Rhea (moon): Difference between revisions

Rhea
	Mosaic of Rhea, assembled from Cassini imagery taken on 26 November 2005
Discovery
Discovered by	G. D. Cassini
Discovery date	December 23, 1672
Designations
Designation	Saturn V
Pronunciation	/ˈriː.ə/
Named after	Ῥέᾱ Rheā
Adjectives	Rhean /ˈriː.ən/
Orbital characteristics
Semi-major axis	527040 km
Eccentricity	0.001
Orbital period (sidereal)	4.518212 d
Average orbital speed	8.48 km/s
Inclination	0.35°
Satellite of	Saturn
Physical characteristics
Dimensions	1532.4 × 1525.6 × 1524.4 km
Mean radius	763.5±0.5 km
Surface area	7325342 km2
Mass	(2.3064854±0.0000522)×1021 kg (~3.9×10−4 Earths)
Mean density	1.2372±0.0029 g/cm3
Surface gravity	0.26 m/s2
Moment of inertia factor	0.3911±0.0045 (disputed/unclear)
Escape velocity	0.635 km/s
Synodic rotation period	4.518212 d ; (synchronous)
Axial tilt	zero
Albedo	0.949±0.003 (geometric)
Apparent magnitude	10

Browse history interactively

← Previous edit

Content deleted Content added

VisualWikitext

Inline

Latest revision as of 09:27, 26 October 2024

Rhea (/ˈriː.ə/) is the second-largest moon of Saturn and the ninth-largest moon in the Solar System, with a surface area that is comparable to the area of Australia. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. Rhea has a nearly circular orbit around Saturn, but it is also tidally locked, like Saturn's other major moons; that is, it rotates with the same period it revolves (orbits), so one hemisphere always faces towards the planet.

The moon itself has a fairly low density, composed of roughly three-quarters ice and only one-quarter rock. The surface of Rhea is heavily cratered, with distinct leading and trailing hemispheres. Like the moon Dione, it has high-albedo ice cliffs that appear as bright wispy streaks visible from space. The surface temperature varies between −174 °C and −220 °C.

Rhea was discovered in 1672 by Giovanni Domenico Cassini. Since then, it has been visited by both Voyager probes and was the subject of close targeted flybys by the Cassini orbiter in 2005, 2007, 2010, 2011, and once more in 2013.

Discovery

Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672, with a 10.4-metre (34 ft) telescope made by Giuseppe Campani.^[1]^[12] Cassini named the four moons he discovered (Tethys, Dione, Rhea, and Iapetus) Sidera Lodoicea (the stars of Louis) to honor King Louis XIV.^[1] Rhea was the second moon of Saturn that Cassini discovered, and the third moon discovered around Saturn overall.^[1]

Name

Rhea is named after the Titan Rhea of Greek mythology, the "mother of the gods" and wife of Kronos, the Greek counterpart of the god Saturn. It is also designated Saturn V (being the fifth major moon going outward from the planet, after Mimas, Enceladus, Tethys, and Dione).^[13]^[14]

Astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V.^[1] Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII, and then to Saturn VIII with the discovery of Hyperion in 1848.^[14]

Rhea was not named until 1847, when John Herschel (son of William Herschel, discoverer of the planet Uranus and two other moons of Saturn, Mimas and Enceladus) suggested in Results of Astronomical Observations made at the Cape of Good Hope that the names of the Titans, sisters and brothers of Kronos (Saturn, in Roman mythology), be used.^[15]^[1]

Orbit

The orbit of Rhea has very low eccentricity (0.001), meaning it is nearly circular. It has a low inclination of less than a degree, inclined by only 0.35° from Saturn's equatorial plane.^[5]

Rhea is tidally locked and rotates synchronously; that is, it rotates at the same speed it revolves (orbits), so one hemisphere is always facing towards Saturn. This is called the near pole. Equally, one hemisphere always faces forward, relative to the direction of movement; this is called the leading hemisphere; the other side is the trailing hemisphere, which faces backwards relative to the moon's motion.^[16]^[17]

Physical characteristics

Size, mass, and internal structure

Rhea is an icy body with a density of about 1.236 g/cm³. This low density indicates that it is made of ~25% rock (density ~3.25 g/cm³) and ~75% water ice (density ~0.93 g/cm³). A layer of Ice II (a high-pressure and extra-low temperature form of ice) is believed, based on the moon's temperature profile, to start around 350 to 450 kilometres (220 to 280 mi) beneath the surface.^[18]^[19]^[20] Rhea is 1,528 kilometres (949 mi) in diameter, but is still only a third of the size of Titan, Saturn's biggest moon.^[21] Although Rhea is the ninth-largest moon, it is only the tenth-most massive moon. Indeed, Oberon, the second-largest moon of Uranus, has almost the same size, but is significantly denser than Rhea (1.63 vs 1.24) and thus more massive, although Rhea is slightly larger by volume.^[d] The surface area of the moon can be estimated at 7,330,000 square kilometres (2,830,000 sq mi), similar to Australia (7,688,287 km²).^[22]^[b]

Before the Cassini–Huygens mission, it was assumed that Rhea had a rocky core.^[23] However, measurements taken during a close flyby by the Cassini orbiter in 2005 cast this into doubt. In a paper published in 2007 it was claimed that the axial dimensionless moment of inertia coefficient was 0.4.^[e]^[24] Such a value indicated that Rhea had an almost homogeneous interior (with some compression of ice in the center) while the existence of a rocky core would imply a moment of inertia of about 0.34.^[23] In the same year another paper claimed the moment of inertia was about 0.37.^[f] Rhea being either partially or fully differentiated would be consistent with the observations of the Cassini probe.^[25] A year later yet another paper claimed that the moon may not be in hydrostatic equilibrium, meaning that the moment of inertia cannot be determined from the gravity data alone.^[26] In 2008 an author of the first paper tried to reconcile these three disparate results. He concluded that there is a systematic error in the Cassini radio Doppler data used in the analysis, but after restricting the analysis to a subset of data obtained closest to the moon, he arrived at his old result that Rhea was in hydrostatic equilibrium and had a moment of inertia of about 0.4, again implying a homogeneous interior.^[9]

The triaxial shape of Rhea is consistent with a homogeneous body in hydrostatic equilibrium rotating at Rhea's angular velocity.^[27] Modelling in 2006 suggested that Rhea could be barely capable of sustaining an internal liquid-water ocean through heating by radioactive decay; such an ocean would have to be at about 176 K, the eutectic temperature for the water–ammonia system.^[28] More recent indications are that Rhea has a homogeneous interior and hence that this ocean does not exist.^[9]

Surface features

Rhea's features resemble those of Dione, with distinct and dissmillar leading and trailing hemispheres, suggesting similar composition and histories. The temperature on Rhea is 99 K (−174 °C) in direct sunlight and between 73 K (−200 °C) and 53 K (−220 °C) in the shade.

Rhea has a rather typical heavily cratered surface,^[29] with the exceptions of a few large Dione-type chasmata or fractures (formerly known as wispy terrain) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit)^[30] and a very faint "line" of material at Rhea's equator that may have been deposited by material deorbiting from its rings.^[31] Rhea has two very large impact basins on its hemisphere facing away from Saturn, which are about 400 and 500 km across.^[30] The more northerly and less degraded of the two, called Tirawa, is roughly comparable in size to the basin Odysseus on Tethys.^[29] There is a 48 km-diameter impact crater at 112°W that is prominent because of an extended system of bright rays, which extend up to 400 km (250 mi) away from the crater, across most of one hemisphere.^[30]^[32] This crater, called Inktomi, is nicknamed "The Splat", and may be one of the youngest craters on the inner moons of Saturn. This was hypothesized in a 2007 paper published by Lunar and Planetary Science.^[30] Rhea's impact craters are more crisply defined than the flatter craters that are pervasive on Ganymede and Callisto; it is theorized that this is due to a much lower surface gravity (0.26 m/s², compared to Ganymede's 1.428 m/s² and Callisto's 1.235 m/s²) and a stiffer crust of ice. Similarly, ejecta blankets – asymmetrical blankets of ejected particles surrounding impact craters – are not present on Rhea, potentially another result of the moon's low surface gravity.^[33]

Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size. This suggests that a major resurfacing event occurred some time during its formation. The leading hemisphere is heavily cratered and uniformly bright. As on Callisto, the craters lack the high relief features seen on the Moon and Mercury. It has been theorized that these cratered plains are up to four billion years old on average.^[34] On the trailing hemisphere there is a network of bright swaths on a dark background, and fewer craters.^[35] It is believed, based on data from the Cassini probe, that these are tectonic features: depressions (graben) and troughs, with ice-covered cliff sides causing the lines' whiteness (more technically their albedo).^[36] The extensive dark areas are thought to be deposited tholins, which are a mix of complex organic compounds generated on the ice by pyrolysis and radiolysis of simple compounds containing carbon, nitrogen and hydrogen.^[37] The trailing side of Rhea's surface is irradiated by Saturn's magnetosphere, which may cause chemical-level changes on the surface, including radiolysis (see § Atmosphere). Particles from Saturn's E-ring are also flung onto the moon's leading hemisphere, coating it.^[38]

Rhea has some evidence of endogenic activity – that is, activity originating from within the moon, such as heating and cryovolcanic activity: there are fault systems and craters with uplifted bases (so-called "relaxed" craters), although the latter is apparently only present in large craters more than 100 km (62 mi) across.^[39]^[40]^[38]

Formation

The moons of Saturn are thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young giant planets formed, they were surrounded by discs of material that gradually coalesced into moons. However, a model proposed by Erik Asphaug and Andreas Reufer for the formation of Titan may also shine a new light on the origin of Rhea and Iapetus. In this model, Titan was formed in a series of giant impacts between pre-existing moons, and Rhea and Iapetus are thought to have formed from part of the debris of these collisions.^[41]

Image of the wispy hemisphere, showing ice cliffs – Powehiwehi (upper center); chasmata stretch from upper left to right center – Onokoro Catenae (lower right)
View of Rhea's leading hemisphere with crater Inktomi and its prominent ray system just below center; impact basin Tirawa is at upper left

Atmosphere

On November 27, 2010, NASA announced the discovery of an extremely tenuous atmosphere—an exosphere. It consists of oxygen and carbon dioxide in proportion of roughly 5 to 2. The surface density of the exosphere is from 10⁵ to 10⁶ molecules in a cubic centimeter, depending on local temperature. The main source of oxygen is radiolysis of water ice at the surface via irradiation from the magnetosphere of Saturn. The source of the carbon dioxide is less clear, but it may be related to oxidation of the organics present in ice or to outgassing of the moon's interior.^[38]^[42]^[43]

Possible ring system

On March 6, 2008, NASA announced that Rhea may have a weak ring system. This would mark the first discovery of rings around a moon. The rings' existence was inferred by observed changes in the flow of electrons trapped by Saturn's magnetic field as Cassini passed by Rhea.^[44]^[45]^[46] Dust and debris could extend out to Rhea's Hill sphere, but were thought to be denser nearer the moon, with three narrow rings of higher density. The case for a ring was strengthened by the subsequent finding of the presence of a set of small ultraviolet-bright spots distributed along Rhea's equator (interpreted as the impact points of deorbiting ring material).^[47] However, when Cassini made targeted observations of the putative ring plane from several angles, there was no evidence of ring material found, suggesting that another explanation for the earlier observations is needed.^[48]^[49]

Exploration

The first images of Rhea were obtained by Voyager 1 & 2 spacecraft in 1980–1981.

There were five close targeted fly-bys by the Cassini orbiter, which was one part of the dual orbiter and lander Cassini–Huygens mission. Launched in 1997, Cassini–Huygens was targeted at the Saturn system; in total it took more than 450 thousand images.^[50] Cassini passed Rhea at a distance of 500 km on November 26, 2005; at a distance of 5,750 km on August 30, 2007; at a distance of 100 km on March 2, 2010; at 69 km flyby on January 11, 2011;^[51] and a last flyby at 992 km on March 9, 2013.^[52]

Notes

^ Calculated on the basis of other parameters.
^ ^a ^b The surface area can be estimated, given the radius, with the formula $4 πr 2$
^ Surface area derived from the radius (r): $4\pi r^2$ .
^ The moons more massive than Rhea are: the Moon, the four Galilean moons, Titan, Triton, Titania, and Oberon. Oberon, Uranus's second-largest moon, has a radius that is ~0.4% smaller than Rhea's, but a density that is ~26% greater. See JPLSSD.
^ More precisely, 0.3911.^[24]
^ More precisely, 0.3721.^[25]

References

^ ^a ^b ^c ^d ^e ^f ^g Tillman, Nola Taylor (2016-06-29). "Rhea: Saturn's dirty snowball moon". space.com.
^ Consulmagno, G.; Ryche, H. (Feb 9, 1982). "Pronouncing the names of the moons of Saturn" (PDF). EOS. 63 (6): 146–147. doi:10.1029/EO063i006p00146. Retrieved Nov 30, 2022.
^ Moore et al. (1984) "The Geomorphology of Rhea", Proceedings of the fifteenth Lunar and Planetary Science, Part 2, p C-791–C-794
^ "Natural Satellites Ephemeris Service". Minor Planet Center.
^ ^a ^b ^c ^d "Saturnian Satellite Fact Sheet". National Space Science Data Center. Archived from the original on 10 Feb 2024.
^ Roatsch, T.; Jaumann, R.; Stephan, K.; Thomas, P. C. (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763–781. doi:10.1007/978-1-4020-9217-6_24. ISBN 978-1-4020-9216-9.
^ ^a ^b ^c Robert. A. Jacobson (18 October 2022). "The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole". The Astronomical Journal. 164 (5). Bibcode:2022AJ....164..199J. doi:10.3847/1538-3881/AC90C9. ISSN 0004-6256. S2CID 252992162. Wikidata Q126389785.
^ Anderson, J. D.; Schubert, G. (2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters. 34 (2): L02202. Bibcode:2007GeoRL..34.2202A. doi:10.1029/2006GL028100.
^ ^a ^b ^c Anderson, John D. (July 2008). Rhea's Gravitational Field and Internal Structure. 37th COSPAR Scientific Assembly. Held 13–20 July 2008, in Montréal, Canada. p. 89. Bibcode:2008cosp...37...89A.
^ Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (9 February 2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science. 315 (5813): 815. Bibcode:2007Sci...315..815V. doi:10.1126/science.1134681. PMID 17289992. S2CID 21932253. (supporting online material, table S1)
^ Observatorio ARVAL (April 15, 2007). "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2011-12-17.
^ David Leverington (29 May 2003). Babylon to Voyager and Beyond: A History of Planetary Astronomy. ISBN 978-0-521-80840-8. LCCN 2002031557. Wikidata Q126386131.
^ "In Depth | Rhea". NASA Solar System Exploration. NASA Science. December 19, 2019. Retrieved January 7, 2020.
^ ^a ^b "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology. July 21, 2006. Retrieved January 7, 2020.
^ As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 (January 14, 1848)
^ "Rhea | 2nd Largest Moon of Saturn | Britannica". www.britannica.com. Retrieved 2024-06-13.
^ "Terms and Definitions". astro.if.ufrgs.br. Retrieved 2024-06-13.
^ Julie Castillo-Rogez (17 November 2006). "Internal structure of Rhea". Journal of Geophysical Research. 111 (E11). doi:10.1029/2004JE002379. ISSN 0148-0227. S2CID 54034624. Wikidata Q126417371.
^ Treatise on Geophysics. Elsevier. 2015-04-17. ISBN 978-0-444-53803-1.
^ Hobbs, Peter Victor (2010-05-06). Ice Physics. OUP Oxford. ISBN 978-0-19-958771-1.
^ "Rhea - NASA Science". science.nasa.gov. Retrieved 2024-06-07.
^ Australia, Geoscience (2014-06-27). "Area of Australia - States and Territories". Geoscience Australia. Retrieved 2024-06-07.
^ ^a ^b Anderson, J. D.; Rappaport, N. J.; Giampieri, G.; et al. (2003). "Gravity field and interior structure of Rhea". Physics of the Earth and Planetary Interiors. 136 (3–4): 201–213. Bibcode:2003PEPI..136..201A. CiteSeerX 10.1.1.7.5250. doi:10.1016/S0031-9201(03)00035-9.
^ ^a ^b Anderson, J. D.; Schubert, J. (2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters. 34 (2): L02202. Bibcode:2007GeoRL..34.2202A. doi:10.1029/2006GL028100.
^ ^a ^b Iess, L.; Rappaport, N.; Tortora, P.; Lunine, Jonathan I.; Armstrong, J.; Asmar, S.; Somenzi, L.; Zingoni, F. (2007). "Gravity field and interior of Rhea from Cassini data analysis". Icarus. 190 (2): 585. Bibcode:2007Icar..190..585I. doi:10.1016/j.icarus.2007.03.027.
^ MacKenzie, R. A.; Iess, L.; Tortora, P.; Rappaport, N. J. (2008). "A non-hydrostatic Rhea". Geophysical Research Letters. 35 (5): L05204. Bibcode:2008GeoRL..35.5204M. doi:10.1029/2007GL032898. hdl:11573/17051.
^ Thomas, P. C.; Burns, J. A.; Helfenstein, P.; Squyres, S.; Veverka, J.; Porco, C.; Turtle, E. P.; McEwen, A.; Denk, T.; Giesef, B.; Roatschf, T.; Johnsong, T. V.; Jacobsong, R. A. (October 2007). "Shapes of the saturnian icy satellites and their significance" (PDF). Icarus. 190 (2): 573–584. Bibcode:2007Icar..190..573T. doi:10.1016/j.icarus.2007.03.012. Retrieved 15 December 2011.
^ Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus. 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.
^ ^a ^b Moore, Jeffrey M.; Schenk, Paul M.; Bruesch, Lindsey S.; Asphaug, Erik; McKinnon, William B. (October 2004). "Large impact features on middle-sized icy satellites" (PDF). Icarus. 171 (2): 421–443. Bibcode:2004Icar..171..421M. doi:10.1016/j.icarus.2004.05.009.
^ ^a ^b ^c ^d Wagner, R.J.; Neukum, G.; et al. (2008). "Geology of Saturn's Satellite Rhea on the Basis of the High-Resolution Images from the Targeted Flyby 049 on Aug. 30, 2007". Lunar and Planetary Science. XXXIX (1391): 1930. Bibcode:2008LPI....39.1930W.
^ Schenk, Paul M.; McKinnon, W. B. (2009). "Global Color Variations on Saturn's Icy Satellites, and New Evidence for Rhea's Ring". American Astronomical Society. 41: 3.03. Bibcode:2009DPS....41.0303S.
^ Schenk, Paul; Kirchoff, Michelle; Hoogenboom, Trudi; Rivera-Valentín, Edgard (2020). "The anatomy of fresh complex craters on the mid-sized icy moons of Saturn and self-secondary cratering at the rayed crater Inktomi (Rhea)". Meteoritics & Planetary Science. 55 (11): 2440–2460. Bibcode:2020M&PS...55.2440S. doi:10.1111/maps.13592. ISSN 1086-9379.
^ Joseph A. Burns; Mildred Shapley Matthews (November 1986). Satellites. University of Arizona Press. ISBN 978-0-8165-0983-6. Wikidata Q126417214.
^ "Rhea – Overview | Planets – NASA Solar System Exploration". NASA Solar System Exploration. Archived from the original on 2016-02-14. Retrieved 2017-09-21.
^ Anderson, J. D.; Schubert, G. (18 January 2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters. 34 (2). Bibcode:2007GeoRL..34.2202A. doi:10.1029/2006GL028100. ISSN 0094-8276.
^ Wagner, R. J.; Giese, B.; Roatsch, T.; Neukum, G.; Denk, T.; Wolf, U.; Porco, C. C. (May 2010). "Tectonic features on Rhea's trailing hemisphere: a first look at the Cassini ISS camera data from orbit 121, Nov. 21, 2009". EGU General Assembly 2010: 6731. Bibcode:2010EGUGA..12.6731W.
^ A spectroscopic study of the surfaces of Saturn's large satellites: H2O ice, tholins, and minor constituents (PDF). Dale P. Cruikshank, Tobias Owen, Cristina Dalle Ore, Thomas R. Geballe, Ted L. Roush, Catherine de Bergh, Scott A. Sandford, Francois Poulet, Gretchen K. Benedix, Joshua P. Emery. Icarus, 175, pages: 268–283, 2 March 2005.
^ ^a ^b ^c Elowitz, Mark; Sivaraman, Bhalamurugan; Hendrix, Amanda; Lo, Jen-Iu; Chou, Sheng-Lung; Cheng, Bing-Ming; Sekhar, B. N. Raja; Mason, Nigel J. (2021-01-22). "Possible detection of hydrazine on Saturn's moon Rhea". Science Advances. 7 (4). Bibcode:2021SciA....7.5749E. doi:10.1126/sciadv.aba5749. ISSN 2375-2548. PMC 10670839. PMID 33523937.
^ Aponte-Hernández, Betzaida; Rivera-Valentín, Edgard G.; Kirchoff, Michelle R.; Schenk, Paul M. (Dec 2012). "Morphometric Study of Craters on Saturn's Moon Rhea". The Planetary Science Journal. 2 (6): 235. doi:10.3847/psj/ac32d4. ISSN 2632-3338. PMC 8670330. PMID 34913034.
^ White, Oliver L.; Schenk, Paul M.; Bellagamba, Anthony W.; Grimm, Ashley M.; Dombard, Andrew J.; Bray, Veronica J. (2017-05-15). "Impact crater relaxation on Dione and Tethys and relation to past heat flow". Icarus. 288: 37–52. Bibcode:2017Icar..288...37W. doi:10.1016/j.icarus.2017.01.025. ISSN 0019-1035.
^ "Giant impact scenario may explain the unusual moons of Saturn". Space Daily. 2012. Retrieved 2012-10-19.
^ "Cassini Finds Ethereal Atmosphere at Rhea". NASA. Archived from the original on September 16, 2011. Retrieved November 27, 2010.
^ Teolis, B. D.; Jones, G. H.; Miles, P. F.; Tokar, R. L.; Magee, B. A.; Waite, J. H.; Roussos, E.; Young, D. T.; Crary, F. J.; Coates, A. J.; Johnson, R. E.; Tseng, W. L.; Baragiola, R. A. (2010). "Cassini Finds an Oxygen–Carbon Dioxide Atmosphere at Saturn's Icy Moon Rhea". Science. 330 (6012): 1813–1815. Bibcode:2010Sci...330.1813T. doi:10.1126/science.1198366. PMID 21109635. S2CID 206530211.
^ "Saturn's Moon Rhea Also May Have Rings". NASA. June 3, 2006. Archived from the original on 2012-10-22.
^ Jones, G. H.; et al. (2008-03-07). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science. 319 (5868): 1380–1384. Bibcode:2008Sci...319.1380J. doi:10.1126/science.1151524. PMID 18323452. S2CID 206509814.
^ Lakdawalla, E. (2008-03-06). "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea". www.planetary.org. Planetary Society. Archived from the original on 2008-03-10. Retrieved 2008-03-09.
^ Lakdawalla, E. (5 October 2009). "Another possible piece of evidence for a Rhea ring". The Planetary Society Blog. Planetary Society. Archived from the original on 2012-02-17. Retrieved 2009-10-06.
^ Matthew S. Tiscareno; Joseph A. Burns; Jeffrey N. Cuzzi; Matthew M. Hedman (2010). "Cassini imaging search rules out rings around Rhea". Geophysical Research Letters. 37 (14): L14205. arXiv:1008.1764. Bibcode:2010GeoRL..3714205T. doi:10.1029/2010GL043663. S2CID 59458559.
^ Kerr, Richard A. (2010-06-25). "The Moon Rings That Never Were". ScienceNow. Archived from the original on 2010-07-01. Retrieved 2010-08-05.
^ "Cassini". science.nasa.gov. Retrieved 2024-06-20.
^ Cook, Jia-Rui C. (13 January 2011). "Cassini Solstice Mission: Cassini Rocks Rhea Rendezvous". saturn.jpl.nasa.gov. NASA/JPL. Archived from the original on 17 January 2011. Retrieved 11 December 2011.
^ "NASA craft snaps last close-up photos of icy Saturn moon". saturn.jpl.nasa.gov. NASA/JPL. Retrieved 17 March 2013.

External links

Listen to this article (14 minutes)

This audio file was created from a revision of this article dated 29 October 2011, and does not reflect subsequent edits.

Rhea Profile at NASA's Solar System Exploration site
The Planetary Society: Rhea
Cassini images of Rhea Archived 2011-08-13 at the Wayback Machine
Images of Rhea at JPL's Planetary Photojournal
Movie of Rhea's rotation from the National Oceanic and Atmospheric Administration site
Rhea global and polar basemaps (March 2012) from Cassini images
Rhea altlas Archived 2013-04-24 at the Wayback Machine (released December 2010) from Cassini images
Rhea nomenclature and Rhea map with feature names from the USGS planetary nomenclature page
Google Rhea 3D, interactive map of the moon
Saturn's moon Rhea has thin atmosphere
An extract of the Journal Des Scavans. of April 22 ft. N. 1686. giving an account of two new satellites of Saturn, discovered lately by Mr. Cassini at the Royal Observatory at Paris

[calculated-6] Calculated on the basis of other parameters.

[surface_area-9] The surface area can be estimated, given the radius, with the formula $4 πr 2$

[10] Surface area derived from the radius (r): $4\pi r^2$ .

[25] The moons more massive than Rhea are: the Moon, the four Galilean moons, Titan, Triton, Titania, and Oberon. Oberon, Uranus's second-largest moon, has a radius that is ~0.4% smaller than Rhea's, but a density that is ~26% greater. See JPLSSD.

[29] More precisely, 0.3911.^[24]

[31] More precisely, 0.3721.^[25]

[space.com.Rhea-1] ^ ^a ^b ^c ^d ^e ^f ^g Tillman, Nola Taylor (2016-06-29). "Rhea: Saturn's dirty snowball moon". space.com.

[2] Consulmagno, G.; Ryche, H. (Feb 9, 1982). "Pronouncing the names of the moons of Saturn" (PDF). EOS. 63 (6): 146–147. doi:10.1029/EO063i006p00146. Retrieved Nov 30, 2022.

[3] Moore et al. (1984) "The Geomorphology of Rhea", Proceedings of the fifteenth Lunar and Planetary Science, Part 2, p C-791–C-794

[NSES-4] "Natural Satellites Ephemeris Service". Minor Planet Center.

[NSSDC-5] "Saturnian Satellite Fact Sheet". National Space Science Data Center. Archived from the original on 10 Feb 2024.

[Roatsch_et_al._2009-7] Roatsch, T.; Jaumann, R.; Stephan, K.; Thomas, P. C. (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763–781. doi:10.1007/978-1-4020-9217-6_24. ISBN 978-1-4020-9216-9.

[Jacobson2022-8] Robert. A. Jacobson (18 October 2022). "The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole". The Astronomical Journal. 164 (5). Bibcode:2022AJ....164..199J. doi:10.3847/1538-3881/AC90C9. ISSN 0004-6256. S2CID 252992162. Wikidata Q126389785.

[Anderson2007-11] Anderson, J. D.; Schubert, G. (2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters. 34 (2): L02202. Bibcode:2007GeoRL..34.2202A. doi:10.1029/2006GL028100.

[Anderson2008-12] Anderson, John D. (July 2008). Rhea's Gravitational Field and Internal Structure. 37th COSPAR Scientific Assembly. Held 13–20 July 2008, in Montréal, Canada. p. 89. Bibcode:2008cosp...37...89A.

[Verbiscer_et_al._2007-13] Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (9 February 2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science. 315 (5813): 815. Bibcode:2007Sci...315..815V. doi:10.1126/science.1134681. PMID 17289992. S2CID 21932253. (supporting online material, table S1)

[Observatorio_ARVAL-14] Observatorio ARVAL (April 15, 2007). "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2011-12-17.

[15] David Leverington (29 May 2003). Babylon to Voyager and Beyond: A History of Planetary Astronomy. ISBN 978-0-521-80840-8. LCCN 2002031557. Wikidata Q126386131.

[16] "In Depth | Rhea". NASA Solar System Exploration. NASA Science. December 19, 2019. Retrieved January 7, 2020.

[:0-17] "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology. July 21, 2006. Retrieved January 7, 2020.

[Lassell1848-18] As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 (January 14, 1848)

[19] "Rhea | 2nd Largest Moon of Saturn | Britannica". www.britannica.com. Retrieved 2024-06-13.

[20] "Terms and Definitions". astro.if.ufrgs.br. Retrieved 2024-06-13.

[21] Julie Castillo-Rogez (17 November 2006). "Internal structure of Rhea". Journal of Geophysical Research. 111 (E11). doi:10.1029/2004JE002379. ISSN 0148-0227. S2CID 54034624. Wikidata Q126417371.

[22] Treatise on Geophysics. Elsevier. 2015-04-17. ISBN 978-0-444-53803-1.

[23] Hobbs, Peter Victor (2010-05-06). Ice Physics. OUP Oxford. ISBN 978-0-19-958771-1.

[24] "Rhea - NASA Science". science.nasa.gov. Retrieved 2024-06-07.

[26] Australia, Geoscience (2014-06-27). "Area of Australia - States and Territories". Geoscience Australia. Retrieved 2024-06-07.

[Anderson2003-27] Anderson, J. D.; Rappaport, N. J.; Giampieri, G.; et al. (2003). "Gravity field and interior structure of Rhea". Physics of the Earth and Planetary Interiors. 136 (3–4): 201–213. Bibcode:2003PEPI..136..201A. CiteSeerX 10.1.1.7.5250. doi:10.1016/S0031-9201(03)00035-9.

[Anderson_2007-28] Anderson, J. D.; Schubert, J. (2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters. 34 (2): L02202. Bibcode:2007GeoRL..34.2202A. doi:10.1029/2006GL028100.

[Iess2007-30] Iess, L.; Rappaport, N.; Tortora, P.; Lunine, Jonathan I.; Armstrong, J.; Asmar, S.; Somenzi, L.; Zingoni, F. (2007). "Gravity field and interior of Rhea from Cassini data analysis". Icarus. 190 (2): 585. Bibcode:2007Icar..190..585I. doi:10.1016/j.icarus.2007.03.027.

[MacKenzie2008-32] MacKenzie, R. A.; Iess, L.; Tortora, P.; Rappaport, N. J. (2008). "A non-hydrostatic Rhea". Geophysical Research Letters. 35 (5): L05204. Bibcode:2008GeoRL..35.5204M. doi:10.1029/2007GL032898. hdl:11573/17051.

[Thomas2007-33] Thomas, P. C.; Burns, J. A.; Helfenstein, P.; Squyres, S.; Veverka, J.; Porco, C.; Turtle, E. P.; McEwen, A.; Denk, T.; Giesef, B.; Roatschf, T.; Johnsong, T. V.; Jacobsong, R. A. (October 2007). "Shapes of the saturnian icy satellites and their significance" (PDF). Icarus. 190 (2): 573–584. Bibcode:2007Icar..190..573T. doi:10.1016/j.icarus.2007.03.012. Retrieved 15 December 2011.

[34] Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus. 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.

[MooreSchenk_2004-35] Moore, Jeffrey M.; Schenk, Paul M.; Bruesch, Lindsey S.; Asphaug, Erik; McKinnon, William B. (October 2004). "Large impact features on middle-sized icy satellites" (PDF). Icarus. 171 (2): 421–443. Bibcode:2004Icar..171..421M. doi:10.1016/j.icarus.2004.05.009.

[Wagner2008-36] Wagner, R.J.; Neukum, G.; et al. (2008). "Geology of Saturn's Satellite Rhea on the Basis of the High-Resolution Images from the Targeted Flyby 049 on Aug. 30, 2007". Lunar and Planetary Science. XXXIX (1391): 1930. Bibcode:2008LPI....39.1930W.

[Schenk2009-37] Schenk, Paul M.; McKinnon, W. B. (2009). "Global Color Variations on Saturn's Icy Satellites, and New Evidence for Rhea's Ring". American Astronomical Society. 41: 3.03. Bibcode:2009DPS....41.0303S.

[38] Schenk, Paul; Kirchoff, Michelle; Hoogenboom, Trudi; Rivera-Valentín, Edgard (2020). "The anatomy of fresh complex craters on the mid-sized icy moons of Saturn and self-secondary cratering at the rayed crater Inktomi (Rhea)". Meteoritics & Planetary Science. 55 (11): 2440–2460. Bibcode:2020M&PS...55.2440S. doi:10.1111/maps.13592. ISSN 1086-9379.

[39] Joseph A. Burns; Mildred Shapley Matthews (November 1986). Satellites. University of Arizona Press. ISBN 978-0-8165-0983-6. Wikidata Q126417214.

[40] "Rhea – Overview | Planets – NASA Solar System Exploration". NASA Solar System Exploration. Archived from the original on 2016-02-14. Retrieved 2017-09-21.

[41] Anderson, J. D.; Schubert, G. (18 January 2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters. 34 (2). Bibcode:2007GeoRL..34.2202A. doi:10.1029/2006GL028100. ISSN 0094-8276.

[42] Wagner, R. J.; Giese, B.; Roatsch, T.; Neukum, G.; Denk, T.; Wolf, U.; Porco, C. C. (May 2010). "Tectonic features on Rhea's trailing hemisphere: a first look at the Cassini ISS camera data from orbit 121, Nov. 21, 2009". EGU General Assembly 2010: 6731. Bibcode:2010EGUGA..12.6731W.

[Cruikshank_2005-43] A spectroscopic study of the surfaces of Saturn's large satellites: H2O ice, tholins, and minor constituents (PDF). Dale P. Cruikshank, Tobias Owen, Cristina Dalle Ore, Thomas R. Geballe, Ted L. Roush, Catherine de Bergh, Scott A. Sandford, Francois Poulet, Gretchen K. Benedix, Joshua P. Emery. Icarus, 175, pages: 268–283, 2 March 2005.

[:1-44] Elowitz, Mark; Sivaraman, Bhalamurugan; Hendrix, Amanda; Lo, Jen-Iu; Chou, Sheng-Lung; Cheng, Bing-Ming; Sekhar, B. N. Raja; Mason, Nigel J. (2021-01-22). "Possible detection of hydrazine on Saturn's moon Rhea". Science Advances. 7 (4). Bibcode:2021SciA....7.5749E. doi:10.1126/sciadv.aba5749. ISSN 2375-2548. PMC 10670839. PMID 33523937.

[45] Aponte-Hernández, Betzaida; Rivera-Valentín, Edgard G.; Kirchoff, Michelle R.; Schenk, Paul M. (Dec 2012). "Morphometric Study of Craters on Saturn's Moon Rhea". The Planetary Science Journal. 2 (6): 235. doi:10.3847/psj/ac32d4. ISSN 2632-3338. PMC 8670330. PMID 34913034.

[46] White, Oliver L.; Schenk, Paul M.; Bellagamba, Anthony W.; Grimm, Ashley M.; Dombard, Andrew J.; Bray, Veronica J. (2017-05-15). "Impact crater relaxation on Dione and Tethys and relation to past heat flow". Icarus. 288: 37–52. Bibcode:2017Icar..288...37W. doi:10.1016/j.icarus.2017.01.025. ISSN 0019-1035.

[47] "Giant impact scenario may explain the unusual moons of Saturn". Space Daily. 2012. Retrieved 2012-10-19.

[atmosphere-48] "Cassini Finds Ethereal Atmosphere at Rhea". NASA. Archived from the original on September 16, 2011. Retrieved November 27, 2010.

[Teolis2010-49] Teolis, B. D.; Jones, G. H.; Miles, P. F.; Tokar, R. L.; Magee, B. A.; Waite, J. H.; Roussos, E.; Young, D. T.; Crary, F. J.; Coates, A. J.; Johnson, R. E.; Tseng, W. L.; Baragiola, R. A. (2010). "Cassini Finds an Oxygen–Carbon Dioxide Atmosphere at Saturn's Icy Moon Rhea". Science. 330 (6012): 1813–1815. Bibcode:2010Sci...330.1813T. doi:10.1126/science.1198366. PMID 21109635. S2CID 206530211.

[rings-50] "Saturn's Moon Rhea Also May Have Rings". NASA. June 3, 2006. Archived from the original on 2012-10-22.

[Jones2008-51] Jones, G. H.; et al. (2008-03-07). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science. 319 (5868): 1380–1384. Bibcode:2008Sci...319.1380J. doi:10.1126/science.1151524. PMID 18323452. S2CID 206509814.

[LakdawallaE-52] Lakdawalla, E. (2008-03-06). "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea". www.planetary.org. Planetary Society. Archived from the original on 2008-03-10. Retrieved 2008-03-09.

[Lakdawalla2-53] Lakdawalla, E. (5 October 2009). "Another possible piece of evidence for a Rhea ring". The Planetary Society Blog. Planetary Society. Archived from the original on 2012-02-17. Retrieved 2009-10-06.

[Tiscareno2010-54] Matthew S. Tiscareno; Joseph A. Burns; Jeffrey N. Cuzzi; Matthew M. Hedman (2010). "Cassini imaging search rules out rings around Rhea". Geophysical Research Letters. 37 (14): L14205. arXiv:1008.1764. Bibcode:2010GeoRL..3714205T. doi:10.1029/2010GL043663. S2CID 59458559.

[norings-55] Kerr, Richard A. (2010-06-25). "The Moon Rings That Never Were". ScienceNow. Archived from the original on 2010-07-01. Retrieved 2010-08-05.

[56] "Cassini". science.nasa.gov. Retrieved 2024-06-20.

[CookJ2011-57] Cook, Jia-Rui C. (13 January 2011). "Cassini Solstice Mission: Cassini Rocks Rhea Rendezvous". saturn.jpl.nasa.gov. NASA/JPL. Archived from the original on 17 January 2011. Retrieved 11 December 2011.

[LastFlybyRhea-58] "NASA craft snaps last close-up photos of icy Saturn moon". saturn.jpl.nasa.gov. NASA/JPL. Retrieved 17 March 2013.

[1]

[2]

[3]

[4]

[5]

[a]

[6]

[7]

[b]

[c]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[d]

[22]

[23]

[e]

[24]

[f]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

v t e Natural satellites of the Solar System
Planetary satellites of	Earth Mars Jupiter Saturn Uranus Neptune
Dwarf planet satellites of	Orcus Pluto Haumea Quaoar Makemake Gonggong Eris
Minor-planet moons	Near-Earth Florence Didymos Dimorphos Moshup Squannit 1994 CC 2001 SN₂₆₃ Main belt Kalliope Linus Euphrosyne Daphne Peneius Eugenia Petit-Prince Sylvia Romulus Remus Minerva Aegis Gorgoneion Camilla Elektra Kleopatra Alexhelios Cleoselene Ida Dactyl Roxane Olympias Pulcova Balam Dinkinesh (Selam) Jupiter trojans Patroclus Menoetius Hektor Skamandrios Eurybates Queta TNOs Lempo Hiisi Paha 2002 UX₂₅ Sila–Nunam Salacia Actaea Varda Ilmarë Gǃkúnǁʼhòmdímà Gǃòʼé ǃHú 2013 FY₂₇
Ranked by size	Ganymede largest: 5268 km / 0.413 Earths Titan Callisto Io Moon Europa Triton Titania Rhea Oberon Iapetus Charon Umbriel Ariel Dione Tethys Dysnomia Enceladus Miranda Vanth Proteus Mimas Ilmarë Nereid Hiʻiaka Actaea Hyperion Phoebe ...
Discovery timeline Inner moons Irregular moons List Planetary-mass moons Naming Subsatellite Regular moons Trojan moons

v t e Saturn
Outline
Geography	Dragon Storm Great White Spot Hexagon Magnetosphere Rings
Moons	S/2009 S 1 Ring moonlets Pan Daphnis Atlas Prometheus Pandora Epimetheus Janus Aegaeon Mimas Methone Anthe Pallene Enceladus Tethys Telesto Calypso Dione Helene Polydeuces Rhea Titan Hyperion Iapetus Inuit group Kiviuq Paaliaq Siarnaq Gallic group Albiorix Norse group Phoebe
Astronomy	Delta Octantis Saturn-crossing minor planets
Exploration	Cassini–Huygens (Huygens) timeline retirement Pioneer 11 Voyager program Voyager 1 Voyager 2
Related	Fiction The Day the Earth Smiled In Saturn's Rings (2018 documentary)
Category

v t e Atmospheres
Stars	Sun
Planets	Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Dwarf planets	Ceres Pluto Makemake Eris
Natural satellites	Moon Io Europa Ganymede Callisto Enceladus Dione Rhea Titan Triton
Exoplanets	GJ 1132 b HD 209458 b Kepler-7b
See also	Prebiotic atmosphere Coma Extraterrestrial atmosphere Stellar atmosphere
Atmospheres in boldface are significant atmospheres; atmospheres in italics are unconfirmed atmospheres.

Authority control databases
International	VIAF
National	Germany United States Israel