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{{Short description|Planet hypothesized to have impacted Earth and created the Moon}}
{{other uses|Theia (disambiguation)}}
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{{Short description|planet that is hypothesized to have impacted Earth and created the Moon}}
[[File:Artist's concept of collision at HD 172555.jpg|thumb|upright=1.3|An artist's depiction of the hypothetical impact of a planet like Theia and the Earth]]
'''Theia''' is a [[List of hypothetical Solar System objects|hypothesized]] ancient [[planet]] in the [[Formation and evolution of the Solar System|early Solar System]] that, according to the [[giant-impact hypothesis]], collided with the [[early Earth]] around 4.5 billion years ago, with some of the resulting ejected debris gathering to form the [[Moon]].<ref name="agerefinement">{{cite web|last1=Wolpert|first1=Stuart|title=UCLA study shows the moon is older than previously thought|url=http://scitechdaily.com/ucla-study-shows-the-moon-is-older-than-previously-thought/|website=scitechdaily.com|access-date=14 January 2017|date=2017-01-12}}</ref><ref name="TheiaPothesis"/>


{{Merge to|Giant-impact hypothesis|date=October 2024}}
In addition to explaining Earth's large satellite, the Theia hypothesis can also explain why Earth's core is larger than would be expected for a body its size; Theia's core and mantle mixed with Earth's core and mantle.<ref>[http://www.seti.org/node/1458 "A New Model for the Origin of the Moon"]. ''SETI Institute''.</ref>
{{Infobox planet

| name = Theia
According to one version of the hypothesis, Theia was an [[Earth trojan]] about the size of [[Mars]], with a diameter of about {{convert|6102|km|abbr=in}}. Additional evidence published in 2019 suggests that Theia might have formed in the [[outer Solar System]] rather than the [[inner Solar System]], and that much of Earth's water originated on Theia.<ref>{{Cite journal|last1=Budde|first1=Gerrit|last2=Burkhardt|first2=Christoph|last3=Kleine|first3=Thorsten|date=2019-05-20|title=Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth|journal=Nature Astronomy|volume=3|issue=8|pages=736–741|language=en|doi=10.1038/s41550-019-0779-y|issn=2397-3366|bibcode=2019NatAs...3..736B|s2cid=181460133}}</ref>
| image = Artist's concept of collision at HD 172555.jpg
| image_alt =
| caption = An artist's depiction of the hypothetical planet impacting the [[Early Earth]].
| background = #d1b56f
| discovered =
| pronounced = {{IPAc-en|ˈ|θ|iː|ə}}
| symbol = [[File:Theia symbol (fixed width).svg]]
| star = [[Sun]]
| surface_area = {{val|1.4437e8|u=km2}}<br />(0.284 Earths<!--144,371,391 km2 in [http://www.peeep.us/7437231b]-->)
| mass = 0.1 - 0.45 [[Earth mass|M<sub>🜨</sub>]]
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}}
'''Theia''' ({{IPAc-en|ˈ|θ|iː|ə}}) is a [[List of hypothetical Solar System objects|hypothesized]] ancient [[planet]] in the [[Formation and evolution of the Solar System|early Solar System]] which, according to the [[giant-impact hypothesis]], collided with the [[early Earth]] around 4.5 billion years ago, with some of the resulting ejected debris coalescing to form the [[Moon]].<ref name="agerefinement" /><ref name="TheiaPothesis" /> Collision simulations support the idea that the [[large low-shear-velocity provinces]] in the lower mantle may be remnants of Theia.<ref>{{Cite news |last=Sample |first=Ian |date=November 1, 2023 |title=Blobs near Earth's core are remnants of collision with another planet, study says |url=https://www.theguardian.com/science/2023/nov/01/blobs-near-earths-core-are-remnants-of-collision-with-another-planet-study-says |work=[[The Guardian]]}}</ref><ref name="NYT-20231101" /> Theia is hypothesized to have been about the size of [[Mars]], and may have formed in the [[outer Solar System]] and [[Origin of water on Earth|provided much of Earth's water]], though this is debated.<ref>{{Cite journal |last1=Meier |first1=M.M.M. |last2=Reufer |first2=A. |last3=Wieler |first3=R. |date=November 2014 |title=On the origin and composition of Theia: Constraints from new models of the Giant Impact |journal=[[Icarus (journal)|Icarus]] |volume=242 |pages=316–328 |arxiv=1410.3819 |bibcode=2014Icar..242..316M |doi=10.1016/j.icarus.2014.08.003 |issn=0019-1035}}</ref>


==Name==
==Name==
Theia was named for the [[Titan (mythology)|titaness]] [[Theia]], who in [[Greek mythology]] was the mother of [[Selene]], the goddess of the Moon,<ref>{{cite book|last1=Murdin|first1=Paul|title=Rock Legends: The Asteroids and Their Discoverers|date=2016|publisher=Springer|isbn=9783319318363|doi=10.1007/978-3-319-31836-3|page=178|url=https://books.google.com/books?id=QDusDAAAQBAJ&pg=PA178}}</ref> which parallels the planet Theia's collision with the early Earth that is theorized to have created the Moon.<ref>{{Cite web | url=https://www.etymonline.com/word/Selene | title=Selene &#124; Origin and meaning of selene by Online Etymology Dictionary}}</ref> In modern [[Greek language|Greek]], it has the same origin with the words "θείος" (theios) and "θεία" (theia) ('uncle' and 'aunt', also meaning 'divine' in ancient greek).
In [[Greek mythology]], [[Theia]] was one of the [[Titans]], the sister of [[Hyperion (Titan)|Hyperion]] whom she later married, and the mother of [[Selene]], the goddess of the Moon:<ref>{{Cite book |last=Murdin |first=Paul |url=https://books.google.com/books?id=QDusDAAAQBAJ&pg=PA178 |title=Rock Legends: The Asteroids and Their Discoverers |date=2016 |publisher=[[Springer International]] |isbn=978-3-319-31836-3 |series=Popular Astronomy |location=Cham |page=178 |bibcode=2016rlat.book.....M |doi=10.1007/978-3-319-31836-3}}</ref> this story parallels the planet Theia's theorized role in creating the Moon.<ref>{{Cite encyclopedia |title=Selene |encyclopedia=Online Etymology Dictionary |url=https://www.etymonline.com/word/Selene}}</ref>


==Orbit==
==Orbit==
Theia is hypothesized to have orbited in the [[Lagrange point#L4 and L5|L<sub>4</sub> or L<sub>5</sub>]] configuration presented by the Earth–Sun system, where it would tend to remain. In that case, it would have grown, potentially to a size comparable to Mars. [[Perturbation (astronomy)|Gravitational perturbations]] by [[Venus]] could have eventually put it onto a collision course with the early Earth.<ref name="Stereo"/>
Theia is hypothesized to have orbited in the [[Lagrange point#L4 and L5|L<sub>4</sub> or L<sub>5</sub>]] configuration presented by the Earth–Sun system, where it would tend to remain. If this were the case it might have grown to a size comparable to [[Mars]], with a diameter of about {{convert|6,102|km|mi|abbr=off}}.{{citation needed|date=December 2022}} [[Perturbation (astronomy)|Gravitational perturbations]] by [[Venus]] could have put it onto a collision course with the [[early Earth]].<ref name="Stereo" />

== Size ==
Theia is often suggested to be around the size of Mars, with a mass about 10% that of current Earth; however, its size is not definitively settled, with some authors suggesting that Theia may have been considerably larger, perhaps 30% or even 40-45% the mass of current Earth making it nearly equal to the mass of proto-Earth.<ref>{{Cite journal |last1=Desch |first1=Steven J. |last2=Robinson |first2=Katharine L. |date=December 2019 |title=A unified model for hydrogen in the Earth and Moon: No one expects the Theia contribution |journal=Geochemistry |volume=79 |issue=4 |pages=125546 |bibcode=2019ChEG...79l5546D |doi=10.1016/j.chemer.2019.125546|doi-access=free }}</ref>


==Collision==
==Collision==
[[File:Big Splash Theia.gif|upright=1.2|Animation of collision event between Earth (blue) and Theia (black), forming the Moon (red and gray). Celestial bodies are not to scale.|thumb]]
[[File:Big Splash Theia.gif|upright=1.2|Animation of collision between Earth (blue) and Theia (black), forming the Moon (red and gray). Bodies are not to scale.|thumb]]


According to the giant-impact hypothesis, Theia orbited the [[Sun]], nearly along the orbit of the [[proto-Earth]], by staying close to one or the other of the Sun–Earth system's two more stable [[Lagrangian point]]s ('' i.e.'', either L<sub>4</sub> or L<sub>5</sub>).<ref name="Stereo"/>
According to the [[Giant-impact hypothesis|giant impact hypothesis]], Theia orbited the [[Sun]], nearly along the orbit of the [[proto-Earth]], by staying close to one or the other of the Sun-Earth system's two more stable [[Lagrangian point]]s (''i.e.'', either L<sub>4</sub> or L<sub>5</sub>).<ref name="Stereo" /> Theia was eventually [[Perturbation (astronomy)|perturbed]] away from that relationship, most likely by the gravitational influence of [[Jupiter]], [[Venus]], or both, resulting in a collision between Theia and Earth.<ref>{{Cite journal |last1=Johansen |first1=Anders |last2=Ronnet |first2=Thomas |last3=Bizzarro |first3=Martin |last4=Schiller |first4=Martin |last5=Lambrechts |first5=Michiel |last6=Nordlund |first6=Åke |last7=Lammer |first7=Helmut |date=2021-02-19 |title=A pebble accretion model for the formation of the terrestrial planets in the Solar System |journal=Science Advances |language=en |volume=7 |issue=8 |doi=10.1126/sciadv.abc0444 |issn=2375-2548 |pmc=7888959 |pmid=33597233|arxiv=2102.08611 |bibcode=2021SciA....7..444J }}</ref>
Theia was eventually [[Perturbation (astronomy)|perturbed]] away from that relationship by the gravitational influence of [[Jupiter]] and/or [[Venus]], resulting in a collision between Theia and Earth.{{citation needed|date=September 2015}}


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Initially, the hypothesis supposed that Theia had struck Earth with a glancing blow<ref name="ReuferMeier2012" /> and ejected many pieces of both the [[proto-Earth]] and Theia, those pieces either forming one body that became the Moon or forming two moons that eventually merged to form the Moon.<ref name="JutziAsphaug2011" /><ref name="Astronomy">{{Cite magazine |last=Emspak |first=Jesse |date=August 2014 |title=FACEOFF! The Moon's oddly different sides |url=https://jesseemspak.com/favorite-stories/faceoff-the-moons-oddly-different-sides-astronomy-august-2014/ |magazine=[[Astronomy (magazine)|Astronomy]] |pages=44–49 |issn=0091-6358}}</ref> Such accounts assumed that a head-on impact would have destroyed both planets, creating a short-lived second [[asteroid belt]] between the orbits of Venus and Mars.
Computer simulations suggest that Theia was traveling no faster than {{cvt|4|km/s|mph}} when it struck Earth at an estimated 45-degree angle.


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In contrast, evidence published in January 2016 suggests that the impact was indeed a head-on collision and that Theia's remains are on Earth and the Moon.<ref name="Nace2016" /><ref>{{Cite journal |last1=Young |first1=Edward D. |last2=Kohl |first2=Issaku E. |last3=Warren |first3=Paul H. |last4=Rubie |first4=David C. |last5=Jacobson |first5=Seth A. |last6=Morbidelli |first6=Alessandro |display-authors=2 |date=29 January 2016 |title=Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact |journal=[[Science (journal)|Science]] |volume=351 |issue=6272 |pages=493–496 |arxiv=1603.04536 |bibcode=2016Sci...351..493Y |doi=10.1126/science.aad0525 |issn=0036-8075 |pmid=26823426 |s2cid=6548599}}</ref><ref>{{Cite web |last=Wolpert |first=Stuart |date=January 28, 2016 |title=Moon was produced by a head-on collision between Earth and a forming planet |url=http://newsroom.ucla.edu/releases/moon-was-produced-by-a-head-on-collision-between-earth-and-a-forming-planet |website=UCLA newsroom |publisher=[[UCLA]]}}</ref>
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Originally, the hypothesis supposed that Theia had struck Earth with a glancing blow<ref name="ReuferMeier2012"/> and ejected many pieces of both the proto-Earth and Theia, those pieces either forming one body that became the Moon or forming two moons that eventually merged to form the Moon.<ref name="JutziAsphaug2011"/><ref name=Astronomy>"Faceoff! The Moon's oddly different sides", ''[[Astronomy (magazine)|Astronomy]]'', August 2014, 44–49.</ref> Such accounts assumed that if Theia had struck the proto-Earth head-on both planets would have been destroyed, creating a short-lived second [[asteroid belt]] between the orbits of Venus and Mars.


Simulations suggest that Theia would be responsible for around 70-90% of the total mass of the Moon under a classic giant impact scenario where Theia is considerably smaller than proto-Earth.<ref name="neture 04-2020">{{Cite journal |last1=Cano |first1=Erick J. |last2=Sharp |first2=Zachary D. |last3=Shearer |first3=Charles K. |date=April 2020 |title=Distinct oxygen isotope compositions of the Earth and Moon |journal=[[Nature Geoscience]] |volume=13 |issue=4 |pages=270–274 |bibcode=2020NatGe..13..270C |doi=10.1038/s41561-020-0550-0 |issn=1752-0894}}</ref>
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In contrast, evidence published in January 2016 suggests that the impact was indeed a head-on collision and that Theia's remains can be found in both the Earth and the Moon.<ref name="Nace2016"/><ref>{{cite journal |last1=Young |first1=E. D. |last2=Kohl |first2=I. E. |last3=Warren |first3=P. H. |last4=Rubie |first4=D. C. |last5=Jacobson |first5=S. A. |last6=Morbidelli |first6=A. |title=Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact |journal=Science |date=28 January 2016 |volume=351 |issue=6272 |pages=493–496 |doi=10.1126/science.aad0525|pmid=26823426 |arxiv=1603.04536 |bibcode=2016Sci...351..493Y |s2cid=6548599 }}</ref><ref>{{cite web|last1=Wolpert|first1=Stuart|title=Moon was produced by a head-on collision between Earth and a forming planet|date=January 28, 2016|url=http://newsroom.ucla.edu/releases/moon-was-produced-by-a-head-on-collision-between-earth-and-a-forming-planet|website=UCLA newsroom|publisher=UCLA}}</ref>


==Hypotheses==
==Hypotheses==
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From the beginning of modern astronomy, there have been at least four hypotheses for the origin of the Moon:
From the beginning of modern astronomy, there have been at least four hypotheses for the origin of the Moon:
# A single body split into Earth and Moon
# A single body split into Earth and Moon
# The Moon was [[sphere of influence (astrodynamics)|captured]] by Earth's gravity (as most of the outer planets' smaller moons were captured)
# The Moon was captured by Earth's gravity (as most of the [[outer planet]]s' smaller moons were captured)
# The Earth and Moon formed at the same time when the [[protoplanetary disk]] [[accretion (astrophysics)|accreted]]
# The Earth and Moon formed at the same time when the [[protoplanetary disk]] [[accretion (astrophysics)|accreted]]
# The Theia-impact scenario described above.
# The Theia-impact scenario described above


The lunar rock samples retrieved by [[Apollo program|Apollo]] [[List of Apollo astronauts|astronauts]] were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event.<ref name="Nace2016"/><ref name="HerwartzPack2014"/><ref name="MeierReufer2014"/>
The lunar rock samples retrieved by [[Apollo program|Apollo]] [[List of Apollo astronauts|astronauts]] were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event.<ref name="Nace2016" /><ref name="HerwartzPack2014" /><ref name="MeierReufer2014" />

It is possible that the [[large low-shear-velocity provinces]] detected deep in Earth's mantle may be fragments of Theia.<ref name="2021LPI....52.1980Y" /><ref name="bbcfuture-20220510-llsvp" /> In 2023, computer simulations reinforced that hypothesis.<ref name="NAT-20231101" /><ref name="NYT-20231101" />

== Composition ==
The composition of Theia and how different it was from Earth is disputed and subject to debate.<ref>{{Cite encyclopedia |last=Pinti |first=Daniele L. |title=Theia |date=2023 |publisher=[[Springer Berlin Heidelberg]] |isbn=978-3-662-65092-9 |editor-last=Gargaud |editor-first=Muriel |location=Berlin, Heidelberg |pages=3021–3022 |doi=10.1007/978-3-662-65093-6_1578 |editor-last2=Irvine |editor-first2=William M. |editor-last3=Amils |editor-first3=Ricardo |editor-last4=Claeys |editor-first4=Philippe |editor-last5=Cleaves |editor-first5=Henderson James |editor-last6=Gerin |editor-first6=Maryvonne |editor-last7=Rouan |editor-first7=Daniel |editor-last8=Spohn |editor-first8=Tilman |editor-last9=Tirard |editor-first9=Stéphane |encyclopedia=Encyclopedia of Astrobiology}}</ref> It is considered unlikely that Theia had an exactly similar isotopic composition to proto-Earth. A key constraint has been that the many isotope ratios of retrieved rocks from the Moon are nearly identical to those from Earth, either implying that the two bodies were extensively homogenized by the collision, or that the isotopic composition of Theia was very similar to Earth. However, a 2020 study showed that lunar rocks were more variable in oxygen isotope composition than previously thought, some differing more from Earth than others, with the more divergent values probably originating deeper in the lunar mantle suggested to be a more true reflection of Theia, and may suggest that Theia formed further away from the Sun than Earth.<ref name="neture 04-2020" />


==See also==
==See also==
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==References==
==References==
{{Spoken Wikipedia|date=2024-12-28|Theia article Narration.mp3}}{{Reflist
{{Reflist
| colwidth = 30em
| colwidth = 30em
| refs =
| refs =
<ref name="TheiaPothesis">
{{cite web
|title = The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same
|publisher = The Daily Galaxy
|url = http://www.dailygalaxy.com/my_weblog/2007/07/the-theia-hypot.html
|date = 2007-07-05
|access-date = 2013-11-13
⚫
}}
</ref>


<ref name="agerefinement">{{Cite web |last=Wolpert |first=Stuart |date=January 12, 2017 |title=UCLA Study Shows the Moon is Older Than Previously Thought |url=https://scitechdaily.com/ucla-study-shows-the-moon-is-older-than-previously-thought/ |access-date=23 March 2022 |website=SciTechDaily |publisher=[[UCLA]]}}</ref>
<ref name="Stereo">

{{cite web
<ref name="TheiaPothesis">{{Cite web |last=K. |first=Yidir |date=5 July 2007 |title=The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same |url=http://www.dailygalaxy.com/my_weblog/2007/07/the-theia-hypot.html |url-status=live |archive-url=http://web.archive.org/web/20170619064755/http://www.dailygalaxy.com/my_weblog/2007/07/the-theia-hypot.html |archive-date=2017-06-19 |access-date=2013-11-13 |website=The Daily Galaxy}}</ref>
|title=STEREO Hunts for Remains of an Ancient Planet near Earth

|publisher=[[NASA]]
<ref name="Stereo">{{Cite web |date=2009-04-09 |title=STEREO Hunts for Remains of an Ancient Planet near Earth |url=http://science1.nasa.gov/science-news/science-at-nasa/2009/09apr_theia/ |url-status=dead |archive-url=https://web.archive.org/web/20131113174213/http://science1.nasa.gov/science-news/science-at-nasa/2009/09apr_theia/ |archive-date=2013-11-13 |access-date=2013-11-13 |publisher=[[NASA]]}}</ref>
|url=http://science1.nasa.gov/science-news/science-at-nasa/2009/09apr_theia/

|date=2009-04-09
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<ref name="ReuferMeier2012">{{Cite journal |last1=Reufer |first1=Andreas |last2=Meier |first2=Matthias M.M. |last3=Benz |first3=Willy |last4=Wieler |first4=Rainer |date=September 2012 |title=A hit-and-run giant impact scenario |journal=[[Icarus (journal)|Icarus]] |volume=221 |issue=1 |pages=296–299 |arxiv=1207.5224 |bibcode=2012Icar..221..296R |doi=10.1016/j.icarus.2012.07.021 |s2cid=118421530}}</ref>
|access-date=2013-11-13

|url-status=dead
<ref name="JutziAsphaug2011">{{Cite journal |last1=Jutzi |first1=M. |last2=Asphaug |first2=E. |date=August 2011 |title=Forming the lunar farside highlands by accretion of a companion moon |journal=[[Nature (journal)|Nature]] |volume=476 |issue=7358 |pages=69–72 |bibcode=2011Natur.476...69J |doi=10.1038/nature10289 |issn=0028-0836 |pmid=21814278 |s2cid=84558}}</ref>
|archive-url=https://web.archive.org/web/20131113174213/http://science1.nasa.gov/science-news/science-at-nasa/2009/09apr_theia/

|archive-date=2013-11-13
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<ref name="HerwartzPack2014">{{Cite journal |last1=Herwartz |first1=Daniel |last2=Pack |first2=Andreas |last3=Friedrichs |first3=Bjarne |last4=Bischoff |first4=Addi |date=6 June 2014 |title=Identification of the giant impactor Theia in lunar rocks |journal=[[Science (journal)|Science]] |volume=344 |issue=6188 |pages=1146–1150 |bibcode=2014Sci...344.1146H |doi=10.1126/science.1251117 |issn=0036-8075 |pmid=24904162 |s2cid=30903580}}</ref>
}}

</ref>
⚫
<ref name="MeierReufer2014">{{Cite journal |last1=Meier |first1=M.M.M. |last2=Reufer |first2=A. |last3=Wieler |first3=R. |date=November 2014 |title=On the origin and composition of Theia: Constraints from new models of the Giant Impact |journal=[[Icarus (journal)|Icarus]] |volume=242 |pages=316–328 |arxiv=1410.3819 |bibcode=2014Icar..242..316M |doi=10.1016/j.icarus.2014.08.003 |s2cid=119226112}}</ref>


⚫
<ref name="Nace2016">{{Cite web |last=Nace |first=Trevor |date=2016-01-30 |title=New Evidence For 4.5 Billion Year Old Impact Formed Our Moon |url=https://www.forbes.com/sites/trevornace/2016/01/30/new-evidence-4-5-billion-year-old-impact-formed-moon/ |access-date=2016-01-30 |website=[[Forbes]]}}</ref>
⚫
<ref name="ReuferMeier2012">{{cite journal |last1=Reufer |first1=Andreas |last2=Meier |first2=Matthias M. M. |last3=Benz |first3=Willy |last4=Wieler |first4=Rainer |title=A hit-and-run giant impact scenario |journal=Icarus |volume=221 |issue=1 |year=2012 |pages=296–299 |doi=10.1016/j.icarus.2012.07.021 |bibcode=2012Icar..221..296R |arxiv = 1207.5224 |s2cid=118421530 }}</ref>


<ref name="JutziAsphaug2011">{{cite journal |last1=Jutzi |first1=M. |last2=Asphaug |first2=E. |title=Forming the lunar farside highlands by accretion of a companion moon |journal=Nature |volume=476 |issue=7358 |year=2011 |pages=69–72 |doi=10.1038/nature10289 |pmid=21814278 |bibcode = 2011Natur.476...69J |s2cid=84558 }}</ref>
<ref name="2021LPI....52.1980Y">{{Cite conference |last1=Yuan |first1=Qian |last2=Li |first2=Mingming |last3=Desch |first3=Steven J. |last4=Ko |first4=Byeongkwan |date=2021 |title=Giant impact origin for the large low shear velocity provinces |url=https://www.hou.usra.edu/meetings/lpsc2021/pdf/1980.pdf |conference=52nd Lunar and Planetary Science Conference |bibcode=2021LPI....52.1980Y |access-date=27 March 2021}}</ref>


<ref name="bbcfuture-20220510-llsvp">{{Cite web |last=Gorvett |first=Zaria |date=12 May 2022 |title=Why are there continent-sized 'blobs' in the deep Earth? |url=https://www.bbc.com/future/article/20220510-why-are-there-continent-sized-blobs-in-the-deep-earth |website=[[BBC Future]]}}</ref>
⚫
<ref name="HerwartzPack2014">{{cite journal |last1=Herwartz |first1=D. |last2=Pack |first2=A. |last3=Friedrichs |first3=B. |last4=Bischoff |first4=A. |title=Identification of the giant impactor Theia in lunar rocks |journal=Science |volume=344 |issue=6188 |year=2014 |pages=1146–1150 |doi=10.1126/science.1251117 |bibcode = 2014Sci...344.1146H |pmid=24904162|s2cid=30903580 }}</ref>


<ref name="NYT-20231101">{{Cite news |last=Chang |first=Kenneth |date=1 November 2023 |title=A 'Big Whack' Formed the Moon and Left Traces Deep in Earth, a Study Suggests - Two enormous blobs deep inside Earth could be remnants of the birth of the moon. |url=https://www.nytimes.com/2023/11/01/science/moon-formation-theia.html |url-status=live |archive-url=https://archive.today/20231101232849/https://www.nytimes.com/2023/11/01/science/moon-formation-theia.html |archive-date=1 November 2023 |access-date=2 November 2023 |work=[[The New York Times]]}}</ref>
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<ref name="MeierReufer2014">{{cite journal |last1=Meier |first1=M. M. M. |last2=Reufer |first2=A. |last3=Wieler |first3=R. |title=On the origin and composition of Theia: Constraints from new models of the Giant Impact |journal=Icarus |volume=242 |year=2014 |pages=316–328 |doi=10.1016/j.icarus.2014.08.003 |bibcode=2014Icar..242..316M |arxiv = 1410.3819 |s2cid=119226112 }}</ref>


<ref name="NAT-20231101">{{Cite journal |last1=Yuan |first1=Qian |last2=Li |first2=Mingming |last3=Desch |first3=Steven J. |last4=Ko |first4=Byeongkwan |last5=Deng |first5=Hongping |last6=Garnero |first6=Edward J. |last7=Gabriel |first7=Travis S. J. |last8=Kegerreis |first8=Jacob A. |last9=Miyazaki |first9=Yoshinori |last10=Eke |first10=Vincent |last11=Asimow |first11=Paul D. |display-authors=2 |date=2 November 2023 |title=Moon-forming impactor as a source of Earth's basal mantle anomalies |url=https://www.nature.com/articles/s41586-023-06589-1 |url-status=live |journal=[[Nature (journal)|Nature]] |volume=623 |issue=7985 |pages=95–99 |bibcode=2023Natur.623...95Y |doi=10.1038/s41586-023-06589-1 |issn=0028-0836 |pmid=37914947 |s2cid=264869152 |archive-url=https://archive.today/20231102061800/https://www.nature.com/articles/s41586-023-06589-1 |archive-date=2 November 2023 |access-date=2 November 2023}}</ref>
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<ref name="Nace2016">{{cite web |last1=Nace |first1=Trevor |title = New Evidence For 4.5 Billion Year Old Impact Formed Our Moon |work = [[Forbes]] |url = https://www.forbes.com/sites/trevornace/2016/01/30/new-evidence-4-5-billion-year-old-impact-formed-moon/ |date = 2016-01-30 |access-date = 2016-01-30 }}</ref>


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Latest revision as of 11:25, 28 December 2024

Theia
An artist's depiction of the hypothetical planet impacting the Early Earth.
Designations
Pronunciation/ˈθiːə/
Symbol
Orbital characteristics
StarSun
Physical characteristics
1.4437×108 km2
(0.284 Earths)
Mass0.1 - 0.45 M🜨

Theia (/ˈθiːə/) is a hypothesized ancient planet in the early Solar System which, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, with some of the resulting ejected debris coalescing to form the Moon.[1][2] Collision simulations support the idea that the large low-shear-velocity provinces in the lower mantle may be remnants of Theia.[3][4] Theia is hypothesized to have been about the size of Mars, and may have formed in the outer Solar System and provided much of Earth's water, though this is debated.[5]

Name

[edit]

In Greek mythology, Theia was one of the Titans, the sister of Hyperion whom she later married, and the mother of Selene, the goddess of the Moon:[6] this story parallels the planet Theia's theorized role in creating the Moon.[7]

Orbit

[edit]

Theia is hypothesized to have orbited in the L4 or L5 configuration presented by the Earth–Sun system, where it would tend to remain. If this were the case it might have grown to a size comparable to Mars, with a diameter of about 6,102 kilometres (3,792 miles).[citation needed] Gravitational perturbations by Venus could have put it onto a collision course with the early Earth.[8]

Size

[edit]

Theia is often suggested to be around the size of Mars, with a mass about 10% that of current Earth; however, its size is not definitively settled, with some authors suggesting that Theia may have been considerably larger, perhaps 30% or even 40-45% the mass of current Earth making it nearly equal to the mass of proto-Earth.[9]

Collision

[edit]
Animation of collision between Earth (blue) and Theia (black), forming the Moon (red and gray). Bodies are not to scale.

According to the giant impact hypothesis, Theia orbited the Sun, nearly along the orbit of the proto-Earth, by staying close to one or the other of the Sun-Earth system's two more stable Lagrangian points (i.e., either L4 or L5).[8] Theia was eventually perturbed away from that relationship, most likely by the gravitational influence of Jupiter, Venus, or both, resulting in a collision between Theia and Earth.[10]

Initially, the hypothesis supposed that Theia had struck Earth with a glancing blow[11] and ejected many pieces of both the proto-Earth and Theia, those pieces either forming one body that became the Moon or forming two moons that eventually merged to form the Moon.[12][13] Such accounts assumed that a head-on impact would have destroyed both planets, creating a short-lived second asteroid belt between the orbits of Venus and Mars.

In contrast, evidence published in January 2016 suggests that the impact was indeed a head-on collision and that Theia's remains are on Earth and the Moon.[14][15][16]

Simulations suggest that Theia would be responsible for around 70-90% of the total mass of the Moon under a classic giant impact scenario where Theia is considerably smaller than proto-Earth.[17]

Hypotheses

[edit]
Main article: Origin of the Moon

From the beginning of modern astronomy, there have been at least four hypotheses for the origin of the Moon:

  1. A single body split into Earth and Moon
  2. The Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured)
  3. The Earth and Moon formed at the same time when the protoplanetary disk accreted
  4. The Theia-impact scenario described above

The lunar rock samples retrieved by Apollo astronauts were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event.[14][18][19]

It is possible that the large low-shear-velocity provinces detected deep in Earth's mantle may be fragments of Theia.[20][21] In 2023, computer simulations reinforced that hypothesis.[22][4]

Composition

[edit]

The composition of Theia and how different it was from Earth is disputed and subject to debate.[23] It is considered unlikely that Theia had an exactly similar isotopic composition to proto-Earth. A key constraint has been that the many isotope ratios of retrieved rocks from the Moon are nearly identical to those from Earth, either implying that the two bodies were extensively homogenized by the collision, or that the isotopic composition of Theia was very similar to Earth. However, a 2020 study showed that lunar rocks were more variable in oxygen isotope composition than previously thought, some differing more from Earth than others, with the more divergent values probably originating deeper in the lunar mantle suggested to be a more true reflection of Theia, and may suggest that Theia formed further away from the Sun than Earth.[17]

See also

[edit]

References

[edit]
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  1. ^ Wolpert, Stuart (January 12, 2017). "UCLA Study Shows the Moon is Older Than Previously Thought". SciTechDaily. UCLA. Retrieved 23 March 2022.
  2. ^ K., Yidir (5 July 2007). "The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same". The Daily Galaxy. Archived from the original on 2017-06-19. Retrieved 2013-11-13.
  3. ^ Sample, Ian (November 1, 2023). "Blobs near Earth's core are remnants of collision with another planet, study says". The Guardian.
  4. ^ a b Chang, Kenneth (1 November 2023). "A 'Big Whack' Formed the Moon and Left Traces Deep in Earth, a Study Suggests - Two enormous blobs deep inside Earth could be remnants of the birth of the moon". The New York Times. Archived from the original on 1 November 2023. Retrieved 2 November 2023.
  5. ^ Meier, M.M.M.; Reufer, A.; Wieler, R. (November 2014). "On the origin and composition of Theia: Constraints from new models of the Giant Impact". Icarus. 242: 316–328. arXiv:1410.3819. Bibcode:2014Icar..242..316M. doi:10.1016/j.icarus.2014.08.003. ISSN 0019-1035.
  6. ^ Murdin, Paul (2016). Rock Legends: The Asteroids and Their Discoverers. Popular Astronomy. Cham: Springer International. p. 178. Bibcode:2016rlat.book.....M. doi:10.1007/978-3-319-31836-3. ISBN 978-3-319-31836-3.
  7. ^ "Selene". Online Etymology Dictionary.
  8. ^ a b "STEREO Hunts for Remains of an Ancient Planet near Earth". NASA. 2009-04-09. Archived from the original on 2013-11-13. Retrieved 2013-11-13.
  9. ^ Desch, Steven J.; Robinson, Katharine L. (December 2019). "A unified model for hydrogen in the Earth and Moon: No one expects the Theia contribution". Geochemistry. 79 (4): 125546. Bibcode:2019ChEG...79l5546D. doi:10.1016/j.chemer.2019.125546.
  10. ^ Johansen, Anders; Ronnet, Thomas; Bizzarro, Martin; Schiller, Martin; Lambrechts, Michiel; Nordlund, Åke; Lammer, Helmut (2021-02-19). "A pebble accretion model for the formation of the terrestrial planets in the Solar System". Science Advances. 7 (8). arXiv:2102.08611. Bibcode:2021SciA....7..444J. doi:10.1126/sciadv.abc0444. ISSN 2375-2548. PMC 7888959. PMID 33597233.
  11. ^ Reufer, Andreas; Meier, Matthias M.M.; Benz, Willy; Wieler, Rainer (September 2012). "A hit-and-run giant impact scenario". Icarus. 221 (1): 296–299. arXiv:1207.5224. Bibcode:2012Icar..221..296R. doi:10.1016/j.icarus.2012.07.021. S2CID 118421530.
  12. ^ Jutzi, M.; Asphaug, E. (August 2011). "Forming the lunar farside highlands by accretion of a companion moon". Nature. 476 (7358): 69–72. Bibcode:2011Natur.476...69J. doi:10.1038/nature10289. ISSN 0028-0836. PMID 21814278. S2CID 84558.
  13. ^ Emspak, Jesse (August 2014). "FACEOFF! The Moon's oddly different sides". Astronomy. pp. 44–49. ISSN 0091-6358.
  14. ^ a b Nace, Trevor (2016-01-30). "New Evidence For 4.5 Billion Year Old Impact Formed Our Moon". Forbes. Retrieved 2016-01-30.
  15. ^ Young, Edward D.; Kohl, Issaku E.; et al. (29 January 2016). "Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact". Science. 351 (6272): 493–496. arXiv:1603.04536. Bibcode:2016Sci...351..493Y. doi:10.1126/science.aad0525. ISSN 0036-8075. PMID 26823426. S2CID 6548599.
  16. ^ Wolpert, Stuart (January 28, 2016). "Moon was produced by a head-on collision between Earth and a forming planet". UCLA newsroom. UCLA.
  17. ^ a b Cano, Erick J.; Sharp, Zachary D.; Shearer, Charles K. (April 2020). "Distinct oxygen isotope compositions of the Earth and Moon". Nature Geoscience. 13 (4): 270–274. Bibcode:2020NatGe..13..270C. doi:10.1038/s41561-020-0550-0. ISSN 1752-0894.
  18. ^ Herwartz, Daniel; Pack, Andreas; Friedrichs, Bjarne; Bischoff, Addi (6 June 2014). "Identification of the giant impactor Theia in lunar rocks". Science. 344 (6188): 1146–1150. Bibcode:2014Sci...344.1146H. doi:10.1126/science.1251117. ISSN 0036-8075. PMID 24904162. S2CID 30903580.
  19. ^ Meier, M.M.M.; Reufer, A.; Wieler, R. (November 2014). "On the origin and composition of Theia: Constraints from new models of the Giant Impact". Icarus. 242: 316–328. arXiv:1410.3819. Bibcode:2014Icar..242..316M. doi:10.1016/j.icarus.2014.08.003. S2CID 119226112.
  20. ^ Yuan, Qian; Li, Mingming; Desch, Steven J.; Ko, Byeongkwan (2021). Giant impact origin for the large low shear velocity provinces (PDF). 52nd Lunar and Planetary Science Conference. Bibcode:2021LPI....52.1980Y. Retrieved 27 March 2021.
  21. ^ Gorvett, Zaria (12 May 2022). "Why are there continent-sized 'blobs' in the deep Earth?". BBC Future.
  22. ^ Yuan, Qian; Li, Mingming; et al. (2 November 2023). "Moon-forming impactor as a source of Earth's basal mantle anomalies". Nature. 623 (7985): 95–99. Bibcode:2023Natur.623...95Y. doi:10.1038/s41586-023-06589-1. ISSN 0028-0836. PMID 37914947. S2CID 264869152. Archived from the original on 2 November 2023. Retrieved 2 November 2023.
  23. ^ Pinti, Daniele L. (2023). "Theia". In Gargaud, Muriel; Irvine, William M.; Amils, Ricardo; Claeys, Philippe; Cleaves, Henderson James; Gerin, Maryvonne; Rouan, Daniel; Spohn, Tilman; Tirard, Stéphane (eds.). Encyclopedia of Astrobiology. Berlin, Heidelberg: Springer Berlin Heidelberg. pp. 3021–3022. doi:10.1007/978-3-662-65093-6_1578. ISBN 978-3-662-65092-9.
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