://web.archive.org/web/20210629003318/https://www.jpl.nasa.gov/news/news.php?feature=7557 |url-status=live }}</ref>

<gallery class="center" widths="380px" heights="360px">
File:ESP 025840 2240-3icecrater.gif|Ice disappearing after being exposed by impact.
</gallery>

<gallery class="center"  widths="380px" heights="360px">
File:50345 1230icelayersangular.jpg|Close view of wall of triangular depression, as seen by HiRISE  layers are visible in the wall.  These layers contain ice.  The lower layers are tilted, while layers near the surface are more or less horizontal.  Such an arrangement of layers is called an  "angular [[unconformity]]".<ref>{{Cite journal |last1=Dundas |first1=Colin M. |last2=Bramson |first2=Ali M. |last3=Ojha |first3=Lujendra |last4=Wray |first4=James J. |last5=Mellon |first5=Michael T. |last6=Byrne |first6=Shane |last7=McEwen |first7=Alfred S. |last8=Putzig |first8=Nathaniel E. |last9=Viola |first9=Donna |last10=Sutton |first10=Sarah |last11=Clark |first11=Erin |last12=Holt |first12=John W. |date=2018-01-12 |title=Exposed subsurface ice sheets in the Martian mid-latitudes |url=https://www.science.org/doi/10.1126/science.aao1619 |journal=Science |language=en |volume=359 |issue=6372 |pages=199–201 |doi=10.1126/science.aao1619 |pmid=29326269 |bibcode=2018Sci...359..199D |issn=0036-8075}}</ref>
File:ESP 053867 2245hotejecta.jpg|Impact crater that may have formed in ice-rich ground, as seen by HiRISE under [[HiWish program]] Location is the [[Ismenius Lacus quadrangle]].
File:53867 2245hotejectamargin.jpg|Close view of impact crater that may have formed in ice-rich ground, as seen by HiRISE under HiWish program.  Note that the [[Ejecta blanket|ejecta]] seems lower than the surroundings. The hot ejecta may have caused some of the ice to go away, thus lowering the level of the ejecta.
File:Icemaplargelabeled454arrows.jpg|Map of near surface ice
</gallery>

====Scalloped topography====
{{main|Scalloped topography}}
Certain regions of Mars display [[scalloped topography|scalloped]]-shaped depressions. The depressions are suspected to be the remains of a degrading ice-rich mantle deposit. Scallops are caused by ice [[Sublimation (phase transition)|sublimating]] from frozen soil. The landforms of scalloped topography can be formed by the subsurface loss of water ice by sublimation under current Martian climate conditions. A model predicts similar shapes when the ground has large amounts of pure ice, up to many tens of meters in depth.<ref>Dundas, C.; Bryrne, S.; McEwen, A. (2015). "Modeling the development of martian sublimation thermokarst landforms". ''Icarus'' 262, 154–169.</ref> This mantle material was probably deposited from the atmosphere as ice formed on dust when the climate was different due to changes in the tilt of the Mars pole (see {{section link||Ice ages}}, below).<ref name=":8">{{Cite journal |last=Christensen |first=Philip R. |date=March 2003 |title=Formation of recent martian gullies through melting of extensive water-rich snow deposits |url=https://www.nature.com/articles/nature01436 |journal=Nature |language=en |volume=422 |issue=6927 |pages=45–48 |doi=10.1038/nature01436 |pmid=12594459 |bibcode=2003Natur.422...45C |s2cid=4385806 |issn=1476-4687 |access-date=July 27, 2022 |archive-date=August 9, 2021 |archive-url=https://web.archive.org/web/20210809152410/https://www.nature.com/articles/nature01436 |url-status=live }}</ref><ref name="Head, J. 2003" /><ref name="hirise">{{cite web |url=http://hirise.lpl.arizona.edu/PSP_002917_2175 |title=HiRISE Dissected Mantled Terrain (PSP_002917_2175) |publisher=Arizona University |access-date=December 19, 2010 |archive-date=August 21, 2017 |archive-url=https://web.archive.org/web/20170821043213/https://hirise.lpl.arizona.edu/PSP_002917_2175 |url-status=live }}</ref> The scallops are typically tens of meters deep and from a few hundred to a few thousand meters across. They can be almost circular or elongated. Some appear to have coalesced causing a large heavily pitted terrain to form. The process of forming the terrain may begin with sublimation from a crack. There are often polygonal cracks where scallops form, and the presence of scalloped topography seems to be an indication of frozen ground.<ref name="Icarus Vol 210" /><ref name="Scalloped terrains in the Peneus an"/>

On November 22, 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars.<ref>{{cite web|url=http://www.space.com/34811-mars-ice-more-water-than-lake-superior.html|title=Huge Underground Ice Deposit on Mars Is Bigger Than New Mexico|website=Space.com|date=November 22, 2016|access-date=November 29, 2016|archive-date=January 12, 2018|archive-url=https://web.archive.org/web/20180112181505/https://www.space.com/34811-mars-ice-more-water-than-lake-superior.html|url-status=live}}</ref> The volume of water detected has been estimated to be equivalent to the volume of water in [[Lake Superior]].<ref name="NASA-20161122"/><ref name="Register-2016"/><ref name="NASA-20161122jpl" />

The volume of water ice in the region were based on measurements from the ground-penetrating radar instrument on [[Mars Reconnaissance Orbiter]], called [[SHARAD]]. From the data obtained from SHARAD, “[[dielectric permittivity]]”, or the dielectric constant was determined. The dielectric constant value was consistent with a large concentration of water ice.<ref>Bramson, A, et al. (2015). "Widespread excess ice in Arcadia Planitia, Mars". ''Geophysical Research Letters'' 42, 6566–6574.</ref><ref>{{cite web |url=https://planetarycassie.com/2016/11/04/widespread-thick-water-ice-found-in-utopia-planitia-mars/ |title=Widespread, Thick Water Ice found in Utopia Planitia, Mars |first=Cassie |last=Stuurman |access-date=November 29, 2016 |url-status=dead |archive-url=https://web.archive.org/web/20161130042608/https://planetarycassie.com/2016/11/04/widespread-thick-water-ice-found-in-utopia-planitia-mars/ |archive-date=November 30, 2016  }}</ref><ref>Stuurman, C., et al. 2016. "SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars". ''Geophysical Research Letters'' 43, 9484–9491.</ref>

These scalloped features are superficially similar to [[Swiss cheese features]], found around the south polar cap. Swiss cheese features are thought to be due to cavities forming in a surface layer of solid [[carbon dioxide]], rather than water ice—although the floors of these holes are probably H<sub>2</sub>O-rich.<ref>{{cite journal |title=A Sublimation Model for the Formation of the Martian Polar Swiss-cheese Features |last1=Byrne |first1=S. |last2=Ingersoll |first2=A. P. |bibcode=2002DPS....34.0301B |volume=34 |date=2002 |page=837 |journal=American Astronomical Society}}</ref>

====Ice patches====

[[file:Mars Viking 21i093.png|thumb|Precipitated water ice covering the Martian plain [[Utopia Planitia]], the water ice precipitated by adhering to [[dry ice]] (observed by the [[Viking 2]] lander)]]
On July 28, 2005, the [[European Space Agency]] announced the existence of a crater partially filled with frozen water;<ref name="lake">{{cite press release |url=http://www.esa.int/SPECIALS/Mars_Express/SEMGKA808BE_0.html |title=Water ice in crater at Martian north pole |date=July 27, 2005 |publisher=[[ESA]] |access-date=October 8, 2009 |archive-date=October 6, 2012 |archive-url=https://web.archive.org/web/20121006122736/http://www.esa.int/SPECIALS/Mars_Express/SEMGKA808BE_0.html |url-status=live }}</ref> some then interpreted the discovery as an "ice lake".<ref name="BBClake">{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/4727847.stm |title=Ice lake found on the Red Planet |date=July 29, 2005 |publisher=[[BBC]] |access-date=October 8, 2009 |archive-date=January 13, 2010 |archive-url=https://web.archive.org/web/20100113160825/http://news.bbc.co.uk/2/hi/science/nature/4727847.stm |url-status=live }}</ref> Images of the crater, taken by the [[High Resolution Stereo Camera]] on board the [[European Space Agency]]'s [[Mars Express]] orbiter, clearly show a broad sheet of ice in the bottom of an unnamed crater located on [[Vastitas Borealis]], a broad plain that covers much of Mars' far northern latitudes, at approximately 70.5° North and 103° East. The crater is {{convert|35|km}} wide and about {{convert|2|km}} deep. The height difference between the crater floor and the surface of the water ice is about {{convert|200|m}}. ESA scientists have attributed most of this height difference to sand dunes beneath the water ice, which are partially visible. While scientists do not refer to the patch as a "lake", the water ice patch is remarkable for its size and for being present throughout the year. Deposits of water ice and layers of frost have been found in many different locations on the planet.

As more and more of the surface of Mars has been imaged by the modern generation of orbiters, it has become gradually more apparent that there are probably many more patches of ice scattered across the Martian surface. Many of these putative patches of ice are concentrated in the Martian mid-latitudes (≈30–60° N/S of the equator). For example, many scientists think that the widespread features in those latitude bands variously described as "latitude dependent mantle" or "pasted-on terrain" consist of dust- or debris-covered ice patches, which are slowly degrading.<ref name="Carr" /> A cover of debris is required both to explain the dull surfaces seen in the images that do not reflect like ice, and also to allow the patches to exist for an extended period of time without subliming away completely. These patches have been suggested as possible water sources for some of the enigmatic channelized flow features like [[Martian Gullies|gullies]] also seen in those latitudes.

Surface features consistent with existing [[pack ice]] have been discovered in the southern [[Elysium Planitia]].<ref name="Cabrol, N 2010">{{cite book |editor-last=Cabrol |editor-first=N. |editor2-first=E. |editor2-last=Grin |date=2010 |title=Lakes on Mars |publisher=Elsevier |location=New York}}</ref> What appear to be plates, ranging in size from {{convert|30|m}} to {{convert|30|km}}, are found in channels leading to a large flooded area. The plates show signs of break up and rotation that clearly distinguish them from lava plates elsewhere on the surface of Mars. The source for the flood is thought to be the nearby geological fault [[Cerberus Fossae]] that spewed water as well as lava aged some 2 to 10&nbsp;million years. It was suggested that the water exited the [[Cerberus Fossae]] then pooled and froze in the low, level plains and that such frozen lakes may still exist.<ref name="Murray2007">{{cite journal |last=Murray |first=John B. |display-authors=etal |date=2005 |title=Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars' equator |journal=Nature |pmid=15772653 |volume=434 |issue=7031 |pages=352–356 |doi=10.1038/nature03379 |quote=Here we present High Resolution Stereo Camera images from the European Space Agency Mars Express spacecraft that indicate that such lakes may still exist. |bibcode=2005Natur.434..352M|s2cid=4373323 }}</ref><ref>{{cite book |last1=Orosei |first1=R. |last2=Cartacci |first2=M. |last3=Cicchetti |first3=A. |last4=Federico |first4=C. |last5=Flamini |first5=E. |last6=Frigeri |first6=A. |last7=Holt |first7=J. W. |last8=Marinangeli |first8=L. |last9=Noschese |first9=R. |last10=Pettinelli |first10=E. |last11=Phillips |first11=R. J. |last12=Picardi |first12=G. |last13=Plaut |first13=J. J. |last14=Safaeinili |first14=A. |last15=Seu |first15=R. |chapter=Radar subsurface sounding over the putative frozen sea in Cerberus Palus, Mars |title=Proceedings of the XIII Internarional Conference on Ground Penetrating Radar |chapter-url=http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1866.pdf |bibcode=2007AGUFM.P14B..05O |doi=10.1109/ICGPR.2010.5550143 |volume=XXXIX |pages=P14B–05 |journal=Lunar and Planetary Science |date=2008 |isbn=978-1-4244-4604-9 |s2cid=23296246 |access-date=January 5, 2010 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327135334/http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1866.pdf |url-status=live }}</ref><ref>{{cite book |title=Mars: an introduction to its interior, surface and atmosphere |last1=Barlow |first1=Nadine G. |publisher=Cambridge University Press |isbn=978-0-521-85226-5|date=2008-01-10 }}</ref>


=== Glaciers ===
{{Main|Glaciers on Mars}}
[[File:Mars glacial-like lobe deposit.jpg|thumb|right|View of a 5-km-wide, glacial-like lobe deposit sloping up into a box canyon. The surface has [[moraine]]s, deposits of rocks that show how the glacier advanced.]]
Many large areas of Mars either appear to host glaciers, or carry evidence that they used to be present. Much of the areas in high latitudes, especially the [[Ismenius Lacus quadrangle]], are suspected to still contain enormous amounts of water ice.<ref name="ISBN 0-8165-1257-4">{{cite book |last1=Strom |first1=R.G. |first2=Steven K. |last2=Croft |first3=Nadine G. |last3=Barlow |title=The Martian Impact Cratering Record, Mars |publisher=University of Arizona Press |isbn=978-0-8165-1257-7 |date=1992 |url-access=registration |url=https://archive.org/details/mars0000unse }}</ref><ref name="Express">{{cite web |url=http://www.esa.int/SPECIALS/Mars_Express/SEMBS5V681F_0.html |title=ESA – Mars Express – Breathtaking views of Deuteronilus Mensae on Mars |publisher=Esa.int |date=March 14, 2005 |access-date=October 9, 2009 |archive-date=October 18, 2012 |archive-url=https://web.archive.org/web/20121018200501/http://www.esa.int/SPECIALS/Mars_Express/SEMBS5V681F_0.html |url-status=live }}</ref> Recent evidence has led many planetary scientists to conclude that water ice still exists as glaciers across much of the Martian mid- and high latitudes, protected from sublimation by thin coverings of insulating rock and/or dust.<ref name="Holt, J. 2008" /><ref name="Plaut, J. 2008" /> An example of this are the glacier-like features called [[lobate debris aprons]] in an area called [[Deuteronilus Mensae]], which display widespread evidence of ice lying beneath a few meters of rock debris.<ref name="Plaut, J. 2008" /> Glaciers are associated with [[fretted terrain]], and many volcanoes. Researchers have described glacial deposits on [[Hecates Tholus]],<ref name="Hauber, E. 2005">{{cite journal |last=Hauber |first=E. |display-authors=etal |date=2005 |title=Discovery of a flank caldera and very young glacial activity at Hecates Tholus, Mars |journal=Nature |volume=434 |pages=356–61 |pmid=15772654 |issue=7031 |doi=10.1038/nature03423 |bibcode=2005Natur.434..356H|s2cid=4427179 }}</ref> [[Arsia Mons]],<ref name="brown">{{cite journal |last1=Shean |first1=David E. |last2=Head |first2=James W. |last3=Fastook |first3=James L. |last4=Marchant |first4=David R. |title=Recent glaciation at high elevations on Arsia Mons, Mars: Implications for the formation and evolution of large tropical mountain glaciers |page=E03004 |date=2007 |issue=E3 |volume=112 |doi=10.1029/2006JE002761 |journal=Journal of Geophysical Research |bibcode=2007JGRE..112.3004S |doi-access=free }}</ref> [[Pavonis Mons]],<ref name="Shean, D. 2005">{{cite journal |last=Shean |first=D. |display-authors=etal |date=2005 |title=Origin and evolution of a cold-based mountain glacier on Mars: The Pavonis Mons fan-shaped deposit |journal=Journal of Geophysical Research |volume=110 |issue=E5 |page=E05001 |doi=10.1029/2004JE002360 |bibcode=2005JGRE..110.5001S|s2cid=14749707 |doi-access=free }}</ref> and [[Olympus Mons]].<ref name="Basilevsky, A. 2006">{{cite journal |last=Basilevsky |first=A. |display-authors=etal |date=2006 |title=Geological recent tectonic, volcanic and fluvial activity on the eastern flank of the Olympus Mons volcano, Mars |journal=Geophysical Research Letters |volume=33 |issue=13 |at=L13201 |doi=10.1029/2006GL026396 |bibcode=2006GeoRL..3313201B|citeseerx=10.1.1.485.770 |s2cid=16847310 }}</ref> Glaciers have also been reported in a number of larger Martian craters in the mid-latitudes and above.
[[File:Reull Vallis lineated deposits.jpg|thumb|left|[[Reull Vallis]] with lineated floor deposits. Location is [[Hellas quadrangle]] ]]

Glacier-like features on Mars are known variously as viscous flow features,<ref>{{cite journal |last=Milliken |first=R. |display-authors=etal |date=2003 |title=Viscous flow features on the surface of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images |journal=Journal of Geophysical Research |volume=108 |issue=E6|page= 5057 | doi = 10.1029/2002je002005 |bibcode=2003JGRE..108.5057M|s2cid=12628857 }}</ref> Martian flow features, lobate debris aprons,<ref name="Plaut, J. 2008" /> or lineated valley fill,<ref name="HeadMarchant2006" /> depending on the form of the feature, its location, the landforms it is associated with, and the author describing it. Many, but not all, small glaciers seem to be associated with gullies on the walls of craters and mantling material.<ref>{{cite journal |doi=10.1016/j.icarus.2004.05.026 |last1=Arfstrom |first1=J. |first2=W. |last2=Hartmann |date=2005 |title=Martian flow features, moraine-like ridges, and gullies: Terrestrial analogs and interrelationships |journal=Icarus |volume=174 |issue=2 |pages=321–35 |bibcode=2005Icar..174..321A}}</ref> The lineated deposits known as lineated valley fill are probably rock-covered glaciers that are found on the floors of most channels within the [[fretted terrain]] found around [[Arabia Terra]] in the northern hemisphere. Their surfaces have ridged and grooved materials that deflect around obstacles. Lineated floor deposits may be related to [[lobate debris apron]]s, which have been proven to contain large amounts of ice by orbiting radar.<ref name="Holt, J. 2008" /><ref name="Plaut, J. 2008" /> For many years, researchers interpreted that features called 'lobate debris aprons' were glacial flows and it was thought that ice existed under a layer of insulating rocks.<ref name="Richard Lewis" /><ref name="Tropical snow">{{cite journal |last1=Head |first1=J. W. |last2=Neukum |first2=G. |last3=Jaumann |first3=R. |last4=Hiesinger |first4=H. |last5=Hauber |first5=E. |last6=Carr |first6=M. |last7=Masson |first7=P. |last8=Foing |first8=B. |last9=Hoffmann |first9=H. |last10=Kreslavsky |first10=M. |last11=Werner |first11=S. |last12=Milkovich |first12=S. |last13=van Gasselt |first13=S. |author14=HRSC Co-Investigator Team |title=Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars |journal=Nature |volume=434 |issue=7031 |pages=346–350 |date=2005 |pmid=15772652 |doi=10.1038/nature03359 |bibcode=2005Natur.434..346H|s2cid=4363630 }}</ref><ref name="in flux">{{cite web |author=Staff |publisher=Brown University |url=http://www.spaceref.com/news/viewpr.html?pid=18050 |archive-url=https://archive.today/20130618031257/http://www.spaceref.com/news/viewpr.html?pid=18050 |url-status=dead |archive-date=June 18, 2013 |title=Mars' climate in flux: Mid-latitude glaciers |work=Marstoday |date=October 17, 2005 }}</ref> With new instrument readings, it has been confirmed that lobate debris aprons contain almost pure ice that is covered with a layer of rocks.<ref name="Holt, J. 2008" /><ref name="Plaut, J. 2008" />
[[File:Glacier close up with hirise.JPG|thumb|A ridge interpreted as the terminal moraine of an alpine glacier. Location is [[Ismenius Lacus quadrangle]]. ]]

Moving ice carries rock material, then drops it as the ice disappears. This typically happens at the snout or edges of the glacier. On Earth, such features would be called [[moraine]]s, but on Mars they are typically known as ''moraine-like ridges'', ''concentric ridges'', or ''arcuate ridges''.<ref>{{cite journal |doi=10.1016/j.icarus.2005.05.011 |last=Berman |first=D. |display-authors=etal |year=2005 |title=The role of arcuate ridges and gullies in the degradation of craters in the Newton Basin region of Mars |journal=Icarus |volume=178 |issue=2 |pages=465–86 |bibcode=2005Icar..178..465B}}</ref> Since ice tends to sublime rather than melt on Mars, and because Mars's low temperatures tend to make glaciers "cold based" (frozen down to their beds, and unable to slide), the remains of these glaciers and the ridges they leave do not appear the exactly same as normal glaciers on Earth. In particular, Martian moraines tend to be deposited without being deflected by the underlying topography, which is thought to reflect the fact that the ice in Martian glaciers is normally frozen down and cannot slide.<ref name="Carr" /> Ridges of debris on the surface of the glaciers indicate the direction of ice movement. The surface of some glaciers have rough textures due to [[Sublimation (phase transition)|sublimation]] of buried ice. The ice evaporates without melting and leaves behind an empty space. Overlying material then collapses into the void.<ref name="fretted">{{cite web |url=http://hirise.lpl.arizona.edu/PSP_009719_2230 |title=Fretted Terrain Valley Traverse |publisher=Hirise.lpl.arizona.edu |access-date=January 16, 2012 |archive-date=October 13, 2017 |archive-url=https://web.archive.org/web/20171013002242/https://hirise.lpl.arizona.edu/PSP_009719_2230 |url-status=live }}</ref> Sometimes chunks of ice fall from the glacier and get buried in the land surface. When they melt, a more or less round hole remains. Many of these "[[kettle hole]]s" have been identified on Mars.<ref>{{cite web |url=http://hirise.lpl.arizona.edu/PSP_006278_2225 |title=Jumbled Flow Patterns |publisher=Arizona University |access-date=January 16, 2012 |archive-date=August 23, 2016 |archive-url=https://web.archive.org/web/20160823204921/http://hirise.lpl.arizona.edu/PSP_006278_2225 |url-status=live }}</ref>

Despite strong evidence for glacial flow on Mars, there is little convincing evidence for [[glacial landforms|landforms]] carved by glacial [[erosion]], e.g., [[U-shaped valley]]s, [[crag and tail]] hills, [[arête]]s, [[drumlin]]s. Such features are abundant in glaciated regions on Earth, so their absence on Mars has proven puzzling. The lack of these landforms is thought to be related to the cold-based nature of the ice in most recent glaciers on Mars. Because the [[solar insolation]] reaching the planet, the temperature and density of the atmosphere, and the [[Geothermal gradient|geothermal heat flux]] are all lower on Mars than they are on Earth, modelling suggests the temperature of the interface between a glacier and its bed stays below freezing and the ice is literally frozen down to the ground. This prevents it from sliding across the bed, which is thought to inhibit the ice's ability to erode the surface.<ref name="Carr" />

=== Groundwater ===
{{See also|Groundwater on Mars}}
In August 2024, a reservoir of liquid water was discovered on Mars - deep in the rocky outer crust of the planet. The findings came from a new analysis of data from Nasa’s Mars Insight Lander, which recorded four years' of vibrations - Mars quakes - from deep inside the Red Planet. The analysis revealed reservoirs of water at depths of about six to 12 miles (10 to 20km) in the Martian crust.<ref>{{Cite web |title=Mars water: Liquid water reservoirs found under Martian crust |url=https://www.bbc.com/news/articles/czxl849j77ko |access-date=2024-08-16 |website=www.bbc.com |language=en-GB}}</ref><ref name=":4">{{Cite web |last=Strickland |first=Ashley |date=2024-08-12 |title=Oceans of water may be trapped deep beneath the Martian surface |url=https://edition.cnn.com/2024/08/12/science/mars-crust-water-reservoir-insight/index.html |access-date=2024-08-16 |website=CNN |language=en}}</ref>

As per estimates, there may be enough water, trapped in tiny cracks and pores of rock in the middle of the Martian crust, to fill oceans on the planet’s surface. The groundwater would likely cover the entirety of Mars to a depth of 1 mile (1.6 kilometers), the study found.<ref name=":4" />

==Development of Mars' water inventory==
The variation in Mars's surface water content is strongly coupled to the evolution of its atmosphere and may have been marked by several key stages.  Head and others put together a detailed history of water on Mars and presented it in March, 2023.<ref>*Head, J., et al.  2023.  GEOLOGICAL AND CLIMATE HISTORY OF MARS: IDENTIFICATION OF POTENTIAL WARM AND
WET CLIMATE 'FALSE POSITIVES'.  54th Lunar and Planetary Science Conference 2023 (LPI Contrib. No. 2806).  1731.pdf</ref>

[[File:Channels near Warrego in Thaumasia.JPG|thumb|250px|right|Dry channels near [[Warrego Valles]]. ]]

===Early Noachian era (4.6 Ga to 4.1 Ga)===
{{further|Noachian}}
The early Noachian era was characterized by atmospheric loss to space from heavy meteoritic bombardment and hydrodynamic escape.<ref name = Jakosky2001>{{cite journal | last1 = Jakosky | first1 = B. M. | last2 = Phillips | first2 = R. J. | year = 2001 |title=Mars' volatile and climate history |journal=Nature |volume =412| issue = 6843| pages =237–244| doi = 10.1038/35084184 | pmid=11449285| doi-access = free | bibcode = 2001Natur.412..237J }}</ref> Ejection by meteorites may have removed ~60% of the [[Earth's atmosphere|early atmosphere]].<ref name = Jakosky2001/><ref name = Chaufray2007>{{cite journal | last1 = Chaufray | first1 = J. Y. | display-authors = etal | year = 2007 | title = Mars solar wind interaction: Formation of the Martian corona and atmospheric loss to space | url = https://hal.archives-ouvertes.fr/hal-00186346/file/Chaufray_et_al-2007-Journal_of_Geophysical_Research__Planets_%281991-2012%29.pdf | journal = Journal of Geophysical Research | volume = 112 | issue = E9 | pages = E09009 | doi = 10.1029/2007JE002915 | bibcode = 2007JGRE..112.9009C | doi-access = free | access-date = November 22, 2019 | archive-date = November 29, 2021 | archive-url = https://web.archive.org/web/20211129022552/https://hal.archives-ouvertes.fr/hal-00186346/file/Chaufray_et_al-2007-Journal_of_Geophysical_Research__Planets_%281991-2012%29.pdf | url-status = live }}</ref> Significant quantities of phyllosilicates may have formed during this period requiring a sufficiently dense atmosphere to sustain surface water, as the spectrally dominant phyllosilicate group, smectite, suggests moderate water-to-rock ratios.<ref name = Chevrier2007>{{cite journal | last1 = Chevrier | first1 = V. | display-authors = etal | year = 2007 | title =Early geochemical environment of Mars as determined from thermodynamics of phyllosilicates| journal = Nature| volume =448| issue = 7149| pages =60–63| doi = 10.1038/nature05961 | pmid=17611538| bibcode =2007Natur.448...60C| s2cid = 1595292 }}</ref> However, the pH-pCO<sub>2</sub> between smectite and carbonate show that the precipitation of smectite would constrain pCO<sub>2</sub> to a value not more than {{convert|1e-2|atm|abbr=on}}.<ref name = Chevrier2007/> As a result, the dominant component of a dense atmosphere on early Mars becomes uncertain, if the clays formed in contact with the Martian atmosphere,<ref name = Catling2007>{{cite journal | last1 = Catling | first1 = D. C. | year = 2007 | title =  Mars: Ancient fingerprints in the clay| journal = Nature| volume =  448| issue = 7149| pages =  31–32| doi = 10.1038/448031a | pmid=17611529| bibcode =2007Natur.448...31C| s2cid = 4387261 }}</ref> particularly given the lack of evidence for [[Carbonates on Mars|carbonate deposits]]. An additional complication is that the ~25% lower brightness of the young Sun would have required an ancient atmosphere with a significant [[greenhouse effect]] to raise surface temperatures to sustain liquid water.<ref name = Catling2007/> Higher CO<sub>2</sub> content alone would have been insufficient, as CO<sub>2</sub> precipitates at [[partial pressure]]s exceeding {{convert|1.5|atm|hPa|abbr=on}}, reducing its effectiveness as a [[greenhouse gas]].<ref name = Catling2007/>

===Middle to late Noachean era (4.1 Ga to 3.8 Ga)===
During the middle to late Noachean era, Mars underwent potential formation of a [[secondary atmosphere]] by outgassing dominated by the Tharsis volcanoes, including significant quantities of H<sub>2</sub>O, CO<sub>2</sub>, and SO<sub>2</sub>.<ref name = Jakosky2001/><ref name = Chaufray2007/> Martian valley networks date to this period, indicating globally widespread and temporally sustained surface water as opposed to catastrophic floods.<ref name = Jakosky2001/> The end of this period coincides with the termination of the internal [[magnetic field]] and a spike in meteoritic bombardment.<ref name = Jakosky2001/><ref name = Chaufray2007/> The cessation of the internal magnetic field and subsequent weakening of any local [[Magnetic field (disambiguation)|magnetic fields]] allowed unimpeded [[atmospheric stripping]] by the solar wind. For example, when compared with their terrestrial counterparts, <sup>38</sup>Ar/<sup>36</sup>Ar, <sup>15</sup>N/<sup>14</sup>N, and <sup>13</sup>C/<sup>12</sup>C ratios of the Martian atmosphere are consistent with ~60% loss of Ar, N<sub>2</sub>, and CO<sub>2</sub> by solar wind stripping of an upper atmosphere enriched in the lighter isotopes via [[Rayleigh fractionation]].<ref name = Jakosky2001/> Supplementing the solar wind activity, impacts would have ejected atmospheric components in bulk without isotopic fractionation. Nevertheless, cometary impacts in particular may have contributed volatiles to the planet.<ref name = Jakosky2001/>

===Hesperian to Amazonian era (present) (~3.8 Ga to present)===
{{further|Hesperian|Amazonian (Mars)}}
Atmospheric enhancement by sporadic outgassing events were countered by solar wind stripping of the atmosphere, albeit less intensely than by the young Sun.<ref name = Chaufray2007/> Catastrophic floods date to this period, favoring sudden subterranean release of volatiles, as opposed to sustained surface flows.<ref name = Jakosky2001/> While the earlier portion of this era may have been marked by aqueous acidic environments and Tharsis-centric groundwater discharge<ref name = AndrewsHanna2007>{{cite journal | last1 = Andrews-Hanna | first1 = J. C. | display-authors = etal   | year = 2007 | title =  Meridiani Planum and the global hydrology of Mars| journal = Nature| volume =  446| issue = 7132| pages =  163–6| doi = 10.1038/nature05594 | pmid=17344848| bibcode =2007Natur.446..163A| s2cid = 4428510 }}</ref> dating to the late Noachian, much of the surface alteration processes during the latter portion is marked by oxidative processes including the formation of Fe<sup>3+</sup> oxides that impart a reddish hue to the Martian surface.<ref name = Chaufray2007/> Such oxidation of primary mineral phases can be achieved by low-pH (and possibly high temperature) processes related to the formation of palagonitic tephra,<ref name = Morris2001>{{cite journal | last1 = Morris | first1 = R. V. | display-authors = etal   | year = 2001 | title =  Phyllosilicate-poor palagonitic dust from Mauna Kea Volcano (Hawaii): A mineralogical analogue for magnetic Martian dust?| journal = Journal of Geophysical Research| volume =  106| issue = E3| pages =  5057–5083| doi = 10.1029/2000JE001328 | bibcode=2001JGR...106.5057M| doi-access = free}}</ref> by the action of H<sub>2</sub>O<sub>2</sub> that forms photochemically in the Martian atmosphere,<ref name = Chevrier2006>{{cite journal | last1 = Chevrier | first1 = V. | display-authors = etal | year = 2006 | title = Iron weathering products in a CO2+(H2O or H2O2) atmosphere: Implications for weathering processes on the surface of Mars | journal = Geochimica et Cosmochimica Acta | volume = 70 | issue = 16 | pages = 4295–4317 | doi = 10.1016/j.gca.2006.06.1368 | bibcode = 2006GeCoA..70.4295C | url = https://hal.archives-ouvertes.fr/hal-01872279/file/Chevrier%20-%202006%20-%20GCA%20-%20experimental%20weathering%20in%20CO2%20%2B%20H2O.pdf | access-date = June 23, 2022 | archive-date = July 13, 2022 | archive-url = https://web.archive.org/web/20220713142018/https://hal.archives-ouvertes.fr/hal-01872279/file/Chevrier%20-%202006%20-%20GCA%20-%20experimental%20weathering%20in%20CO2%20+%20H2O.pdf | url-status = live }}</ref> and by the action of water,<ref name = Chevrier2007/> none of which require free O<sub>2</sub>. The action of H<sub>2</sub>O<sub>2</sub> may have dominated temporally given the drastic reduction in aqueous and igneous activity in this recent era, making the observed Fe<sup>3+</sup> oxides volumetrically small, though pervasive and spectrally dominant.<ref name = Bibring2006>{{cite journal | last1 = Bibring | first1 = J-P. | display-authors = etal   | year = 2006 | title =  Global mineralogical and aqueous mars history derived from OMEGA/Mars Express data| journal = Science| volume =  312| issue = 5772| pages =  400–4| doi = 10.1126/science.1122659 | pmid = 16627738 | bibcode = 2006Sci...312..400B| doi-access = free}}</ref> Nevertheless, aquifers may have driven sustained, but highly localized surface water in recent geologic history, as evident in the geomorphology of craters such as Mojave.<ref name = McEwen2007>{{cite journal | last1 = McEwen | first1 = A. S. | display-authors = etal   | year = 2007 | title =  A Closer Look at Water-Related Geologic Activity on Mars| journal = Science| volume =  317| issue = 5845| pages =  1706–1709| doi = 10.1126/science.1143987 | pmid=17885125| bibcode =2007Sci...317.1706M| s2cid = 44822691 }}</ref> Furthermore, the [[Lafayette meteorite|Lafayette]] [[Martian meteorite]] shows evidence of aqueous alteration as recently as 650 Ma.<ref name = Jakosky2001/>

[[File:PIA22487-Mars-BeforeAfterDust-20180719.gif|thumb|right|200px|Mars before and after/during the 2018 global dust storm]]
In 2020 scientists reported that Mars' current loss of atomic hydrogen from water is largely driven by seasonal processes and [[Climate of Mars#Dust storms|dust storms]] that transport water directly to the upper atmosphere and that this has influenced the planet's climate likely during the last 1 Ga.<ref>{{cite news |title=Escape from Mars: How water fled the red planet |url=https://phys.org/news/2020-11-mars-fled-red-planet.html |access-date=8 December 2020 |work=phys.org |language=en |archive-date=October 9, 2021 |archive-url=https://web.archive.org/web/20211009114212/https://phys.org/news/2020-11-mars-fled-red-planet.html |url-status=live }}</ref><ref>{{cite journal |last1=Stone |first1=Shane W. |last2=Yelle |first2=Roger V. |last3=Benna |first3=Mehdi |last4=Lo |first4=Daniel Y. |last5=Elrod |first5=Meredith K. |last6=Mahaffy |first6=Paul R. |title=Hydrogen escape from Mars is driven by seasonal and dust storm transport of water |journal=Science |date=13 November 2020 |volume=370 |issue=6518 |pages=824–831 |doi=10.1126/science.aba5229 |pmid=33184209 |bibcode=2020Sci...370..824S |s2cid=226308137 |url=https://www.science.org/doi/10.1126/science.aba5229 |access-date=8 December 2020 |language=en |issn=0036-8075 |archive-date=September 16, 2022 |archive-url=https://web.archive.org/web/20220916121529/https://www.science.org/doi/10.1126/science.aba5229 |url-status=live }}</ref> More recent studies have suggested that upward propagating atmospheric gravity waves can play an important role during global dust storms in modulating water escape.<ref>{{Cite journal|last=Yiğit|first=Erdal|date=2021-12-10|title=Martian water escape and internal waves|url=https://www.science.org/doi/10.1126/science.abg5893|journal=Science|language=en|volume=374|issue=6573|pages=1323–1324|doi=10.1126/science.abg5893|pmid=34882460|bibcode=2021Sci...374.1323Y|s2cid=245012567|issn=0036-8075|access-date=December 16, 2021|archive-date=December 16, 2021|archive-url=https://web.archive.org/web/20211216001442/https://www.science.org/doi/10.1126/science.abg5893|url-status=live}}</ref><ref>{{Cite journal|last1=Yiğit|first1=Erdal|last2=Medvedev|first2=Alexander S.|last3=Benna|first3=Mehdi|last4=Jakosky|first4=Bruce M.|date=2021-03-16|title=Dust Storm-Enhanced Gravity Wave Activity in the Martian Thermosphere Observed by MAVEN and Implication for Atmospheric Escape|url=https://onlinelibrary.wiley.com/doi/10.1029/2020GL092095|journal=Geophysical Research Letters|language=en|volume=48|issue=5|doi=10.1029/2020GL092095|arxiv=2101.07698|bibcode=2021GeoRL..4892095Y|s2cid=234356651|issn=0094-8276|access-date=December 16, 2021|archive-date=June 11, 2024|archive-url=https://web.archive.org/web/20240611062827/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL092095|url-status=live}}</ref>

==== Ice ages ====
[[File:Conical mound in trough on Mars' north pole.jpg|thumb|North polar layered deposits of ice and dust.]]
Mars has experienced about 40 large scale changes in the amount and distribution of ice on its surface over the past five million years,<ref>{{cite journal | last1 = Schorghofer | first1 = Norbert | year = 2007 | title = Dynamics of ice ages on Mars | url = http://depts.washington.edu/marsweb/papers/PDFs/Schorghofer-2007-Mars-ice-ages.pdf | journal = Nature | volume = 449 | issue = 7159 | pages = 192–194 | bibcode = 2007Natur.449..192S | doi = 10.1038/nature06082 | pmid = 17851518 | s2cid = 4415456 | access-date = January 12, 2018 | archive-url = https://web.archive.org/web/20180113121555/http://depts.washington.edu/marsweb/papers/PDFs/Schorghofer-2007-Mars-ice-ages.pdf | archive-date = January 13, 2018 | url-status = dead }}</ref><ref name="Shean, D. 2005" /> with the most recent happening about 2.1 to 0.4 Myr ago, during the Late [[Amazonian (Mars)|Amazonian]] glaciation at the [[Martian dichotomy|dichotomy]] boundary.<ref name="Dickson2008">{{cite journal |last1=Dickson |first1=James L. |last2=Head |first2=James W. |last3=Marchant |first3=David R. |date=2008 |title=Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases |journal=[[Geology (journal)|Geology]] |volume=36 |issue=5 |pages=411–4 |doi=10.1130/G24382A.1|bibcode=2008Geo....36..411D |s2cid=14291132 }}</ref><ref>{{cite journal | last1 = Head | first1 = J. W. | last2 = III | last3 = Mustard | first3 = J. F. | last4 = Kreslavsky | first4 = M. A. | last5 = Milliken | first5 = R. E. | last6 = Marchant | first6 = D. R. | year = 2003 | title = Recent ice ages on Mars | journal = Nature | volume = 426 | issue = 6968| pages = 797–802 | doi=10.1038/nature02114| pmid = 14685228 | bibcode = 2003Natur.426..797H | s2cid = 2355534 }}</ref> These changes are known as ice ages.<ref name="Smith 2016">{{cite journal |title=An ice age recorded in the polar deposits of Mars |journal=Science |date= May 27, 2016 |last1=Smith |first1=Isaac B. |last2=Putzig |first2=Nathaniel E. |last3=Holt |first3=John W. |last4=Phillips |first4=Roger J. |volume=352 |issue=6289 |pages= 1075–1078 |doi=10.1126/science.aad6968 |pmid=27230372|bibcode=2016Sci...352.1075S |doi-access=free }}</ref> Ice ages on Mars are very different from the ones that the Earth experiences. Ice ages are driven by changes in Mars's orbit and [[Axial tilt|tilt]] —also known as obliquity. Orbital calculations show that Mars wobbles on its axis far more than Earth does. The Earth is stabilized by its proportionally large moon, so it only wobbles a few degrees. Mars may change its tilt by many tens of degrees.<ref name="hirise" /><ref>{{cite journal | last1 = Levrard | first1 = B. | last2 = Forget | first2 = F. | last3 = Montmessian | first3 = F. | last4 = Laskar | first4 = J. | year = 2004 | title = Recent ice-rich deposits formed at high latitudes on Mars by sublimation of unstable equatorial ice during low obliquity | journal = Nature | volume = 431 | issue = 7012| pages = 1072–1075 | doi=10.1038/nature03055| pmid = 15510141 | bibcode = 2004Natur.431.1072L | s2cid = 4420650 }}</ref> When this obliquity is high, its poles get much more direct sunlight and heat; this causes the ice caps to warm and become smaller as ice sublimes. Adding to the variability of the climate, the [[Orbital eccentricity|eccentricity]] of the orbit of Mars changes twice as much as Earth's eccentricity. As the poles sublime, the ice is redeposited closer to the equator, which receive somewhat less [[solar insolation]] at these high obliquities.<ref name="IceAge&shy;" /> Computer simulations have shown that a 45° tilt of the Martian axis would result in ice accumulation in areas that display glacial landforms.<ref>{{cite journal |last=Forget |first=F. |display-authors=etal |date=2006 |title=Formation of Glaciers on Mars by Atmospheric Precipitation at High Obliquity |journal=Science |volume=311 |pages=368–71 |pmid=16424337 |issue=5759 |doi=10.1126/science.1120335 |bibcode=2006Sci...311..368F|s2cid=5798774 }}</ref>

The moisture from the ice caps travels to lower latitudes in the form of deposits of frost or snow mixed with dust. The atmosphere of Mars contains a great deal of fine dust particles, the water vapor condenses on these particles that then fall down to the ground due to the additional weight of the water coating. When ice at the top of the mantling layer returns to the atmosphere, it leaves behind dust that serves to insulate the remaining ice.<ref name="IceAge&shy;" /> The total volume of water removed is a few percent of the ice caps, or enough to cover the entire surface of the planet under one meter of water. Much of this moisture from the ice caps results in a thick smooth mantle with a mixture of ice and dust.<ref name=":8" /><ref name="Head, J. 2003" /><ref>{{cite journal |last=Mustard |first=J. |display-authors=etal |date=2001 |title=Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice |journal=Nature |volume=412 |pages=411–4 |pmid=11473309 |issue=6845 |doi=10.1038/35086515 |bibcode=2001Natur.412..411M |s2cid=4409161 }}</ref><ref>{{cite journal |last1=Kreslavsky |first1=M. |first2=J. |last2=Head |date=2002 |title=Mars: Nature and evolution of young latitude-dependent water-ice-rich mantle |issue=15 |journal=Geophysical Research Letters |volume=29 |doi=10.1029/2002GL015392 |bibcode=2002GeoRL..29.1719K |pages=14–1–14–4 |doi-access=free }}</ref> This ice-rich mantle, that can be 100 meters thick at mid-latitudes,<ref name='exposed 100m'>{{cite web |last1=Beatty |first1=Kelly |title=Water Ice Found Exposed in Martian Cliffs - Sky & Telescope |url=https://www.skyandtelescope.com/astronomy-news/cliffs-reveal-water-ice-on-mars/ |website=Sky & Telescope |access-date=3 October 2018 |date=23 January 2018}}</ref> smoothes the land at lower latitudes, but in places it displays a bumpy texture or patterns that give away the presence of former water ice underneath.

== Habitability assessments ==
{{Main|Life on Mars}}
[[File:Mars rover being tested near the Paranal Observatory.jpg|thumb|[[ExoMars rover]]  prototype being tested in the [[Atacama Desert]], 2013.]]
Since the [[Viking lander biological experiments|''Viking'' landers]] that searched for current microbial life in 1976, NASA has pursued a "follow the water" strategy on Mars. However, liquid water is a necessary but not sufficient condition for life as we know it because [[Planetary habitability|habitability]] is a function of a multitude of environmental parameters.<ref name='NASA Astrobio Strategy 2015'>[http://nai.nasa.gov/media/medialibrary/2016/04/NASA_Astrobiology_Strategy_2015_FINAL_041216.pdf Astrobiology Strategy 2015] {{Webarchive|url=https://web.archive.org/web/20161222190939/https://nai.nasa.gov/media/medialibrary/2016/04/NASA_Astrobiology_Strategy_2015_FINAL_041216.pdf |date=December 22, 2016 }} (PDF) NASA.</ref> Chemical, physical, geological, and geographic attributes shape the environments on Mars. Isolated measurements of these factors may be insufficient to deem an environment habitable, but the sum of measurements can help predict locations with greater or lesser habitability potential.<ref name="2013 LPS">{{cite journal |bibcode=2013LPI....44.2185C |title=Habitability Assessment at Gale Crater: Implications from Initial Results |last1=Conrad |first1=P. G. |last2=Archer |first2=D. |last3=Coll |first3=P. |last4=De La Torre |first4=M. |last5=Edgett |first5=K. |last6=Eigenbrode |first6=J. L. |last7=Fisk |first7=M. |last8=Freissenet |first8=C. |last9=Franz |first9=H. |last10=Glavin |first10=D. P. |last11=Gómez |first11=F. |last12=Haberle |first12=R. |last13=Hamilton |first13=V. |last14=Jones |first14=J. H. |last15=Kah |first15=L. C. |last16=Leshin |first16=L. A. |last17=Mahaffy |first17=P. M. |last18=McAdam |first18=A. |last19=McKay |first19=C. P. |last20=Navarro-González |first20=R. |last21=Steele |first21=A. |last22=Stern |first22=J. |last23=Sumner |first23=D. |last24=Treiman |first24=A. H. |last25=Wong |first25=M. H. |last26=Wray |first26=J. |last27=Yingst |first27=R. A. |author28=MSL Science Team |display-authors=9 |volume=1719 |issue=1719 |date=2013 |page=2185 |journal=44th Lunar and Planetary Science Conference }}</ref>

Habitable environments need not be inhabited, and for purposes of [[planetary protection]], scientists are trying to identify potential habitats where stowaway bacteria from Earth on spacecraft could contaminate Mars.<ref name="strategy">{{cite book | author1=Committee on an Astrobiology Strategy for the Exploration of Mars | author2=National Research Council | date=2007 | chapter=Planetary Protection for Mars Missions | chapter-url=http://www.nap.edu/openbook.php?record_id=11937&page=95 | pages=95–98 | title=An Astrobiology Strategy for the Exploration of Mars | publisher=The National Academies Press | isbn=978-0-309-10851-5 }}</ref> If life exists—or existed—on Mars, evidence or [[biosignature]]s could be found in the subsurface, away from present-day harsh surface conditions such as [[perchlorate]]s,<ref name="SM-20170706">{{cite news |last=Daley |first=Jason |title=Mars Surface May Be Too Toxic for Microbial Life - The combination of UV radiation and perchlorates common on Mars could be deadly for bacteria |url=http://www.smithsonianmag.com/smart-news/mars-surface-may-be-toxic-bacteria-180963966/ |date=6 July 2017 |work=[[Smithsonian (magazine)|Smithsonian]] |access-date=8 July 2017 }}</ref><ref name="NAT-20170706">{{cite journal|last1=Wadsworth |first1=Jennifer |last2=Cockell |first2=Charles S. |title=Perchlorates on Mars enhance the bacteriocidal effects of UV light |date=6 July 2017 |journal=[[Scientific Reports]] |volume=7 |page=4662 |number=4662 |doi=10.1038/s41598-017-04910-3 |bibcode = 2017NatSR...7.4662W |pmid=28684729 |pmc=5500590}}</ref> ionizing radiation, desiccation and freezing.<ref name='NASA strategy 2015'>{{cite web|url=https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf|title=NASA Astrobiology Strategy|year=2015|work=NASA.|access-date=September 5, 2018|archive-url=https://web.archive.org/web/20161222190306/https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf|archive-date=December 22, 2016|url-status=dead}}</ref> Habitable locations could occur kilometers below the surface in a hypothetical hydrosphere, or it could occur near the sub-surface in contact with permafrost.<ref name="Dartnell-1"/><ref name="ionising radiation"/><ref name="subsurface habitability model"/><ref name="Parnell"/><ref name=Steigerwald/>

The ''Curiosity'' rover is assessing Mars' past and present habitability potential. The European-Russian [[ExoMars]] programme is an astrobiology project dedicated to the search for and identification of biosignatures on Mars. It includes the [[ExoMars Trace Gas Orbiter]] that started mapping the [[atmospheric methane]] in April 2018, and the 2022 [[ExoMars rover]] that will drill and analyze subsurface samples 2 meters deep. NASA's [[Mars 2020]] rover will cache dozens of drilled core samples for their potential transport to Earth laboratories in the late 2020s or 2030s.

== Findings by probes ==
{{Main|Chronology of discoveries of water on Mars}}
<!--Note: as this section is linked to its parent article, then by policy: only a related summary of it (or of each mission) must be presented in this article. Only SOME representative images may be used as part of this summary.-->

=== Mariner 9 ===
[[File:Scamander Vallis from Mars Global Surveyor.jpg|thumb|Meander in [[Scamander Vallis]], as seen by [[Mars Global Surveyor]]. Such images implied that large amounts of water once flowed on the surface of Mars.]]
The images acquired by the [[Mariner 9]] Mars orbiter, launched in 1971, revealed the first direct evidence of past water in the form of dry river beds, [[canyon]]s (including the [[Valles Marineris]], a system of canyons over about {{convert|4020|km|mi|-1}} long), evidence of water [[erosion]] and deposition, weather fronts, fogs, and more.<ref>{{cite web |url=http://marsprogram.jpl.nasa.gov/missions/past/mariner8-9.html |archive-url=https://web.archive.org/web/20040411165457/http://marsprogram.jpl.nasa.gov/missions/past/mariner8-9.html |url-status=dead |archive-date=April 11, 2004 |title=Mars Exploration: Missions |publisher=Marsprogram.jpl.nasa.gov |access-date=December 19, 2010}}</ref> The findings from the Mariner 9 missions underpinned the later [[Viking program]]. The enormous [[Valles Marineris]] canyon system is named after Mariner 9 in honor of its achievements.

=== Viking program ===
{{Main|Viking program}}
[[File:Streamlined Islands in Maja Valles.jpg|thumb|left|Streamlined islands in [[Maja Valles]] suggest that large floods occurred on Mars.]]

By discovering many geological forms that are typically formed from large amounts of water, the two [[Viking program|Viking]] orbiters and the two landers caused a revolution in our knowledge about water on Mars. Huge [[outflow channels]] were found in many areas. They showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers.<ref name="history.nasa.gov">{{cite web |url=https://history.nasa.gov/SP-441/ch4.htm |title=Viking Orbiter Views of Mars |date=January 1980 |publisher=History.nasa.gov |access-date=December 19, 2010 |last1=Carr |first1=M. H. |last2=Baum |first2=W. A. |last3=Blasius |first3=K. R. |last4=Briggs |first4=G. A. |last5=Cutts |first5=J. A. |last6=Duxbury |first6=T. C. |last7=Greeley |first7=R. |last8=Guest |first8=J. |last9=Masursky |first9=H. |last10=Smith |first10=B. A. }}</ref> Large areas in the southern hemisphere contained branched [[Valley networks (Mars)|valley networks]], suggesting that rain once fell.<ref>{{cite web |url=https://history.nasa.gov/SP-441/ch5.htm |title=ch5 |work=NASA History |date=January 1980 |publisher=NASA |access-date=December 19, 2010 |last1=Carr |first1=M. H. |last2=Baum |first2=W. A. |last3=Blasius |first3=K. R. |last4=Briggs |first4=G. A. |last5=Cutts |first5=J. A. |last6=Duxbury |first6=T. C. |last7=Greeley |first7=R. |last8=Guest |first8=J. |last9=Masursky |first9=H. |last10=Smith |first10=B. A. }}</ref> Many craters look as if the impactor fell into mud. When they were formed, ice in the soil may have melted, turned the ground into mud, then the mud flowed across the surface.<ref name="Raeburn">{{cite journal |last=Raeburn |first=P. |date=1998 |title=Uncovering the Secrets of the Red Planet Mars |journal=National Geographic |location=Washington D.C.}}</ref><ref name="Moore">{{cite book |last=Moore |first=P. |display-authors=etal |date=1990 |title=The Atlas of the Solar System |publisher=Mitchell Beazley Publishers |location=New York}}</ref><ref name="Kieffer1992" /><ref>{{cite web |url=https://history.nasa.gov/SP-441/ch7.htm |title=Craters |date=January 1980 |publisher=NASA |access-date=December 19, 2010 |last1=Carr |first1=M. H. |last2=Baum |first2=W. A. |last3=Blasius |first3=K. R. |last4=Briggs |first4=G. A. |last5=Cutts |first5=J. A. |last6=Duxbury |first6=T. C. |last7=Greeley |first7=R. |last8=Guest |first8=J. |last9=Masursky |first9=H. |last10=Smith |first10=B. A. }}</ref> Regions, called "Chaotic Terrain," seemed to have quickly lost great volumes of water that caused large channels to form downstream. Estimates for some channel flows run to ten thousand times the flow of the [[Mississippi River]].<ref name="Morton, O 2002">{{cite book |last=Morton |first=O. |date=2002 |title=Mapping Mars |url=https://archive.org/details/mappingmarsscien00mort_0 |url-access=registration |publisher=Picador, NY|isbn=9780312245511 }}</ref> Underground volcanism may have melted frozen ice; the water then flowed away and the ground collapsed to leave chaotic terrain. Also, general chemical analysis by the two Viking landers suggested the surface has been either exposed to or submerged in water in the past.<ref name="Arvidson, R 1989">{{cite journal |doi=10.1029/RG027i001p00039 |last1=Arvidson |first1=R |last2=Gooding |first2=James L. |last3=Moore |first3=Henry J. |date=1989 |title=The Martian surface as Imaged, Sampled, and Analyzed by the Viking Landers |journal=Reviews of Geophysics |volume=27 |issue=1 |pages=39–60 |bibcode=1989RvGeo..27...39A}}</ref><ref>{{cite journal |doi=10.1126/science.194.4271.1283 |last1=Clark |first1=B. |last2=Baird |first2=AK |last3=Rose |first3=HJ Jr. |last4=Toulmin P |first4=3rd |last5=Keil |first5=K |last6=Castro |first6=AJ |last7=Kelliher |first7=WC |last8=Rowe |first8=CD |last9=Evans |first9=PH |date=1976 |title=Inorganic Analysis of Martian Samples at the Viking Landing Sites |journal=Science |volume=194 |issue=4271 |pages=1283–1288 |pmid=17797084 |bibcode=1976Sci...194.1283C |s2cid=21349024 }}</ref>

=== Mars Global Surveyor ===
{{Main|Mars Global Surveyor}}
[[File:Hematite region Sinus Meridiani sur Mars.jpg|thumb|Map showing the distribution of [[hematite]] in Sinus Meridiani. This data was used to target the landing of the ''Opportunity'' rover that found definite evidence of past water.]]
The [[Mars Global Surveyor]]'s [[Thermal Emission Spectrometer]] (TES) is an instrument able to determine the mineral composition on the surface of Mars. Mineral composition gives information on the presence or absence of water in ancient times. TES identified a large ({{convert|30000|km²}}) area in the [[Nili Fossae]] formation that contains the mineral [[olivine]].<ref>{{cite journal | last1 = Hoefen | first1 = T.M. | display-authors = etal   | year = 2003 | title = Discovery of Olivine in the Nili Fossae Region of Mars | journal = Science | volume = 302 | issue = 5645| pages = 627–630 | doi=10.1126/science.1089647 | pmid=14576430| bibcode = 2003Sci...302..627H | s2cid = 20122017 | url = https://zenodo.org/record/1230836 }}</ref> It is thought that the ancient asteroid impact that created the [[Isidis basin]] resulted in faults that exposed the olivine. The discovery of olivine is strong evidence that parts of Mars have been extremely dry for a long time. Olivine was also discovered in many other small outcrops within 60 degrees north and south of the equator.<ref>{{cite journal |doi=10.1126/science.1089647 |last1=Hoefen |first1=T. |last2=Clark |first2=RN |last3=Bandfield |first3=JL |last4=Smith |first4=MD |last5=Pearl |first5=JC |last6=Christensen |first6=PR |date=2003 |title=Discovery of Olivine in the Nili Fossae Region of Mars |journal=Science |volume=302 |issue=5645 |pages=627–630 |pmid=14576430 |bibcode=2003Sci...302..627H|s2cid=20122017 |url=https://zenodo.org/record/1230836 }}</ref> The probe has imaged several channels that suggest past sustained liquid flows, two of them are found in [[Nanedi Valles]] and in [[Nirgal Vallis]].<ref name="Malin, M 2001">{{cite journal |last1=Malin |first1=Michael C. |last2=Edgett |first2=Kenneth S. |title=Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission |pages=23429–23570 |journal=Journal of Geophysical Research |date=2001 |doi=10.1029/2000JE001455 |volume=106 |bibcode=2001JGR...10623429M |issue=E10|s2cid=129376333 |doi-access=free }}</ref>

[[File:Nanedi channel.JPG|thumb|Inner channel (near top of the image) on floor of Nanedi Valles that suggests that water flowed for a fairly long period. Image from [[Lunae Palus quadrangle]].]]

=== Mars Pathfinder ===
{{Main|Mars Pathfinder}}
The [[Mars Pathfinder|Pathfinder]] lander recorded the variation of diurnal temperature cycle. It was coldest just before sunrise, about {{convert|−78|C|F K}}, and warmest just after Mars noon, about {{convert|−8|C|F K}}. At this location, the highest temperature never reached the freezing point of water ({{convert|0|C|F K}}), too cold for pure liquid water to exist on the surface.

The atmospheric pressure measured by the Pathfinder on Mars is very low —about 0.6% of Earth's, and it would not permit pure liquid water to exist on the surface.<ref>{{cite web |url=http://mars.jpl.nasa.gov/MPF/science/atmospheric.html |title=Atmospheric and Meteorological Properties |publisher=NASA}}</ref>

Other observations were consistent with water being present in the past. Some of the rocks at the Mars Pathfinder site leaned against each other in a manner geologists term imbricated. It is suspected that strong flood waters in the past pushed the rocks around until they faced away from the flow. Some pebbles were rounded, perhaps from being tumbled in a stream. Parts of the ground are crusty, maybe due to cementing by a fluid containing minerals.<ref name="Golombek, M 1997" /> There was evidence of clouds and maybe fog.<ref name="Golombek, M 1997">{{cite journal |doi=10.1126/science.278.5344.1743 |last1=Golombek |first1=M. P. |last2=Cook |first2=R. A. |last3=Economou |first3=T. |last4=Folkner |first4=W. M. |last5=Haldemann |first5=A. F. C. |last6=Kallemeyn |first6=P. H. |last7=Knudsen |first7=J. M. |last8=Manning |first8=R. M. |last9=Moore |first9=H. J. |last10=Parker |first10=T. J. |last11=Rieder |first11=R. |last12=Schofield |first12=J. T. |last13=Smith |first13=P. H. |last14=Vaughan |first14=R. M. |title=Overview of the Mars Pathfinder Mission and Assessment of Landing Site Predictions |journal=Science |volume=278 |issue=5344 |pages=1743–1748 |pmid=9388167 |bibcode=1997Sci...278.1743G |date=1997|doi-access=free }}</ref>

=== Mars Odyssey ===
{{Main|Evidence of water on Mars from Mars Odyssey}}
[[File:Semeykin Crater Drainage.JPG|thumb|left|Complex drainage system in [[Semeykin Crater]]. Location is [[Ismenius Lacus quadrangle]]]]
The [[2001 Mars Odyssey]] found much evidence for water on Mars in the form of images, and with its [[Neutron spectroscopy|neutron spectrometer]], it proved that much of the ground is loaded with water ice. Mars has enough ice just beneath the surface to fill [[Lake Michigan]] twice.<ref name="mars.jpl.nasa.gov">{{cite web |url=http://mars.jpl.nasa.gov/odyssey/newsroom/pressreleases/20020528a.html |title=Mars Odyssey: Newsroom |publisher=Mars.jpl.nasa.gov |date=May 28, 2002}}</ref> In both hemispheres, from 55° latitude to the poles, Mars has a high density of ice just under the surface; one kilogram of soil contains about {{convert|500|g}} of water ice. But close to the equator, there is only 2% to 10% of water in the soil.<ref name="Feildman, T. 2004">{{cite journal |date=2004 |last=Feldman |first=W.C. |display-authors=etal |title=Global Distribution of Near-Surface Hydrogen on Mars |journal=Journal of Geophysical Research |volume=109 |issue=E9 |doi=10.1029/2003JE002160 |bibcode=2004JGRE..109.9006F |doi-access=free }}</ref> Scientists think that much of this water is also locked up in the chemical structure of minerals, such as [[clay]] and [[sulfate]]s.<ref>{{cite journal |doi=10.1006/icar.1993.1141 |last1=Murche |first1=S. |last2=Mustard |first2=John |date=1993 |title=Spatial Variations in the Spectral Properties of Bright Regions on Mars |journal=Icarus |volume=105 |pages=454–468 |bibcode=1993Icar..105..454M |issue=2 |last3=Bishop |first3=Janice |author-link3=Janice Bishop|last4=Head |first4=James |last5=Pieters |first5=Carle |last6=Erard |first6=Stephane}}</ref><ref>{{cite web |url=http://marswatch.tn.cornell.edu/burns.html |title=Home Page for Bell (1996) Geochemical Society paper |publisher=Marswatch.tn.cornell.edu |access-date=December 19, 2010}}</ref> Although the upper surface contains a few percent of chemically-bound water, ice lies just a few meters deeper, as it has been shown in [[Arabia Terra]], [[Amazonis quadrangle]], and [[Elysium quadrangle]] that contain large amounts of water ice.<ref>{{cite journal |doi=10.1126/science.1073541 |last1=Feldman |first1=W. C. |last2=Boynton |first2=W. V. |last3=Tokar |first3=R. L. |last4=Prettyman |first4=T. H. |last5=Gasnault |first5=O. |last6=Squyres |first6=S. W. |last7=Elphic |first7=R. C. |last8=Lawrence |first8=D. J. |last9=Lawson |first9=S. L. |last10=Maurice |first10=S. |last11=McKinney |first11=G. W. |last12=Moore |first12=K. R. |last13=Reedy |first13=R. C. |title=Global Distribution of Neutrons from Mars: Results from Mars Odyssey |journal=Science |volume=297 |issue=5578 |pages=75–78 |pmid=12040088 |bibcode=2002Sci...297...75F |date=2002|s2cid=11829477 |doi-access=free }}</ref> The orbiter also discovered vast deposits of bulk water ice near the surface of equatorial regions.<ref name="ICRS-20170928"/> Evidence for equatorial hydration is both morphological and compositional and is seen at both the [[Medusae Fossae]] formation and the [[Tharsis Montes]].<ref name="ICRS-20170928"/> Analysis of the data suggests that the southern hemisphere may have a layered structure, suggestive of stratified deposits beneath a now extinct large water mass.<ref>{{cite journal |doi=10.1126/science.1073616 |last1=Mitrofanov |first1=I. |last2=Anfimov |first2=D. |last3=Kozyrev |first3=A. |last4=Litvak |first4=M. |last5=Sanin |first5=A. |last6=Tret'yakov |first6=V. |last7=Krylov |first7=A. |last8=Shvetsov |first8=V. |last9=Boynton |first9=W. |last10=Shinohara |first10=C. |last11=Hamara |first11=D. |last12=Saunders |first12=R. S. |title=Maps of Subsurface Hydrogen from the High Energy Neutron Detector, Mars Odyssey |journal=Science |volume=297 |issue=5578 |pages=78–81 |pmid=12040089 |bibcode=2002Sci...297...78M |date=2002|s2cid=589477 |doi-access=free }}</ref>
[[File:Blocks in Aram.JPG|thumb|Blocks in Aram showing a possible ancient source of water. Location is [[Oxia Palus quadrangle]].]]

The instruments aboard the ''Mars Odyssey'' are able to study the top meter of soil. In 2002, available data were used to calculate that if all soil surfaces were covered by an even layer of water, this would correspond to a global layer of water (GLW) {{convert|0.5-1.5|km}}.<ref>{{cite journal |doi=10.1126/science.1073722 |last1=Boynton |first1=W. V. |last2=Feldman |first2=W. C. |last3=Squyres |first3=S. W. |last4=Prettyman |first4=T. H. |last5=Brückner |first5=J. |last6=Evans |first6=L. G. |last7=Reedy |first7=R. C. |last8=Starr |first8=R. |last9=Arnold |first9=J. R. |last10=Drake |first10=D. M. |last11=Englert |first11=P. A. J. |last12=Metzger |first12=A. E. |last13=Mitrofanov |first13=Igor |last14=Trombka |first14=J. I. |last15=d'Uston |first15=C. |last16=Wänke |first16=H. |last17=Gasnault |first17=O. |last18=Hamara |first18=D. K. |last19=Janes |first19=D. M. |last20=Marcialis |first20=R. L. |last21=Maurice |first21=S. |last22=Mikheeva |first22=I. |last23=Taylor |first23=G. J. |last24=Tokar |first24=R. |last25=Shinohara |first25=C. |title=Distribution of Hydrogen in the Near Surface of Mars: Evidence for Subsurface Ice Deposits |journal=Science |volume=297 |issue=5578 |pages=81–85 |pmid=12040090 |bibcode=2002Sci...297...81B |date=2002|s2cid=16788398 |doi-access=free }}</ref>

Thousands of images returned from ''Odyssey'' orbiter also support the idea that Mars once had great amounts of water flowing across its surface. Some images show patterns of branching valleys; others show layers that may have been formed under lakes; even river and lake [[river|deltas]] have been identified.<ref name="Irwin III 2005">{{cite journal |last1=Irwin |first1=Rossman P. |last2=Howard |first2=Alan D. |last3=Craddock |first3=Robert A. |last4=Moore |first4=Jeffrey M. |title=An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development |journal=Journal of Geophysical Research |volume=110 |issue=E12 |pages=E12S15 |date=2005 |doi=10.1029/2005JE002460 |bibcode=2005JGRE..11012S15I|doi-access=free }}</ref><ref name="themis.asu.edu">{{cite web |url=http://themis.asu.edu/zoom-20020807a |title=Dao Vallis |date=August 7, 2002 |work=Mars Odyssey Mission |publisher=THEMIS |access-date=December 19, 2010 }}</ref>
For many years researchers suspected that glaciers exist under a layer of insulating rocks.<ref name="Holt, J. 2008" /><ref name="Richard Lewis" /><ref name="Plaut, J. 2008" /> [[Lineated valley fill]] is one example of these rock-covered glaciers. They are found on the floors of some channels. Their surfaces have ridged and grooved materials that deflect around obstacles. Lineated floor deposits may be related to [[lobate debris apron]]s, which have been shown by orbiting radar to contain large amounts of ice.<ref name="Holt, J. 2008" /><ref name="Plaut, J. 2008" />

=== ''Phoenix'' ===
{{Main|Phoenix (spacecraft)}}
[[File:Phoenix Sol 0 horizon.jpg|thumb|[[Permafrost]] polygons imaged by the ''Phoenix'' lander.]]
The ''[[Phoenix (spacecraft)|Phoenix]]'' lander also confirmed the existence of large amounts of water ice in the northern region of Mars.<ref name="Arvidson, R. 2008">{{cite journal |last1=Smith |first1=P. H. |last2=Tamppari |first2=L. |last3=Arvidson |first3=R. E. |last4=Bass |first4=D. |last5=Blaney |first5=D.|author5-link= Diana Blaney |last6=Boynton |first6=W. |last7=Carswell |first7=A. |last8=Catling |first8=D. |last9=Clark |first9=B. |last10=Duck |first10=T. |last11=DeJong |first11=E. |last12=Fisher |first12=D. |last13=Goetz |first13=W. |last14=Gunnlaugsson |first14=P. |last15=Hecht |first15=M. |last16=Hipkin |first16=V. |last17=Hoffman |first17=J. |last18=Hviid |first18=S. |last19=Keller |first19=H. |last20=Kounaves |first20=S. |last21=Lange |first21=C. F. |last22=Lemmon |first22=M. |last23=Madsen |first23=M. |last24=Malin |first24=M. |last25=Markiewicz |first25=W. |last26=Marshall |first26=J. |last27=McKay |first27=C. |last28=Mellon |first28=M. |last29=Michelangeli |first29=D. |last30=Ming |first30=D. |last31=Morris |first31=R. |last32=Renno |first32=N. |last33=Pike |first33=W. T. |last34=Staufer |first34=U. |last35=Stoker |first35=C. |last36=Taylor |first36=P. |last37=Whiteway |first37=J. |last38=Young |first38=S. |last39=Zent |first39=A. |title=Introduction to special section on the phoenix mission: Landing site characterization experiments, mission overviews, and expected science |journal=Journal of Geophysical Research |volume=113 |issue=E12 |pages=E00A18 |doi=10.1029/2008JE003083 |bibcode=2008JGRE..113.0A18S |date=2008|display-authors=29 |hdl=2027.42/94752 |s2cid=38911896 |hdl-access=free }}</ref><ref>{{cite web |url=http://www.nasa.gov/mission_pages/phoenix/news/phx20100909.html |title=NASA Data Shed New Light About Water and Volcanoes on Mars |publisher=NASA |date=September 9, 2010 |access-date=March 21, 2014 |archive-date=January 26, 2021 |archive-url=https://web.archive.org/web/20210126063250/http://www.nasa.gov/mission_pages/phoenix/news/phx20100909.html |url-status=dead }}</ref> This finding was predicted by previous orbital data and theory,<ref>{{cite journal |last1=Mellon |first1=M. |last2=Jakosky |first2=B. |date=1993 |title=Geographic variations in the thermal and diffusive stability of ground ice on Mars |journal=Journal of Geophysical Research |volume=98 |issue=E2 |pages=3345–3364 |doi=10.1029/92JE02355 |bibcode=1993JGR....98.3345M }}</ref> and was measured from orbit by the Mars Odyssey instruments.<ref name="Feildman, T. 2004" /> On June 19, 2008, NASA announced that dice-sized clumps of bright material in the "Dodo-Goldilocks" trench, dug by the robotic arm, had vaporized over the course of four days, strongly indicating that the bright clumps were composed of water ice that [[Sublimation (phase transition)|sublimes]] following exposure. Recent radiative transfer modeling has shown that this water ice was snow with a grain size of ~350&nbsp;μm with 0.015% dust.<ref>{{Cite journal |last1=Khuller |first1=Aditya R. |last2=Christensen |first2=Philip R. |last3=Warren |first3=Stephen G. |date=September 2021 |title=Spectral Albedo of Dusty Martian H 2 O Snow and Ice |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=9 |doi=10.1029/2021JE006910 |bibcode=2021JGRE..12606910K |s2cid=238721489 |issn=2169-9097|doi-access=free }}</ref> Even though CO<sub>2</sub> ([[dry ice]]) also sublimes under the conditions present, it would do so at a rate much faster than observed.<ref name="Confirmation of Water on Mars">{{cite web |url=http://www.nasa.gov/mission_pages/phoenix/news/phoenix-20080620.html |title=Confirmation of Water on Mars |publisher=Nasa.gov |date=June 20, 2008 |access-date=October 8, 2009 |archive-date=July 1, 2008 |archive-url=https://web.archive.org/web/20080701104400/http://www.nasa.gov/mission_pages/phoenix/news/phoenix-20080620.html |url-status=dead }}</ref> On July 31, 2008, NASA announced that ''Phoenix'' further confirmed the presence of water ice at its landing site. During the initial heating cycle of a sample, the mass spectrometer detected water vapor when the sample temperature reached {{convert|0|C|F K}}.<ref name="LATimes">{{cite news |last=Johnson |first=John |title=There's water on Mars, NASA confirms |work=Los Angeles Times |date=August 1, 2008 |url=https://www.latimes.com/news/science/la-sci-phoenix1-2008aug01,0,3012423.story}}</ref> Stable liquid water cannot exist on the surface of Mars with its present low atmospheric pressure and temperature (it would boil), except at the lowest elevations for short periods.<ref name="Kostama">{{cite journal |journal=Geophysical Research Letters |volume=33 |issue=11 |pages=L11201 |date=June 3, 2006 |last1=Kostama |first1=V.-P. |last2=Kreslavsky |first2=M. A. |last3=Head |first3=J. W. |title=Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement |url=http://www.agu.org/pubs/crossref/2006/2006GL025946.shtml |doi=10.1029/2006GL025946 |bibcode=2006GeoRL..3311201K |citeseerx=10.1.1.553.1127 |s2cid=17229252 |access-date=October 8, 2009 |archive-date=March 18, 2009 |archive-url=https://web.archive.org/web/20090318010946/http://www.agu.org/pubs/crossref/2006/2006GL025946.shtml |url-status=dead }}</ref><ref name="flows">{{cite journal |journal=Journal of Geophysical Research |date=May 7, 2005 |last=Heldmann |first=Jennifer L. |display-authors=etal |title=Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions |url=http://daleandersen.seti.org/Dale_Andersen/Science_articles_files/Heldmann%20et%20al.2005.pdf |volume=110 |issue=E5 |pages=Eo5004 |doi=10.1029/2004JE002261 |bibcode=2005JGRE..110.5004H |hdl=2060/20050169988 |s2cid=1578727 |hdl-access=free |access-date=October 8, 2009 |archive-date=October 1, 2008 |archive-url=https://web.archive.org/web/20081001162643/http://daleandersen.seti.org/Dale_Andersen/Science_articles_files/Heldmann%20et%20al.2005.pdf |url-status=dead }} 'conditions such as now occur on Mars, outside of the temperature-pressure stability regime of liquid water' … 'Liquid water is typically stable at the lowest elevations and at low latitudes on the planet, because the atmospheric pressure is greater than the vapor pressure of water and surface temperatures in equatorial regions can reach {{convert|220|K|C F}} for parts of the day.</ref><ref name="Arvidson, R. 2008"/><ref name="Dirt find">{{cite web |url=http://www.space.com/scienceastronomy/090702-phoenix-soil.html |title=The Dirt on Mars Lander Soil Findings |date=July 2, 2009 |publisher=SPACE.com |access-date=December 19, 2010}}</ref>

The presence of the [[perchlorate]] (ClO<sub>4</sub><sup>–</sup>) anion, a strong [[Oxidizing agent|oxidizer]], in the martian soil was confirmed. This salt can considerably lower the water [[Melting point|freezing point]].
[[File:PIA10741 Possible Ice Below Phoenix.jpg|thumb|left|View underneath ''Phoenix'' lander showing water ice exposed by the landing retrorockets.]]

When ''Phoenix'' landed, the [[retrorocket]]s splashed soil and melted ice onto the vehicle.<ref name="martinez2013">{{cite journal | author = Martínez, G. M. | author2 = Renno, N. O. | name-list-style = amp | date = 2013 | title = Water and brines on Mars: current evidence and implications for MSL | journal = Space Science Reviews | volume = 175 | issue = 1–4 | pages = 29–51 | doi = 10.1007/s11214-012-9956-3 | bibcode = 2013SSRv..175...29M | doi-access = free }}</ref> Photographs showed the landing had left blobs of material stuck to the landing struts.<ref name="martinez2013" /> The blobs expanded at a rate consistent with [[deliquescence]], darkened before disappearing (consistent with [[liquefaction]] followed by dripping), and appeared to merge. These observations, combined with [[thermodynamic]] evidence, indicated that the blobs were likely liquid [[brine]] droplets.<ref name="martinez2013" /><ref name="Renno2009">{{cite journal |doi=10.1029/2009JE003362 |title=Possible physical and thermodynamical evidence for liquid water at the Phoenix landing site |date=2009 |last1=Rennó |first1=Nilton O. |last2=Bos |first2=Brent J. |last3=Catling |first3=David |last4=Clark |first4=Benton C. |last5=Drube |first5=Line |last6=Fisher |first6=David |last7=Goetz |first7=Walter |last8=Hviid |first8=Stubbe F. |last9=Keller |first9=Horst Uwe |last10=Kok |first10=Jasper F. |last11=Kounaves |first11=Samuel P. |last12=Leer |first12=Kristoffer |last13=Lemmon |first13=Mark |last14=Madsen |first14=Morten Bo |last15=Markiewicz |first15=Wojciech J. |last16=Marshall |first16=John |last17=McKay |first17=Christopher |last18=Mehta |first18=Manish |last19=Smith |first19=Miles |last20=Zorzano |first20=M. P. |last21=Smith |first21=Peter H. |last22=Stoker |first22=Carol |last23=Young |first23=Suzanne M. M. |journal=Journal of Geophysical Research |volume=114 |issue=E1 |pages=E00E03 |bibcode=2009JGRE..114.0E03R|hdl=2027.42/95444 |s2cid=55050084 |hdl-access=free }}</ref> Other researchers suggested the blobs could be "clumps of frost."<ref name="NYTimes20090316">{{cite web |last=Chang |first=Kenneth |url=https://www.nytimes.com/2009/03/17/science/17mars.html |title=Blobs in Photos of Mars Lander Stir a Debate: Are They Water? |publisher=New York Times (online) |date=March 16, 2009 }}</ref><ref name="Sciencedaily.com">{{cite web |url=https://www.sciencedaily.com/releases/2009/03/090319232438.htm |title=Liquid Saltwater Is Likely Present On Mars, New Analysis Shows |website=ScienceDaily |date=March 20, 2009 }}</ref><ref>{{cite web |url=http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=3350 |title=Astrobiology Top 10: Too Salty to Freeze |work=Astrobiology Magazine |access-date=December 19, 2010 |archive-url=https://web.archive.org/web/20110604121445/http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=3350 |archive-date=2011-06-04 |url-status=usurped}}</ref> In 2015 it was confirmed that perchlorate plays a role in forming [[recurring slope lineae]] on steep [[Gully (Mars)|gullies]].<ref name="Ojhaetal2015"/><ref>{{cite journal |last1=Hecht |first1=M. H. |last2=Kounaves |first2=S. P. |last3=Quinn |first3=R. C. |last4=West |first4=S. J. |last5=Young |first5=S. M. M. |last6=Ming |first6=D. W. |last7=Catling |first7=D. C. |last8=Clark |first8=B. C. |last9=Boynton |first9=W. V. |last10=Hoffman |first10=J. |last11=DeFlores |first11=L. P. |last12=Gospodinova |first12=K. |last13=Kapit |first13=J. |last14=Smith |first14=P. H. |title=Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site |journal=Science |volume=325 |issue=5936 |pages=64–67 |pmid=19574385 |doi=10.1126/science.1172466 |bibcode=2009Sci...325...64H |date=2009 |s2cid=24299495 }}</ref>

For about as far as the camera can see, the landing site is flat, but shaped into polygons between {{convert|2-3|m}} in diameter which are bounded by troughs that are {{convert|20-50|cm}} deep. These shapes are due to ice in the soil expanding and contracting due to major temperature changes. The microscope showed that the soil on top of the polygons is composed of rounded particles and flat particles, probably a type of clay.<ref>{{cite journal |last1=Smith |first1=P. H. |last2=Tamppari |first2=L. K. |last3=Arvidson |first3=R. E. |last4=Bass |first4=D. |last5=Blaney |first5=D.|author5-link= Diana Blaney |last6=Boynton |first6=W. V. |last7=Carswell |first7=A. |last8=Catling |first8=D. C. |last9=Clark |first9=B. C. |last10=Duck |first10=T. |last11=DeJong |first11=E. |last12=Fisher |first12=D. |last13=Goetz |first13=W. |last14=Gunnlaugsson |first14=H. P. |last15=Hecht |first15=M. H. |last16=Hipkin |first16=V. |last17=Hoffman |first17=J. |last18=Hviid |first18=S. F. |last19=Keller |first19=H. U. |last20=Kounaves |first20=S. P. |last21=Lange |first21=C. F. |last22=Lemmon |first22=M. T. |last23=Madsen |first23=M. B. |last24=Markiewicz |first24=W. J. |last25=Marshall |first25=J. |last26=McKay |first26=C. P. |last27=Mellon |first27=M. T. |last28=Ming |first28=D. W. |last29=Morris |first29=R. V. |last30=Pike |first30=W. T. |last31=Renno |first31=N. |last32=Staufer |first32=U. |last33=Stoker |first33=C. |last34=Taylor |first34=P. |last35=Whiteway |first35=J. A. |last36=Zent |first36=A. P. |title=H<sub>2</sub>O at the Phoenix Landing Site |journal=Science |volume=325 |issue=5936 |pages=58–61 |date=2009 |pmid=19574383 |doi=10.1126/science.1172339 |bibcode=2009Sci...325...58S |s2cid=206519214 |display-authors=29 }}</ref> Ice is present a few inches below the surface in the middle of the polygons, and along its edges, the ice is at least {{convert|8|in}} deep.<ref name="Dirt find" />

Snow was observed to fall from cirrus clouds. The clouds formed at a level in the atmosphere that was around {{convert|−65|C|F K}}, so the clouds would have to be composed of water-ice, rather than carbon dioxide-ice (CO<sub>2</sub> or dry ice), because the temperature for forming carbon dioxide ice is much lower than {{convert|−120|C|F K}}. As a result of mission observations, it is now suspected that water ice (snow) would have accumulated later in the year at this location.<ref name="Witeway2009">{{cite journal |last1=Whiteway |first1=J. A. |last2=Komguem |first2=L. |last3=Dickinson |first3=C. |last4=Cook |first4=C. |last5=Illnicki |first5=M. |last6=Seabrook |first6=J. |last7=Popovici |first7=V. |last8=Duck |first8=T. J. |last9=Davy |first9=R. |last10=Taylor |first10=P. A. |last11=Pathak |first11=J. |last12=Fisher |first12=D. |last13=Carswell |first13=A. I. |last14=Daly |first14=M. |last15=Hipkin |first15=V. |last16=Zent |first16=A. P. |last17=Hecht |first17=M. H. |last18=Wood |first18=S. E. |last19=Tamppari |first19=L. K. |last20=Renno |first20=N. |last21=Moores |first21=J. E. |last22=Lemmon |first22=M. T. |last23=Daerden |first23=F. |last24=Smith |first24=P. H. |title=Mars Water-Ice Clouds and Precipitation |journal=Science |volume=325 |issue=5936 |pages=68–70 |pmid=19574386 |doi=10.1126/science.1172344 |bibcode=2009Sci...325...68W |date=2009 |s2cid=206519222 }}</ref> The highest temperature measured during the mission, which took place during the Martian summer, was {{convert|−19.6|C|F K}}, while the coldest was {{convert|−97.7|C|F K}}. So, in this region the temperature remained far below the freezing point ({{convert|0|C|F K}}) of water.<ref>{{cite web |url=http://www.asc-csa.gc.ca/eng/media/news_releases/2009/0702.asp |title=CSA – News Release |publisher=Asc-csa.gc.ca |date=July 2, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20110705011110/http://www.asc-csa.gc.ca/eng/media/news_releases/2009/0702.asp |archive-date=July 5, 2011  }}</ref>

=== Mars Exploration Rovers ===
{{Main|Mars Exploration Rover}}
[[File:Opportunity photo of Mars outcrop rock.jpg|thumb|Close-up of a rock outcrop.]]
[[File:Opp layered sol17-B017R1 br.jpg|thumb|left|Thin rock layers, not all parallel to each other.]]
[[File:07-ml-3-soil-mosaic-B019R1 br.jpg|thumb|Hematite [[Martian spherules|spherules]].]]
[[File:nasa mars opportunity rock water 150 eng 02mar04.jpg|thumb|Partly embedded [[Martian spherules|spherules]].]]

The [[Mars Exploration Rover]]s, ''[[Spirit rover|Spirit]]'' and ''[[Opportunity rover|Opportunity]]'' found a great deal of evidence for past water on Mars. The [[Spirit rover]] landed in what was thought to be a large lake bed. The lake bed had been covered over with lava flows, so evidence of past water was initially hard to detect. On March 5, 2004, NASA announced that ''Spirit'' had found hints of water history on Mars in a rock dubbed "Humphrey".<ref>{{cite web |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040305a.html |title=Mars Exploration Rover Mission: Press Releases |publisher=Marsrovers.jpl.nasa.gov |date=March 5, 2004 }}</ref>

As ''Spirit'' traveled in reverse in December 2007, pulling a seized wheel behind, the wheel scraped off the upper layer of soil, uncovering a patch of white ground rich in [[Silicon dioxide|silica]]. Scientists think that it must have been produced in one of two ways.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/mer/mer-20070521.html |title=NASA – Mars Rover Spirit Unearths Surprise Evidence of Wetter Past |publisher=NASA |date=May 21, 2007 |access-date=January 17, 2012 |archive-date=March 8, 2013 |archive-url=https://web.archive.org/web/20130308054606/http://www.nasa.gov/mission_pages/mer/mer-20070521.html |url-status=dead }}</ref> One: [[hot spring]] deposits produced when water dissolved silica at one location and then carried it to another (i.e. a [[geyser]]). Two: acidic steam rising through cracks in rocks stripped them of their mineral components, leaving silica behind.<ref name="20071210a">{{cite web |last=Bertster |first=Guy |title=Mars Rover Investigates Signs of Steamy Martian Past |work=Press Release |publisher=Jet Propulsion Laboratory, Pasadena, California |date=December 10, 2007 |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20071210a.html}}</ref> The ''Spirit'' rover also found evidence for water in the Columbia Hills of Gusev crater. In the Clovis group of rocks the [[Mössbauer spectrometer]] (MB) detected [[goethite]],<ref>{{cite journal |last=Klingelhofer |first=G. |display-authors=etal |date=2005 |journal=Lunar Planet. Sci. |title=volume XXXVI |type=abstr. |page=2349}}</ref> that forms only in the presence of water,<ref>{{cite journal |last=Schroder |first=C. |display-authors=etal |publisher=European Geosciences Union, General Assembly |title=Journal of Geophysical Research |type=abstr. |volume=7 |page=10254 |date=2005}}</ref><ref>{{cite journal |last=Morris |first=S. |display-authors=etal |title=Mössbauer mineralogy of rock, soil, and dust at Gusev crater, Mars: Spirit's journal through weakly altered olivine basalt on the plains and pervasively altered basalt in the Columbia Hills |journal=J. Geophys. Res. |volume=111|issue=E2 |doi=10.1029/2005je002584 |bibcode=2006JGRE..111.2S13M|year=2006 |pages=n/a |hdl=1893/17159 |hdl-access=free }}</ref><ref>{{cite journal |last1=Ming |first1=D. |last2=Mittlefehldt |first2=D. W.|date=2006 |title=Geochemical and mineralogical indicators for aqueous processes in the Columbia Hills of Gusev crater, Mars |journal=J. Geophys. Res.|volume=111 |issue=E2 |pages=E02S12 |bibcode=2006JGRE..111.2S12M |last3=Morris |first3=R. V. |last4=Golden |first4=D. C. |last5=Gellert |first5=R. |last6=Yen |first6=A. |last7=Clark |first7=B. C. |last8=Squyres |first8=S. W. |last9=Farrand |first9=W. H. |last10=Ruff |first10=S. W. |last11=Arvidson |first11=R. E. |last12=Klingelhöfer |first12=G. |last13=McSween |first13=H. Y. |last14=Rodionov |first14=D. S. |last15=Schröder |first15=C. |last16=De Souza |first16=P. A. |last17=Wang |first17=A. |doi=10.1029/2005JE002560|hdl=1893/17114 |hdl-access=free }}</ref> iron in the oxidized form Fe<sup>3+</sup>,<ref>{{cite book |editor-last=Bell |editor-first=J |title=The Martian Surface |date=2008 |publisher=Cambridge University Press. |isbn=978-0-521-86698-9}}</ref> [[carbonate]]-rich rocks, which means that regions of the planet once harbored water.<ref>{{cite journal|url=https://www.sciencedaily.com/releases/2010/06/100603140959.htm |title=Outcrop of long-sought rare rock on Mars found |doi=10.1126/science.1189667 |pmid=20522738 |publisher=Sciencedaily.com |date=June 4, 2010 |journal=Science |volume=329 |issue=5990 |pages=421–424 |first1=R. V. |last1=Morris|last2=Ruff |first2=S. W. |last3=Gellert |first3=R. |last4=Ming |first4=D. W. |last5=Arvidson |first5=R. E. |last6=Clark |first6=B. C. |last7=Golden |first7=D. C. |last8=Siebach |first8=K. |last9=Klingelhofer |first9=G. |last10=Schroder |first10=C. |last11=Fleischer |first11=I. |last12=Yen |first12=A. S. |last13=Squyres |first13=S. W.|bibcode=2010Sci...329..421M|s2cid=7461676 |doi-access=free }}</ref><ref>{{cite journal |first1=Richard V. |last1=Morris |first2=Steven W. |last2=Ruff |first3=Ralf |last3=Gellert |first4=Douglas W. |last4=Ming |first5=Raymond E. |last5=Arvidson |first6=Benton C. |last6=Clark |first7=D. C. |last7=Golden |first8=Kirsten |last8=Siebach |first9=Göstar |last9=Klingelhöfer |first10=Christian |last10=Schröder |first11=Iris |last11=Fleischer |first12=Albert S. |last12=Yen |first13=Steven W. |last13=Squyres |display-authors=8 |title=Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover |journal=Science |date=June 3, 2010 |doi=10.1126/science.1189667 |pmid=20522738 |volume=329 |issue=5990 |pages=421–424|bibcode=2010Sci...329..421M|s2cid=7461676 |doi-access=free }}</ref>

The [[Opportunity rover|''Opportunity'' rover]] was directed to a site that had displayed large amounts of [[hematite]] from orbit. Hematite often forms from water. The rover indeed found layered rocks and marble- or blueberry-like hematite [[concretion]]s. Elsewhere on its traverse, ''Opportunity'' investigated aeolian dune [[stratigraphy]] in Burns Cliff in [[Endurance (crater)|Endurance Crater]]. Its operators concluded that the preservation and cementation of these outcrops had been controlled by flow of shallow groundwater.<ref name="BurnsCliff" /> In its years of continuous operation, ''Opportunity'' sent back evidence that this area on Mars was soaked in liquid water in the past.<ref name="marsrovers">{{cite web |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040302a.html |title=Opportunity Rover Finds Strong Evidence Meridiani Planum Was Wet |access-date=July 8, 2006}}</ref><ref name="SFN-20130125">{{cite web |last=Harwood |first=William |title=Opportunity rover moves into 10th year of Mars operations |url=http://www.spaceflightnow.com/news/n1301/25opportunity/ |date=January 25, 2013 |publisher=Space Flight Now}}</ref>

The MER rovers found evidence for ancient wet environments that were very acidic. In fact, what ''Opportunity'' found evidence of [[sulfuric acid]], a harsh chemical for life.<ref name="Amos June 2013" /><ref name="Clay clues" /><ref>{{cite journal |last1=Benison |first1=KC |last2=Laclair |first2=DA |title=Modern and ancient extremely acid saline deposits: terrestrial analogs for martian environments? |journal=Astrobiology |volume=3 |issue=3 |pages=609–618 |date=2003 |pmid=14678669 |doi=10.1089/153110703322610690 |bibcode=2003AsBio...3..609B|s2cid=36757620 }}</ref><ref>{{cite journal |last1=Benison |first1=K |last2=Bowen |first2=B |title=Acid saline lake systems give clues about past environments and the search for life on Mars |journal=Icarus |volume=183 |issue=1 |pages=225–229 |bibcode=2006Icar..183..225B |date=2006 |doi=10.1016/j.icarus.2006.02.018}}</ref> But on May 17, 2013, NASA announced that ''Opportunity'' found [[clay]] deposits that typically form in wet environments that are near neutral [[pH|acidity]]. This find provides additional evidence about a wet ancient environment possibly favorable for [[Life on Mars|life]].<ref name="Amos June 2013" /><ref name="Clay clues" />

=== Mars Reconnaissance Orbiter ===
{{Main|Evidence of water on Mars found by Mars Reconnaissance Orbiter}}
[[File:Springs in Vernal Crater.jpg|left|thumb|Springs in [[Vernal Crater]], as seen by [[HIRISE]]. These springs may be good places to look for evidence of past life, because hot springs can preserve evidence of life forms for a long time. Location is [[Oxia Palus quadrangle]]. ]]
The [[Mars Reconnaissance Orbiter]]'s [[HiRISE]] instrument has taken many images that strongly suggest that Mars has had a rich history of water-related processes. A major discovery was finding evidence of ancient [[hot spring]]s. If they have hosted microbial life, they may contain [[biosignature]]s.<ref name="Osterloo2008">{{cite journal |doi=10.1126/science.1150690 |last1=Osterloo |first1=MM |last2=Hamilton |date=2008 |first2=VE |last3=Bandfield |first3=JL |last4=Glotch |first4=TD |last5=Baldridge |first5=AM |last6=Christensen |first6=PR |last7=Tornabene |first7=LL |last8=Anderson |first8=FS |title=Chloride-Bearing Materials in the Southern Highlands of Mars |journal=Science |volume=319 |issue=5870 |pages=1651–1654 |pmid=18356522 |bibcode=2008Sci...319.1651O|citeseerx=10.1.1.474.3802 |s2cid=27235249 }}</ref> Research published in January 2010, described strong evidence for sustained precipitation in the area around [[Valles Marineris]].<ref name=Weitz>{{cite journal |doi=10.1016/j.icarus.2009.04.017 |last1=Weitz |first1=C. |last2=Milliken |date=2010 |first2=R.E. |last3=Grant |first3=J.A. |last4=McEwen |first4=A.S. |last5=Williams |first5=R.M.E. |last6=Bishop |first6=J.L. |author-link6=Janice Bishop|last7=Thomson |first7=B.J. |title=Mars Reconnaissance Orbiter observations of light-toned layered deposits and associated fluvial landforms on the plateaus adjacent to Valles Marineris |journal=Icarus |volume=205 |issue=1 |pages=73–102 |bibcode=2010Icar..205...73W}}</ref><ref name="Icarus Vol 210">{{cite journal  |title=Atmospheric mass loss by stellar wind from planets around main sequence M stars|volume=210 |issue=2 |pages=539–1000 |date=December 2010 |doi=10.1016/j.icarus.2010.07.013 |bibcode=2010Icar..210..539Z |journal=Icarus|last1=Zendejas|first1=J.|last2=Segura|first2=A.|last3=Raga|first3=A.C.|arxiv=1006.0021|s2cid=119243879 }}</ref> The types of minerals there are associated with water. Also, the high density of small branching channels indicates a great deal of precipitation.

Rocks on Mars have been found to frequently occur as layers, called strata, in many different places.<ref>{{cite book |editor-last=Grotzinger |editor-first=J. |editor2-first=R. |editor2-last=Milliken |date=2012 |title=Sedimentary Geology of Mars |publisher=SEPM}}</ref> Layers form by various ways, including volcanoes, wind, or water.<ref>{{cite web |url=http://hirise.lpl.arizona.edu?PSP_008437_1750 |title=HiRISE – High Resolution Imaging Science Experiment |publisher=HiriUniversity of Arizona |access-date=December 19, 2010}}</ref> Light-toned rocks on Mars have been associated with [[Mineral hydration|hydrated minerals]] like sulfates and clay.<ref>{{cite web |url=http://themis.asu.edu/features/nilosyrtis |title=Target Zone: Nilosyrtis? &#124; Mars Odyssey Mission THEMIS |publisher=Themis.asu.edu |access-date=December 19, 2010}}</ref>
[[File:Asimov Layers Close-up.JPG|thumb|Layers on the west slope of Asimov Crater. Location is [[Noachis quadrangle]].]]

The orbiter helped scientists determine that much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.<ref name="Head, J. 2003">{{cite journal |last1=Head |first1=James W. |last2=Mustard |first2=John F. |last3=Kreslavsky |first3=Mikhail A. |last4=Milliken |first4=Ralph E. |last5=Marchant |first5=David R. |title=Recent ice ages on Mars |journal=Nature |volume=426 |issue=6968 |pages=797–802 |date=2003 |pmid=14685228 |doi=10.1038/nature02114 |bibcode=2003Natur.426..797H|s2cid=2355534 }}</ref><ref>{{Cite journal |last1=Mellon |first1=M. T. |first2=B. M. |last2=Jakosky |first3=S. E. |last3=Postawko |date=1997 |title=The persistence of equatorial ground ice on Mars |publisher=onlinelibrary.wiley.com |journal=J. Geophys. Res. |volume=102 |issue=E8 |pages=19357–19369 |doi=10.1029/97JE01346 |bibcode=1997JGR...10219357M|doi-access=free }}</ref><ref>{{cite web |first=John D. |last=Arfstrom |url=http://www.lpi.usra.edu/meetings/climatology2012/pdf/8001.pdf |title=A Conceptual Model of Equatorial Ice Sheets on Mars. J |publisher=Lunar and Planetary Institute |work=Comparative Climatology of Terrestrial Planets |date=2012}}</ref>

The ice mantle under the shallow subsurface is thought to result from frequent, major climate changes. Changes in Mars' orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas. During certain climate periods water vapor leaves polar ice and enters the atmosphere. The water returns to the ground at lower latitudes as deposits of frost or snow mixed generously with dust. The atmosphere of Mars contains a great deal of fine dust particles.<ref name="Head 2008 PNAS" /> Water vapor condenses on the particles, then they fall down to the ground due to the additional weight of the water coating. When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.<ref name="IceAge&shy;">{{cite news |publisher=MLA NASA/Jet Propulsion Laboratory |date=December 18, 2003 |title=Mars may be emerging from an ice age |work=ScienceDaily |url=https://www.sciencedaily.com/releases/2003/12/031218075443.htm}}</ref>

In 2008, research with the Shallow Radar on the Mars Reconnaissance Orbiter provided strong evidence that the [[lobate debris aprons]] (LDA) in [[Hellas Planitia]] and in mid northern latitudes are [[glacier]]s that are covered with a thin layer of rocks. Its radar also detected a strong reflection from the top and base of LDAs, meaning that pure water ice made up the bulk of the formation.<ref name="Holt, J. 2008" /> The discovery of water ice in LDAs demonstrates that water is found at even lower latitudes.<ref name="Kieffer1992">{{cite book |first=Hugh H. |last=Kieffer |title=Mars |url=https://books.google.com/books?id=NoDvAAAAMAAJ |access-date=March 7, 2011 |date=1992 |publisher=University of Arizona Press |isbn=978-0-8165-1257-7}}</ref>

Research published in September 2009, demonstrated that some new craters on Mars show exposed, pure water ice.<ref>{{cite journal |last1=Byrne |pmid=19779195 |date=2009 |first1=Shane |last2=Dundas |first2=Colin M. |last3=Kennedy |first3=Megan R. |last4=Mellon |first4=Michael T. |last5=McEwen |first5=Alfred S. |last6=Cull |first6=Selby C. |last7=Daubar |first7=Ingrid J. |last8=Shean |first8=David E. |last9=Seelos |first9=Kimberly D. |last10=Murchie |first10=Scott L. |last11=Cantor |first11=Bruce A. |last12=Arvidson |first12=Raymond E. |last13=Edgett |first13=Kenneth S. |last14=Reufer |first14=Andreas |last15=Thomas |first15=Nicolas |last16=Harrison |first16=Tanya N. |last17=Posiolova |first17=Liliya V. |last18=Seelos |first18=Frank P. |title=Distribution of mid-latitude ground ice on Mars from new impact craters |journal=Science |volume=325 |issue=5948 |pages=1674–1676 |doi=10.1126/science.1175307 |bibcode=2009Sci...325.1674B|s2cid=10657508 }}</ref> After a time, the ice disappears, evaporating into the atmosphere. The ice is only a few feet deep. The ice was confirmed with the Compact Imaging Spectrometer (CRISM) on board the Mars Reconnaissance Orbiter.<ref>{{cite web |url=http://www.space.com/scienceastronomy/090924-mars-crater-ice.html |title=Water Ice Exposed in Mars Craters |date=September 24, 2009 |publisher=SPACE.com |access-date=December 19, 2010}}</ref> Similar exposures of ice have been detected within the mid-latitude mantle (originally proposed to contain buried dusty snow covered with dust and regolith;<ref name=":8"/>) that drapes most pole-facing slopes in the mid-latitudes using spectral analysis of HiRISE images.<ref>{{Cite journal |last1=Khuller |first1=Aditya |last2=Christensen |first2=Philip |date=February 2021 |title=Evidence of Exposed Dusty Water Ice within Martian Gullies |url=https://onlinelibrary.wiley.com/doi/10.1029/2020JE006539 |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=2 |doi=10.1029/2020JE006539 |bibcode=2021JGRE..12606539R |s2cid=234174382 |issn=2169-9097}}</ref>

Additional collaborating reports published in 2019 evaluated the amount of water ice located at the northern pole. One report used data from the MRO's [[SHARAD]] (SHAllow RADar sounder) probes. SHARAD has the capability scanning up to about {{convert|2|km|miles}} below the surface at {{convert|15|m|ft}} intervals. The analysis of past SHARAD runs showed evidence of strata of water ice and sand below the [[Planum Boreum]], with as much as 60% to 88% of the volume being water ice. This supports the theory of the long-term global weather of Mars consisting of cycles of global warming and cooling; during cooling periods, water gathered at the poles to form the ice layers, and then as global warming occurred, the unthawed water ice was covered by dust and dirt from Mars' frequent dust storms. The total ice volume determine by this study indicated that there was approximately {{convert|2.2e5|km3|cumi}}, or enough water, if melted, to fully cover the Mars surface with a {{convert|1.5|m|ft}} layer of water.<ref>{{cite journal | title = Buried ice and sand caps at the north pole of Mars: revealing a record of climate change in the cavi unit with SHARAD | author1 = S. Nerozzi |author2=J.W. Holt | date = May 22, 2019 | doi = 10.1029/2019GL082114 | journal = [[Geophysical Research Letters]] | volume = 46 | issue = 13 | pages = 7278–7286 | bibcode = 2019GeoRL..46.7278N | hdl = 10150/634098 | s2cid = 182153656 | hdl-access = free }}</ref> The work was corroborated by a separate study that used recorded gravity data to estimate the density of the Planum Boreum, indicating that on average, it contained up to 55% by volume of water ice.<ref>{{cite journal | title = Compositional Constraints on the North Polar Cap of Mars from Gravity and Topography | author1 = Lujendra Ojha |author2=Stefano Nerozzi |author3=Kevin Lewis | date = May 22, 2019 | doi = 10.1029/2019GL082294 | journal = [[Geophysical Research Letters]] | volume = 46 | issue = 15 | pages = 8671–8679 | bibcode = 2019GeoRL..46.8671O | s2cid = 181334027 }}</ref>

Many features that look like the [[pingo]]s on the Earth were found in Utopia Planitia (~35-50° N; ~80-115° E) by examining photos from HiRISE. Pingos contain a core of ice.<ref>Soare, E., et al.  2019.     
Possible (closed system) pingo and ice-wedge/thermokarst complexes at the mid latitudes of Utopia Planitia, Mars. Icarus.  https://doi.org/10.1016/j.icarus.2019.03.010</ref>
{{Clear}}

=== ''Curiosity'' rover ===
{{Main|Timeline of Mars Science Laboratory}}
[[File:PIA16156-Mars Curiosity Rover-Water-AncientStreambed.jpg|thumb|left|"[[Hottah (Mars)|Hottah]]" [[rock outcrop]] – an ancient [[streambed]] discovered by the [[Curiosity (rover)|''Curiosity'' rover]] team (September 14, 2012) ([http://photojournal.jpl.nasa.gov/figures/PIA16156_fig1.jpg close-up]) ([https://web.archive.org/web/20130521042719/http://mars.jpl.nasa.gov/msl/images/pia16223-stereoHattah-Mastcam-br2.jpg 3-D version]).]]
[[File:PIA16189 fig1-Curiosity Rover-Rock Outcrops-Mars and Earth.jpg|thumb|[[Rock outcrop]] on Mars – compared with a terrestrial [[Conglomerate (geology)#Fluvial|fluvial conglomerate]] – suggesting water "vigorously" flowing in a stream.<ref name="NASA-20120927">{{cite web |last1=Brown |first1=Dwayne |last2=Cole |first2=Steve |last3=Webster |first3=Guy |last4=Agle |first4=D.C. |title=NASA Rover Finds Old Streambed On Martian Surface |url=http://www.nasa.gov/home/hqnews/2012/sep/HQ_12-338_Mars_Water_Stream.html |date=September 27, 2012 |publisher=[[NASA]]}}</ref><ref name="NASA-20120927a">{{cite web |author=NASA |author-link=NASA |title=NASA's Curiosity Rover Finds Old Streambed on Mars – video (51:40) |url=https://www.youtube.com/watch?v=fYo31XjoXOk | archive-url=https://ghostarchive.org/varchive/youtube/20211113/fYo31XjoXOk| archive-date=2021-11-13 | url-status=live|date=September 27, 2012 |publisher=[[NASA]]television}}{{cbignore}}</ref><ref name="AP-20120927">{{cite news |last=Chang |first=Alicia |title=Mars rover Curiosity finds signs of ancient stream |url=http://apnews.excite.com/article/20120927/DA1IDOO00.html |date=September 27, 2012 |agency=Associated Press}}</ref>]]

Very early in its ongoing mission, [[NASA]]'s [[Curiosity (rover)|''Curiosity'']] rover discovered unambiguous [[fluvial]] sediments on Mars. The properties of the pebbles in these outcrops suggested former vigorous flow on a streambed, with flow between ankle- and waist-deep. These rocks were found at the foot of an [[alluvial fan]] system descending from the crater wall, which had previously been identified from orbit.<ref name="NASA-20120927" /><ref name="NASA-20120927a" /><ref name="AP-20120927" />

In October 2012, the first [[X-ray crystallography#Mineralogy and metallurgy|X-ray diffraction analysis]] of a [[Martian soil]] was performed by ''Curiosity''. The results revealed the presence of several minerals, including [[feldspar]], [[pyroxene]]s and [[olivine]], and suggested that the Martian soil in the sample was similar to the weathered basaltic soils of [[Hawaii Volcanoes|Hawaiian volcanoes]]. The sample used is composed of dust distributed from [[Martian dust|global dust storms]] and local fine sand. So far, the materials ''Curiosity'' has analyzed are consistent with the initial ideas of deposits in [[Gale Crater]] recording a transition through time from a wet to dry environment.<ref name="NASA-20121030">{{cite web |last=Brown |first=Dwayne |title=NASA Rover's First Soil Studies Help Fingerprint Martian Minerals |url=http://www.nasa.gov/home/hqnews/2012/oct/HQ_12-383_Curiosity_CheMin.html |date=October 30, 2012 |publisher=[[NASA]] |access-date=June 16, 2013 |archive-date=June 3, 2016 |archive-url=https://web.archive.org/web/20160603091908/http://www.nasa.gov/home/hqnews/2012/oct/HQ_12-383_Curiosity_CheMin.html |url-status=dead }}</ref>

In December 2012, NASA reported that ''Curiosity'' performed its first extensive [[soil analysis]], revealing the presence of water molecules, [[sulfur]] and [[chlorine]] in the [[Martian soil]].<ref name="NASA-20121203">{{cite web |last1=Brown |first1=Dwayne |last2=Webster |first2=Guy |last3=Neal-Jones |first3=Nance |title=NASA Mars Rover Fully Analyzes First Martian Soil Samples |url=http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1399 |archive-url=https://web.archive.org/web/20121205005911/http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1399 |url-status=dead |archive-date=December 5, 2012 |date=December 3, 2012 |publisher=[[NASA]] }}</ref><ref name="NYT-20121203">{{cite web |last=Chang |first=Ken |title=Mars Rover Discovery Revealed |url=http://thelede.blogs.nytimes.com/2012/12/03/mars-rover-discovery-revealed |date=December 3, 2012 |work=[[New York Times]]}}</ref> And in March 2013, NASA reported evidence of [[mineral hydration]], likely hydrated [[calcium sulfate]], in several [[Rock (geology)|rock samples]] including the broken fragments of [[Tintina (rock)|"Tintina" rock]] and [[List of rocks on Mars#Curiosity|"Sutton Inlier" rock]] as well as in [[Vein (geology)|veins]] and [[Nodule (geology)|nodules]] in other rocks like [[List of rocks on Mars#Curiosity|"Knorr" rock]] and [[List of rocks on Mars#Curiosity|"Wernicke" rock]].<ref name="NASA-20130318">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=Curiosity Mars Rover Sees Trend In Water Presence |url=http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1446 |archive-url=https://web.archive.org/web/20130322065943/http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1446 |url-status=dead |archive-date=March 22, 2013 |date=March 18, 2013 |work=[[NASA]]}}</ref><ref name="BBC-20130319">{{cite web |last=Rincon |first=Paul |title=Curiosity breaks rock to reveal dazzling white interior |url=https://www.bbc.co.uk/news/science-environment-21340279 |date=March 19, 2013 |publisher=BBC}}</ref><ref name="MSN-20130120">{{cite web |author=Staff |title=Red planet coughs up a white rock, and scientists freak out |url=http://now.msn.com/white-mars-rock-called-tintina-found-by-curiosity-rover |date=March 20, 2013 |publisher=[[MSN]] |url-status=dead |archive-url=https://web.archive.org/web/20130323164757/http://now.msn.com/white-mars-rock-called-tintina-found-by-curiosity-rover |archive-date=March 23, 2013  }}</ref> Analysis using the rover's [[Curiosity (rover)#Dynamic Albedo of Neutrons (DAN)|DAN instrument]] provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of {{convert|60|cm|ft|abbr=on}}, in the rover's traverse from the ''[[Bradbury Landing]]'' site to the ''Yellowknife Bay'' area in the [[Glenelg, Mars|''Glenelg'']] terrain.<ref name="NASA-20130318" />

On September 26, 2013, NASA scientists reported the [[Mars]] ''Curiosity'' rover detected abundant chemically-bound water (1.5 to 3 weight percent) in [[Martian soil|soil samples]] at the [[Rocknest (Mars)|Rocknest region]] of [[Aeolis Palus]] in [[Gale Crater]].<ref name="ST-20130926">{{cite web |last=Lieberman |first=Josh |title=Mars Water Found: Curiosity Rover Uncovers 'Abundant, Easily Accessible' Water In Martian Soil |url=http://www.isciencetimes.com/articles/6131/20130926/mars-water-soil-nasa-curiosity-rover-martian.htm |date=September 26, 2013 |work=iSciencetimes}}</ref><ref name="Science-20130926a">{{cite journal |last=Leshin |first=L. A. |display-authors=etal |title=Volatile, Isotope, and Organic Analysis of Martian Fines with the Mars Curiosity Rover |date=September 27, 2013 |journal=[[Science (journal)|Science]] |volume=341 |issue=6153 |doi=10.1126/science.1238937 |pages=1238937 |pmid=24072926|bibcode=2013Sci...341E...3L |s2cid=206549244 |url=https://semanticscholar.org/paper/7f3089e0c3e10eb39e48ff007e04a778811683dd }}</ref><ref name="Science-20130926">{{cite journal |last=Grotzinger |first=John |title=Introduction To Special Issue: Analysis of Surface Materials by the Curiosity Mars Rover |date=September 26, 2013 |journal=[[Science (journal)|Science]] |volume=341 |issue=6153 |page=1475 |doi=10.1126/science.1244258|pmid=24072916 |bibcode=2013Sci...341.1475G |doi-access=free }}</ref><ref name="NASA-20130926a">{{cite web |last1=Neal-Jones |first1=Nancy |last2=Zubritsky |first2=Elizabeth |last3=Webster |first3=Guy |last4=Martialay |first4=Mary |title=Curiosity's SAM Instrument Finds Water and More in Surface Sample |url=http://www.nasa.gov/content/goddard/curiositys-sam-instrument-finds-water-and-more-in-surface-sample/ |date=September 26, 2013 |work=[[NASA]]}}</ref><ref name="NASA-20130926b">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=Science Gains From Diverse Landing Area of Curiosity |url=http://www.nasa.gov/mission_pages/msl/news/msl20130926.html |date=September 26, 2013 |work=[[NASA]] |access-date=September 27, 2013 |archive-date=May 2, 2019 |archive-url=https://web.archive.org/web/20190502194152/http://www.nasa.gov/mission_pages/msl/news/msl20130926.html |url-status=dead }}</ref><ref name="NYT-20131001">{{cite news |last=Chang |first=Kenneth |title=Hitting Pay Dirt on Mars |url=https://www.nytimes.com/2013/10/01/science/space/hitting-pay-dirt-on-mars.html |date=October 1, 2013 |work=[[New York Times]]}}</ref> In addition, NASA reported the rover found two principal soil types: a fine-grained [[Mafic|mafic type]] and a locally derived, coarse-grained [[Felsic|felsic type]].<ref name="Science-20130926" /><ref name="NASA-20130926b" /><ref name="Science-20130913b">{{cite journal |last=Meslin |first=P.-Y. |display-authors=etal |title=Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars |date=September 26, 2013 |journal=[[Science (journal)|Science]] |volume=341 |issue=6153 |doi=10.1126/science.1238670 |pages=1238670 |pmid=24072924|bibcode=2013Sci...341E...1M |s2cid=7418294 }}</ref> The mafic type, similar to other [[martian soil]]s and [[martian dust]], was associated with hydration of the amorphous phases of the soil.<ref name="Science-20130913b" /> Also, [[perchlorates]], the presence of which may make detection of life-related [[Organic matter|organic molecules]] difficult, were found at the ''Curiosity'' rover landing site (and earlier at the more polar site of the [[Phoenix (spacecraft)|Phoenix lander]]) suggesting a "global distribution of these salts".<ref name="NYT-20131001" /> NASA also reported that [[Jake M (rock)|Jake M rock]], a rock encountered by ''Curiosity'' on the way to [[Glenelg, Mars|Glenelg]], was a [[mugearite]] and very similar to terrestrial mugearite rocks.<ref name="Science-20130926c">{{cite journal |last1=Stolper |first1=E.M. |last2=Baker |first2=M.B. |last3=Newcombe |first3=M.E. |last4=Schmidt |first4=M.E. |last5=Treiman |first5=A.H. |last6=Cousin |first6=A. |last7=Dyar |first7=M.D. |last8=Fisk |first8=M.R. |last9=Gellert |first9=R. |last10=King |first10=P.L. |last11=Leshin |first11=L. |last12=Maurice |first12=S. |last13=McLennan |first13=S.M. |last14=Minitti |first14=M.E. |last15=Perrett |first15=G. |last16=Rowland |first16=S. |last17=Sautter |first17=V. |author17-link=Violaine Sautter |last18=Wiens |first18=R.C. |last19=MSL ScienceTeam |title=The Petrochemistry of Jake_M: A Martian Mugearite |journal=[[Science (journal)|Science]] |volume=341 |issue=6153 |doi=10.1126/science.1239463 |publisher=[[American Association for the Advancement of Science|AAAS]] |date=2013 |pages=1239463 |pmid=24072927 |bibcode=2013Sci...341E...4S |s2cid=16515295 |url=https://authors.library.caltech.edu/41547/13/Jake_M%20Stolper%20et%20al.%20%282013%29%20Science.pdf |access-date=July 23, 2019 |archive-date=August 11, 2021 |archive-url=https://web.archive.org/web/20210811150621/https://authors.library.caltech.edu/41547/13/Jake_M%20Stolper%20et%20al.%20(2013)%20Science.pdf |url-status=dead }}</ref>

On December 9, 2013, NASA reported that Mars once had a large [[freshwater lake]] inside [[Gale (crater)|Gale Crater]],<ref name="NYT-20131209" /><ref name="SCI-20131209" /> that could have been a hospitable environment for [[microbial life]].

On December 16, 2014, NASA reported detecting an unusual increase, then decrease, in the amounts of [[methane]] in the [[Atmosphere of Mars|atmosphere]] of the planet [[Mars]]; in addition, [[organic chemical]]s were detected in powder drilled from a [[List of rocks on Mars|rock]] by the [[Curiosity (rover)|''Curiosity'' rover]]. Also, based on [[deuterium]] to [[hydrogen]] ratio studies, much of the water at [[Gale Crater]] on Mars was found to have been lost during ancient times, before the lake bed in the crater was formed; afterwards, large amounts of water continued to be lost.<ref name="NASA-20141216-GW">{{cite web |last1=Webster |first1=Guy |last2=Neal-Jones |first2=Nancy |last3=Brown |first3=Dwayne |title=NASA Rover Finds Active and Ancient Organic Chemistry on Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-432 |date=December 16, 2014 |work=[[NASA]] |access-date=December 16, 2014 }}</ref><ref name="NYT-20141216-KC">{{cite news |last=Chang |first=Kenneth |title='A Great Moment': Rover Finds Clue That Mars May Harbor Life |url=https://www.nytimes.com/2014/12/17/science/a-new-clue-in-the-search-for-life-on-mars.html |date=December 16, 2014 |work=[[New York Times]] |access-date=December 16, 2014 }}</ref><ref name="SCI-20141216-PRM">{{cite journal |title=Mars Atmosphere – The imprint of atmospheric evolution in the D/H of Hesperian clay minerals on Mars |date=December 16, 2014 |journal=[[Science (journal)|Science]] |volume=347 |issue=6220 |pages=412–414 |doi=10.1126/science.1260291 |display-authors=1 |last1=Mahaffy |first1=P. R. |last2=Webster|first2=C. R.|last3=Stern |first3=J. C. |last4=Brunner |first4=A. E. |last5=Atreya |first5=S. K. |last6=Conrad |first6=P. G. |last7=Domagal-Goldman |first7=S. |last8=Eigenbrode |first8=J. L. |last9=Flesch |first9=G. J. |last10=Christensen |first10=L. E. |last11=Franz |first11=H. B. |last12=Freissinet |first12=C. |last13=Glavin |first13=D. P. |last14=Grotzinger |first14=J. P. |last15=Jones |first15=J. H. |last16=Leshin |first16=L. A. |last17=Malespin |first17=C. |last18=McAdam |first18=A. C. |last19=Ming |first19=D. W. |last20=Navarro-Gonzalez |first20=R. |last21=Niles |first21=P. B. |last22=Owen |first22=T. |last23=Pavlov |first23=A. A. |last24=Steele |first24=A. |last25=Trainer |first25=M. G. |last26=Williford |first26=K. H. |last27=Wray |first27=J. J. |bibcode=2015Sci...347..412M |pmid=25515119|s2cid=37075396 |url=https://authors.library.caltech.edu/52528/7/Mahaffy-SM.pdf }}</ref>

On April 13, 2015, ''[[Nature (journal)|Nature]]'' published an analysis of humidity and ground temperature data collected by ''Curiosity'', showing evidence that films of liquid brine water form in the upper 5&nbsp;cm of Mars's subsurface at night. The water activity and temperature remain below the requirements for reproduction and [[metabolism]] of known terrestrial microorganisms.<ref name="Torres 2015"/><ref>{{cite news |last=Rincon |first=Paul |url=https://www.bbc.com/news/science-environment-32287609 |title=Evidence of liquid water found on Mars |work=BBC News |date=April 13, 2015 |access-date=April 15, 2015 }}</ref>

On October 8, 2015, NASA confirmed that lakes and streams existed in [[Gale (crater)|Gale crater]] 3.3 – 3.8 billion years ago delivering sediments to build up the lower layers of [[Mount Sharp]].<ref name="NASA-20151008">{{cite web |last=Clavin |first=Whitney |title=NASA's Curiosity Rover Team Confirms Ancient Lakes on Mars |url=http://www.jpl.nasa.gov/news/news.php?feature=4734 |date=October 8, 2015 |work=[[NASA]] |access-date=October 9, 2015 }}</ref><ref name="SCI-20151009">{{cite journal |author=Grotzinger, J.P. |title=Deposition, exhumation, and paleoclimate of an ancient lake deposit, Gale crater, Mars |date=October 9, 2015 |journal=[[Science (journal)|Science]] |volume=350 |issue=6257 |pages=aac7575 |doi=10.1126/science.aac7575|pmid=26450214 |bibcode=2015Sci...350.7575G |s2cid=586848 |url=https://resolver.caltech.edu/CaltechAUTHORS:20151009-084255932 }}</ref>

On November 4, 2018, geologists presented evidence, based on studies in [[Gale (crater)|Gale Crater]] by the [[Curiosity (rover)|''Curiosity'' rover]], that there was plenty of [[water]] on early [[Mars]].<ref name="EA-20181103-gsa">{{cite news |author=Geological Society of America |title=Evidence of outburst flooding indicates plentiful water on early Mars |url=https://www.eurekalert.org/pub_releases/2018-11/gsoa-eoo110318.php |date= November 3, 2018 |work=[[EurekAlert!]] |access-date=November 5, 2018 |author-link=Geological Society of America }}</ref><ref name="GSA-20181104">{{cite journal |author=Heydari, Ezat|display-authors=etal |title=Significance of Flood Depositis in Gale Crater, Mars |url=https://gsa.confex.com/gsa/2018AM/webprogram/Paper319960.html |date=November 4, 2018 |journal=[[Geological Society of America]] |access-date=November 5, 2018 }}</ref>

===''Mars Express''===
The ''[[Mars Express]] Orbiter'', launched by the [[European Space Agency]], has been mapping the surface of Mars and using radar equipment to look for evidence of sub-surface water. Between 2012 and 2015, the ''Orbiter'' scanned the area beneath the ice caps on the [[Planum Australe]]. Scientists determined by 2018 that the readings indicated a sub-surface lake bearing water about {{convert|20|km|mi}} wide. The top of the lake is located {{convert|1.5|km}} under the planet's surface; how much deeper the liquid water extends remains unknown.<ref>{{cite journal |vauthors=Orosei R, Lauro SE, Pettinelli E, Cicchetti A, Coradini M, Cosciotti B, Di Paolo F, Flamini E, Mattei E, Pajola M, Soldovieri F, Cartacci M, Cassenti F, Frigeri A, Giuppi S, Martufi R, Masdea A, Mitri G, Nenna C, Noschese R, Restano M, Seu R|date=July 25, 2018 |title=Radar evidence of subglacial liquid water on Mars |journal= [[Science (journal)|Science]]|volume=361 |issue=3699 |pages= 490–493|doi=10.1126/science.aar7268 |pmid= 30045881|arxiv=2004.04587 |bibcode=2018Sci...361..490O |hdl=11573/1148029 |s2cid=206666385 |hdl-access=free }}</ref><ref>{{cite news | url = https://www.bbc.com/news/science-environment-44952710 | title = Liquid water 'lake' revealed on Mars | first = Mary | last = Halton | date = July 25, 2018 | access-date = July 25, 2018 |work=BBC News}}</ref>

===Zhurong Rover===

China's Zhurong touched down on Mars in the area called Utopia Planitia on May 14, 2021. Its six scientific instruments including two panoramic cameras, a ground-penetrating radar and a magnetic field detector. Zhurong  used a laser to zap rocks to study their compositions.<ref>{{cite web | url=https://www.space.com/china-mars-rover-landing-success-tianwen-1-zhurong | title=China's 1st Mars rover 'Zhurong' lands on the Red Planet | website=[[Space.com]] | date=May 15, 2021 }}</ref>

Zhurong found evidence of water when it examined the crust at the surface, called "duricrust."  The crust
contained hydrated sulfate/silica materials in the Amazonian-age terrain of the landing site. The duricrust was produced  either by subsurface ice melting or groundwater rising.<ref>{{cite journal | doi=10.1126/sciadv.abn8555 | title=Zhurong reveals recent aqueous activities in Utopia Planitia, Mars | date=2022 | last1=Liu | first1=Yang | last2=Wu | first2=Xing | last3=Zhao | first3=Yu-Yan Sara | last4=Pan | first4=Lu | last5=Wang | first5=Chi | last6=Liu | first6=Jia | last7=Zhao | first7=Zhenxing | last8=Zhou | first8=Xiang | last9=Zhang | first9=Chaolin | last10=Wu | first10=Yuchun | last11=Wan | first11=Wenhui | last12=Zou | first12=Yongliao | journal=Science Advances | volume=8 | issue=19 | pages=eabn8555 | pmid=35544566 | pmc=9094648 | bibcode=2022SciA....8N8555L }}</ref><ref>Liu, Y., et al.  2022.  Zhurong reveals recent aqueous activities in Utopia Planitia, Mars.  Science Advances.  VOL. 8, NO. 19</ref>

Looking at the dunes at Zhurong's landing site, researchers found a large shift in wind direction (as evidenced in the dune directions) that occurred about the same time that layers in the Martian northern ice caps changed.  It was suggested that these events happened when the rotational tilt of the planet changed.<ref>Liu, J., et al.  2023. Martian dunes indicative of wind regime shift in line with end of ice age.  Nature</ref>

===InSight===
NASA's [[InSight]] lander discovered [[groundwater on Mars]] in 2024 by measuring the [[seismic wave]]s from [[Marsquake]]s with its [[seismometer]]. At the area it was measuring, it is estimated that there is water 7 to 13 miles beneath the [[ Martian surface|surface of Mars]]. It is estimated that there is enough groundwater on Mars that could theoretically cover all of Mars surface in water between 0.62 and 1.24 miles deep, if it was all surface water.<ref>https://www.space.com/the-universe/mars/oceans-worth-of-water-may-be-buried-within-mars-but-can-we-get-to-it</ref><ref>https://www.smithsonianmag.com/smart-news/mars-hosts-a-giant-reservoir-of-water-underground-we-just-cant-easily-reach-it-study-finds-180984888/#:~:text=Data%20from%20NASA's%20InSight%20lander,7%20and%2013%20miles%20deep.</ref>

== Interactive map ==
{{Mars map}}

== See also ==
{{div col|colwidth=30em}}
* {{annotated link|Atmosphere of Mars#Water|Atmospheric water on Mars}}
* {{annotated link|Climate of Mars}}
* {{annotated link|Colonization of Mars}}
* {{annotated link|Evolution of water on Mars and Earth}}
* {{annotated link|Extraterrestrial liquid water}}
* {{annotated link|Lakes on Mars}}
* {{annotated link|Life on Mars}}
* [[Mars Express#Scientific discoveries and important events|Mars Express §&nbsp;Scientific discoveries and important events]]
* [[Mars Global Surveyor#Discovery of water ice on Mars|Mars Global Surveyor §&nbsp;Discovery of water ice on Mars]]{{Broken anchor|date=2024-07-17|bot=User:Cewbot/log/20201008/configuration|target_link=Mars Global Surveyor#Discovery of water ice on Mars|reason= The anchor (Discovery of water ice on Mars) [[Special:Diff/997485043|has been deleted]].}}
* {{annotated link|Martian canals}}
* {{annotated link|Mud cracks on Mars}}
{{div col end}}

== References ==
{{reflist|colwidth=30em}}

== Bibliography ==
* Boyce, Joseph, M. (2008). ''The Smithsonian Book of Mars;'' Konecky & Konecky: Old Saybrook, CT, {{ISBN|978-1-58834-074-0}}
* Carr, Michael, H. (1996). ''Water on Mars;'' Oxford University Press: New York, {{ISBN|0-19-509938-9}}.
* Carr, Michael, H. (2006). ''The Surface of Mars;'' Cambridge University Press: Cambridge, UK, {{ISBN|978-0-521-87201-0}}.
* Hartmann, William, K. (2003). ''A Traveler's Guide to Mars: The Mysterious Landscapes of the Red Planet;'' Workman: New York, {{ISBN|0-7611-2606-6}}.
* Hanlon, Michael (2004). ''The Real Mars: Spirit, Opportunity, Mars Express and the Quest to Explore the Red Planet;'' Constable: London, {{ISBN|1-84119-637-1}}.
* Kargel, Jeffrey, S. (2004). ''Mars: A Warmer Wetter Planet;'' Springer-Praxis: London, {{ISBN|1-85233-568-8}}.
* Morton, Oliver (2003). ''Mapping Mars: Science, Imagination, and the Birth of a World;'' Picador: New York, {{ISBN|0-312-42261-X}}.
* Sheehan, William (1996). ''The Planet Mars: A History of Observation and Discovery;'' University of Arizona Press: Tucson, AZ, {{ISBN|0-8165-1640-5}}.
* Viking Orbiter Imaging Team (1980). ''Viking Orbiter Views of Mars,'' C.R. Spitzer, Ed.; NASA SP-441: Washington DC.

== External links ==
{{Commons|Water on Mars}}

*Head, J., et al.  2023.  GEOLOGICAL AND CLIMATE HISTORY OF MARS: IDENTIFICATION OF POTENTIAL WARM AND WET CLIMATE 'FALSE POSITIVES'.  54th Lunar and Planetary Science Conference 2023 (LPI Contrib. No. 2806).  1731.pdf
* [https://www.youtube.com/watch?v=D-SCOHj8u-A  Water on Mars - James Secosky - 2021 Mars Society Virtual Convention -- Tells where water was and where ice is today on Mars (34 minutes)]
* [https://science.nasa.gov/science-news/science-at-nasa/2012/27sep_streambed/ NASA – ''Curiosity'' Rover Finds Evidence For An Ancient Streambed – September, 2012] {{Webarchive|url=https://web.archive.org/web/20211009115723/https://science.nasa.gov/science-news/science-at-nasa/2012/27sep_streambed/ |date=October 9, 2021 }}
* [http://marsoweb.nas.nasa.gov/HiRISE/hirise_images/ Images – Signs Of Water On Mars] ([[HiRISE]])
* [https://www.youtube.com/watch?v=HQKnDdB36zY Video (02:01) – Liquid Flowing Water Discovered on Mars – August, 2011]
* [https://www.youtube.com/watch?v=Jr1Xu2i-Uc0 Video (04:32) – Evidence: Water "Vigorously" Flowed On Mars – September, 2012]
* [https://www.youtube.com/watch?v=WH8kHncLZwM Video (03:56) – Measuring Mars' Ancient Ocean – March, 2015]
* [https://www.youtube.com/watch?v=m2ERsEXAq_s  - Jeffrey Plaut - Subsurface Ice - 21st Annual International Mars Society Convention-2018]
*[https://www.youtube.com/watch?v=1plIgTG9x-A  Chris McKay: Results of the Phoenix Mission to Mars and Analog Sites on Earth]
* {{Cite news|url=https://www.nasa.gov/press-release/goddard/2018/mars-terraforming|title=Mars Terraforming Not Possible Using Present-Day Technology|last=Steigerwald|first=Bill|date=2018-07-25|work=NASA|access-date=2018-11-26|language=en}}

{{Mars}}
{{Geography of Mars}}
{{Water}}
{{Portal bar|Astronomy|Biology|Solar System|Water}}

[[Category:Water on Mars| ]]
[[Category:Extraterrestrial water|Mars]]