View source for Rhea (moon)

{{Short description|Moon of Saturn}}
{{Distinguish|text=the asteroid named [[577 Rhea]]}}
{{Infobox planet
| name = Rhea
| mpc_name = Saturn V
| pronounced = {{IPAc-en|ˈ|r|iː|.|ə}}<ref>{{cite journal |last1=Consulmagno |first1=G. |last2=Ryche |first2=H. |date=Feb 9, 1982 |title=Pronouncing the names of the moons of Saturn |url=https://www.vaticanobservatory.org/wp-content/uploads/2019/12/Consolmagno-Reiche-Pronouncing-Moons-of-Saturn.pdf |journal=EOS |volume=63 |issue=6 |pages=146–147 |doi=10.1029/EO063i006p00146 |access-date=Nov 30, 2022}}</ref>
| named_after = [[Rhea (mythology)|Ῥέᾱ]] ''Rheā''
| adjectives = Rhean {{IPAc-en|ˈ|r|iː|.|ən}}<ref>Moore et al. (1984) "The Geomorphology of Rhea", ''Proceedings of the fifteenth Lunar and Planetary Science'', Part 2, p C-791–C-794</ref>
| image = PIA07763 Rhea full globe5.jpg
| caption = Mosaic of Rhea, assembled from ''[[Cassini–Huygens|Cassini]]'' imagery taken on 26 November 2005
| discoverer = [[Giovanni Domenico Cassini|G. D. Cassini]]<ref name=space.com.Rhea/>
| discovered = December 23, 1672<ref name=space.com.Rhea>{{cite web |url=http://www.space.com/20577-rhea-saturn-s-dirty-snowball-moon.html |title=Rhea: Saturn's dirty snowball moon |last=Tillman |first=Nola Taylor |date=2016-06-29 |website=space.com}}</ref>
| orbit_ref =  <ref name="NSES" />
| semimajor = {{val|527040|u=km}}<ref name="NSSDC"/>
| eccentricity = {{val|0.001}}<ref name="NSSDC"/>
| period = {{val|4.518212|u=[[Day|d]]}}
| avg_speed = 8.48 km/s{{efn|name=calculated|Calculated on the basis of other parameters.}}
| inclination = 0.35°<ref name="NSSDC"/>
| satellite_of = [[Saturn]]
| mean_radius = {{val|763.5|0.5|u=km}}<ref name="Jacobson2022">{{cite Q|Q126389785|doi-access=free}}</ref>
| dimensions = 1532.4&thinsp;×&thinsp;1525.6&thinsp;×&thinsp;1524.4 km <ref name="Roatsch et al. 2009" />
| surface_area = {{val|7325342|u=km<sup>2</sup>}}{{efn|name=surface_area}}
| mass = {{val|2.3064854|0.0000522|e=21|u=kg}}<ref name="Jacobson2022"/> (~3.9{{e|-4}} Earths)
| density = {{val|1.2372|0.0029|u=g/cm<sup>3</sup>}}<ref name="Jacobson2022"/>
| surface_grav = {{Gr|2.306|763.8|2}} [[Acceleration|m/s<sup>2</sup>]]{{efn|Surface area derived from the radius (''r''): {{math|4\pi r^2}}.}}
| moment_of_inertia_factor = {{val|0.3911|0.0045}}<ref name="Anderson2007">{{cite journal |last1= Anderson|first1=J. D.|last2= Schubert|first2= G.|title= Saturn's satellite Rhea is a homogeneous mix of rock and ice|journal= Geophysical Research Letters |volume= 34|issue= 2|pages=L02202|year= 2007|doi= 10.1029/2006GL028100|bibcode= 2007GeoRL..34.2202A|doi-access= free}}</ref> (disputed/unclear<ref name="Anderson2008" />)
| escape_velocity = {{V2|2.306|763.8|3}} km/s
| rotation = {{val|4.518212|u=d}} <br /> ([[Synchronous rotation|synchronous]])
| axial_tilt = zero
| albedo = {{val|0.949|0.003}} ([[geometric albedo|geometric]]) <ref name="Verbiscer et al. 2007" />
| magnitude = 10 <ref name="Observatorio ARVAL" />
| temp_name1 = [[Kelvin]]
| min_temp_1 = 53 [[Kelvin|K]]
| mean_temp_1 = &nbsp;
| max_temp_1 = 99 K
}}

'''Rhea''' ({{IPAc-en|ˈ|r|iː|.|ə}}) is the second-largest [[natural satellite|moon]] of [[Saturn]] and the [[List of natural satellites by diameter|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 [[Tidal locking|tidally locked]], like Saturn's other major moons; that is, it [[Rotation|rotates]] with the same period it revolves ([[orbit]]s), 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 (moon)|Dione]], it has high-[[albedo]] ice cliffs that appear as bright wispy streaks visible from space. The surface temperature varies between −174&nbsp;°C and −220&nbsp;°C.

Rhea was discovered in 1672 by [[Giovanni Domenico Cassini]]. Since then, it has been visited by both [[Voyager program|Voyager probes]] and was the subject of close targeted [[flyby (spaceflight)|flyby]]s by the [[Cassini–Huygens|''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 {{Convert|10.4|m|ft|adj=on}} telescope made by [[Giuseppe Campani]].<ref name=space.com.Rhea/><ref>{{Cite Q|Q126386131}}</ref> Cassini named the four moons he discovered (Tethys, Dione, Rhea, and [[Iapetus (moon)|Iapetus]]) ''[[Sidera Lodoicea]]'' (the stars of Louis) to honor King [[Louis XIV of France|Louis XIV]].<ref name="space.com.Rhea" />  Rhea was the second [[moons of Saturn|moon of Saturn]] that Cassini discovered, and the third moon discovered around Saturn overall.<ref name=space.com.Rhea/>

==Name==
Rhea is named after the [[titan (mythology)|Titan]] [[Rhea (mythology)|Rhea]] of [[Greek mythology]], the "mother of the gods" and wife of [[Cronus|Kronos]], the [[Interpretatio graeca|Greek counterpart]] of the god [[Saturn (mythology)|Saturn]]. It is also designated '''Saturn&nbsp;V''' (being the fifth major moon going outward from the planet, after [[Mimas (moon)|Mimas]], [[Enceladus (moon)|Enceladus]], [[Tethys (moon)|Tethys]], and [[Dione (moon)|Dione]]).<ref>{{Cite web|url=https://solarsystem.nasa.gov/moons/saturn-moons/rhea/in-depth/|title=In Depth {{!}} Rhea|date=December 19, 2019|website=NASA Solar System Exploration|publisher=NASA Science|access-date=January 7, 2020}}</ref><ref name=":0">{{Cite web|url=https://planetarynames.wr.usgs.gov/Page/Planets|title=Planet and Satellite Names and Discoverers|date=July 21, 2006|work=Gazetteer of Planetary Nomenclature|publisher=USGS Astrogeology|access-date=January 7, 2020}}</ref>

Astronomers fell into the habit of referring to them and [[Titan (moon)|Titan]] as ''Saturn&nbsp;I'' through ''Saturn&nbsp;V''.<ref name=space.com.Rhea/>  Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to ''Saturn&nbsp;VII'', and then to ''Saturn&nbsp;VIII'' with the discovery of [[Hyperion (moon)|Hyperion]] in 1848.<ref name=":0" />

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 (moon)|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.<ref name="Lassell1848" /><ref name=space.com.Rhea/>

== Orbit ==
The orbit of Rhea has very low [[Orbital eccentricity|eccentricity]] (0.001), meaning it is nearly circular. It has a low [[Orbital inclination|inclination]] of less than a degree, inclined by only 0.35° from Saturn's equatorial plane.<ref name="NSSDC"/>

Rhea is [[Tidal locking|tidally locked]] and rotates synchronously; that is, it [[Rotation|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.<ref>{{Cite web |title=Rhea {{!}} 2nd Largest Moon of Saturn {{!}} Britannica |url=https://www.britannica.com/topic/Rhea-astronomy |access-date=2024-06-13 |website=www.britannica.com |language=en}}</ref><ref>{{Cite web |title=Terms and Definitions |url=http://astro.if.ufrgs.br/solar/terms.htm |access-date=2024-06-13 |website=astro.if.ufrgs.br}}</ref>

== Physical characteristics ==

=== Size, mass, and internal structure ===
[[File:Rhea, Earth & Moon size comparison.jpg|left|thumb|250x250px|Size comparison of [[Earth]] (right), the [[Moon]] (left top), and Rhea (left down)]]
Rhea is an icy body with a [[density]] of about 1.236 g/cm<sup>3</sup>. This low density indicates that it is made of ~25% rock (density ~3.25 g/cm<sup>3</sup>) and ~75% water ice (density ~0.93 g/cm<sup>3</sup>). 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 {{Convert|350 to 450|km|mi}} beneath the surface.<ref>{{Cite Q|Q126417371}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=Rg6dBAAAQBAJ&dq=%22surface+gravity%22+of+%22rhea%22&pg=RA4-PA628 |title=Treatise on Geophysics |date=2015-04-17 |publisher=Elsevier |isbn=978-0-444-53803-1 |language=en}}</ref><ref>{{Cite book |last=Hobbs |first=Peter Victor |url=https://books.google.com/books?id=7Is6AwAAQBAJ&pg=PA61 |title=Ice Physics |date=2010-05-06 |publisher=OUP Oxford |isbn=978-0-19-958771-1 |language=en}}</ref> Rhea is {{Convert|1528|km|mi}} in diameter, but is still only a third of the size of [[Titan (moon)|Titan]], Saturn's biggest moon.<ref>{{Cite web |title=Rhea - NASA Science |url=https://science.nasa.gov/saturn/moons/rhea/ |access-date=2024-06-07 |website=science.nasa.gov |language=en-US}}</ref> Although Rhea is the ninth-largest moon, it is only the tenth-most massive moon. Indeed, [[Oberon (moon)|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.<ref group="lower-alpha">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 [http://ssd.jpl.nasa.gov/?sat_phys_par JPLSSD.]</ref> The surface area of the moon can be estimated at {{Convert|7330000|km2|mi2|sigfig=3|round=10}}, similar to Australia (7,688,287&nbsp;km<sup>2</sup>).<ref>{{Cite web |last=Australia |first=Geoscience |date=2014-06-27 |title=Area of Australia - States and Territories |url=https://www.ga.gov.au/scientific-topics/national-location-information/dimensions/area-of-australia-states-and-territories |access-date=2024-06-07 |website=Geoscience Australia |language=en}}</ref>{{Efn|The [[surface area]] can be estimated, given the radius, with the formula {{math|4''&pi;r''<sup>2</sup>}}|name=surface_area}}

Before the ''[[Cassini–Huygens]]'' mission, it was assumed that Rhea had a rocky core.<ref name="Anderson2003" />  However, measurements taken during a close flyby by the [[Cassini–Huygens|''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.{{refn | group = lower-alpha | More precisely, 0.3911.<ref name="Anderson_2007" /> }}<ref name="Anderson_2007" />  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.<ref name="Anderson2003" />  In the same year another paper claimed the moment of inertia was about 0.37.{{refn | group = lower-alpha | More precisely, 0.3721.<ref name="Iess2007" /> }} Rhea being either partially or fully differentiated would be consistent with the observations of the ''Cassini'' probe.<ref name="Iess2007" /> 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.<ref name="MacKenzie2008" />  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.<ref name="Anderson2008" />

The [[Triaxial ellipsoid|triaxial]] shape of Rhea is consistent with a homogeneous body in [[hydrostatic equilibrium]] rotating at Rhea's angular velocity.<ref name="Thomas2007" /> Modelling in 2006 suggested that Rhea could be barely capable of sustaining an [[subsurface ocean|internal liquid-water ocean]] through heating by [[radioactive decay]]; such an ocean would have to be at about 176&nbsp;K, the [[eutectic temperature]] for the water–ammonia system.<ref>
{{cite journal| doi = 10.1016/j.icarus.2006.06.005| last1 = Hussmann| first1 = Hauke| last2 = Sohl| first2 = Frank| last3 = Spohn| first3 = Tilman| date = November 2006| title = Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects| journal = [[Icarus (journal)|Icarus]]| volume = 185| issue = 1| pages = 258–273| url = https://www.researchgate.net/publication/225019299| bibcode = 2006Icar..185..258H| ref = {{sfnRef|Hussmann Sohl et al.|2006}}}}</ref> More recent indications are that Rhea has a homogeneous interior and hence that this ocean does not exist.<ref name=Anderson2008/>

=== Surface features ===
{{main|List of geological features on Rhea}}
Rhea's features resemble those of [[Dione (moon)|Dione]], with distinct and dissmillar leading and trailing hemispheres, suggesting similar composition and histories. The temperature on Rhea is 99&nbsp;K (−174&nbsp;°C) in direct sunlight and between 73&nbsp;K (−200&nbsp;°C) and 53&nbsp;K (−220&nbsp;°C) in the shade.
[[File:PIA18310-SaturnMoon-Rhea-20150210.jpg|thumb|Surface features on Rhea well defined due to the lighting]]
Rhea has a rather typical heavily [[Impact crater|cratered]] surface,<ref name="MooreSchenk_2004" /> with the exceptions of a few large Dione-type chasmata or fractures (formerly known as [[Dione (moon)#Ice cliffs (formerly 'wispy terrain')|wispy terrain]]) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit)<ref name="Wagner2008" /> and a very faint "line" of material at Rhea's equator that may have been deposited by material deorbiting from its rings.<ref name="Schenk2009" />  Rhea has two very large impact basins on its hemisphere facing away from Saturn, which are about 400 and 500&nbsp;km across.<ref name="Wagner2008" />  The more northerly and less degraded of the two, called [[Tirawa (crater)|Tirawa]], is roughly comparable in size to the basin Odysseus on [[Tethys (moon)|Tethys]].<ref name="MooreSchenk_2004" />  There is a 48&nbsp;km-diameter impact crater at 112°W that is prominent because of an extended system of bright [[ray system|ray]]s, which extend up to {{Convert|400|km|mi|abbr=on}} away from the crater, across most of one hemisphere.<ref name="Wagner2008" /><ref>{{Cite journal |last1=Schenk |first1=Paul |last2=Kirchoff |first2=Michelle |last3=Hoogenboom |first3=Trudi |last4=Rivera-Valentín |first4=Edgard |date=2020 |title=The anatomy of fresh complex craters on the mid-sized icy moons of Saturn and self-secondary cratering at the rayed crater Inktomi (Rhea) |url=https://onlinelibrary.wiley.com/doi/10.1111/maps.13592 |journal=Meteoritics & Planetary Science |language=en |volume=55 |issue=11 |pages=2440–2460 |doi=10.1111/maps.13592 |bibcode=2020M&PS...55.2440S |issn=1086-9379}}</ref>  This crater, called [[Inktomi (crater)|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 Conference|''Lunar and Planetary Science'']].<ref name="Wagner2008" /> Rhea's [[impact crater]]s are more crisply defined than the flatter craters that are pervasive on [[Ganymede (moon)|Ganymede]] and [[Callisto (moon)|Callisto]]; it is theorized that this is due to a much lower [[surface gravity]] (0.26 [[Metre per second squared|m/s<sup>2</sup>]], compared to Ganymede's 1.428&nbsp;m/s<sup>2</sup> and Callisto's 1.235&nbsp;m/s<sup>2</sup>) and a stiffer crust of ice. Similarly, [[ejecta blanket]]s – asymmetrical blankets of ejected particles surrounding impact craters – are not present on Rhea, potentially another result of the moon's low surface gravity.<ref>{{Cite Q|Q126417214}}</ref>
[[File:Rhea at approximately 2,348 miles (3,778 kilometers) away.jpg|thumb|Closeup showing two craters on Rhea's surface taken in 2013 by [[Cassini–Huygens|''Cassini'' spacecraft]]]]
Its surface can be divided into two geologically different areas based on [[impact crater|crater]] density; the first area contains craters which are larger than 40&nbsp;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 (moon)|Callisto]], the craters lack the high relief features seen on the [[Moon]] and [[Mercury (planet)|Mercury]]. It has been theorized that these cratered plains are up to four billion years old on average.<ref>{{Cite web|url=https://solarsystem.nasa.gov/planets/rhea|archive-url=https://web.archive.org/web/20160214000303/http://solarsystem.nasa.gov/planets/rhea|url-status=dead|archive-date=2016-02-14|title=Rhea – Overview {{!}} Planets – NASA Solar System Exploration|website=NASA Solar System Exploration|access-date=2017-09-21}}</ref> On the trailing hemisphere there is a network of bright swaths on a dark background, and fewer craters.<ref>{{Cite journal |last1=Anderson |first1=J. D. |last2=Schubert |first2=G. |date=18 January 2007 |title=Saturn's satellite Rhea is a homogeneous mix of rock and ice |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006GL028100 |journal=Geophysical Research Letters |language=en |volume=34 |issue=2 |doi=10.1029/2006GL028100 |bibcode=2007GeoRL..34.2202A |issn=0094-8276}}</ref> 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]]).<ref>{{Cite journal |last1=Wagner |first1=R. J. |last2=Giese |first2=B. |last3=Roatsch |first3=T. |last4=Neukum |first4=G. |last5=Denk |first5=T. |last6=Wolf |first6=U. |last7=Porco |first7=C. C. |date=May 2010 |title=Tectonic features on Rhea's trailing hemisphere: a first look at the Cassini ISS camera data from orbit 121, Nov. 21, 2009 |url=https://ui.adsabs.harvard.edu/abs/2010EGUGA..12.6731W |journal=EGU General Assembly 2010 |pages=6731|bibcode=2010EGUGA..12.6731W }}</ref> The extensive dark areas are thought to be deposited [[tholin]]s, which are a mix of complex [[organic compound]]s generated on the ice by [[pyrolysis]] and [[radiolysis]] of simple compounds containing carbon, nitrogen and hydrogen.<ref name='Cruikshank 2005'>[http://astrochemistry.org/docs/Cruikshanketal2005.pdf 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.</ref> The trailing side of Rhea's surface is [[Magnetosphere of Saturn#Sources and transport of plasma|irradiated by Saturn's magnetosphere]], which may cause chemical-level changes on the surface, including [[radiolysis]] (see {{Section link|2=Atmosphere|nopage=y}}). Particles from Saturn's [[E Ring (ring of Saturn)|E-ring]] are also flung onto the moon's leading hemisphere, coating it.<ref name=":1" />

Rhea has some evidence of endogenic activity – that is, activity originating from within the moon, such as heating and [[Cryovolcano|cryovolcanic]] activity: there are [[Fault (geology)|fault]] systems and craters with uplifted bases (so-called "relaxed" craters), although the latter is apparently only present in large craters more than {{Convert|100|km|mi|abbr=on}} across.<ref>{{Cite journal |last1=Aponte-Hernández |first1=Betzaida |last2=Rivera-Valentín |first2=Edgard G. |last3=Kirchoff |first3=Michelle R. |last4=Schenk |first4=Paul M. |date=Dec 2012 |title=Morphometric Study of Craters on Saturn's Moon Rhea |journal=The Planetary Science Journal |volume=2 |issue=6 |pages=235 |doi=10.3847/psj/ac32d4 |issn=2632-3338 |pmc=8670330 |pmid=34913034 |doi-access=free}}</ref><ref>{{Cite journal |last1=White |first1=Oliver L. |last2=Schenk |first2=Paul M. |last3=Bellagamba |first3=Anthony W. |last4=Grimm |first4=Ashley M. |last5=Dombard |first5=Andrew J. |last6=Bray |first6=Veronica J. |date=2017-05-15 |title=Impact crater relaxation on Dione and Tethys and relation to past heat flow |url=https://www.sciencedirect.com/science/article/pii/S0019103516305048 |journal=Icarus |volume=288 |pages=37–52 |doi=10.1016/j.icarus.2017.01.025 |bibcode=2017Icar..288...37W |issn=0019-1035}}</ref><ref name=":1">{{Cite journal |last1=Elowitz |first1=Mark |last2=Sivaraman |first2=Bhalamurugan |last3=Hendrix |first3=Amanda |last4=Lo |first4=Jen-Iu |last5=Chou |first5=Sheng-Lung |last6=Cheng |first6=Bing-Ming |last7=Sekhar |first7=B. N. Raja |last8=Mason |first8=Nigel J. |date=2021-01-22 |title=Possible detection of hydrazine on Saturn's moon Rhea |journal=Science Advances |language=en |volume=7 |issue=4 |doi=10.1126/sciadv.aba5749 |issn=2375-2548 |pmc=10670839 |pmid=33523937|bibcode=2021SciA....7.5749E }}</ref>

==Formation==
The moons of Saturn are thought to have formed through [[Accretion disc|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 Ian Asphaug|Erik Asphaug]] and [[Andreas Reufer]] for the formation of [[Titan (moon)|Titan]] may also shine a new light on the origin of Rhea and [[Iapetus (moon)|Iapetus]]. In this model, Titan was formed in a series of [[giant impact]]s between pre-existing moons, and Rhea and Iapetus are thought to have formed from part of the debris of these collisions.<ref>{{cite web |title=Giant impact scenario may explain the unusual moons of Saturn |work=Space Daily |date=2012 |url=http://www.spacedaily.com/reports/Giant_impact_scenario_may_explain_the_unusual_moons_of_Saturn_999.html |access-date=2012-10-19 }}</ref><gallery class="center" widths="100" heights="300" mode="packed">
File:Rhea ice cliffs.jpg|Image of the wispy hemisphere, showing ice cliffs – Powehiwehi (upper center); chasmata stretch from upper left to right center – Onokoro Catenae (lower right)
File:PIA08148 (Rhea-Splat).jpg|View of Rhea's leading hemisphere with crater Inktomi and its prominent [[ray system]] just below center; [[impact basin]] [[Tirawa (crater)|Tirawa]] is at upper left
</gallery>

== 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 10<sup>5</sup> to 10<sup>6</sup> 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.<ref name=":1" /><ref name="atmosphere" /><ref name="Teolis2010" />

== Possible ring system ==
{{main|Rings of Rhea}}
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.<ref name="rings" /><ref name="Jones2008" /><ref name="LakdawallaE" />  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).<ref name="Lakdawalla2" />  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.<ref name="Tiscareno2010" /><ref name="norings" />

== Exploration ==
The first images of Rhea were obtained by ''[[Voyager program|Voyager 1 & 2]]'' spacecraft in 1980–1981.

There were five close targeted fly-bys by the ''[[Cassini–Huygens|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.<ref>{{Cite web |title=Cassini |url=https://science.nasa.gov/mission/cassini/quick-facts/ |access-date=2024-06-20 |website=science.nasa.gov |language=en-US}}</ref> ''Cassini'' passed Rhea at a distance of 500&nbsp;km on November 26, 2005; at a distance of 5,750&nbsp;km on August 30, 2007; at a distance of 100&nbsp;km on March 2, 2010; at 69&nbsp;km flyby on January 11, 2011;<ref name="CookJ2011" /> and a last flyby at 992&nbsp;km on March 9, 2013.<ref name="LastFlybyRhea" />

== See also ==
* [[Planet#Objects formerly considered planets|Former classification of planets]]
* [[List of natural satellites]]
* [[Saturn's moons in fiction#Rhea|Rhea in fiction]]
* [[Rings of Rhea]]
* [[Subsatellite]]
* [[Moons of Saturn]]

== Notes ==
{{Reflist
| colwidth = 30em
| group = lower-alpha
}}

== References ==
{{Reflist
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<ref name="Roatsch et al. 2009">
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<ref name="Verbiscer et al. 2007">
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<ref name="Observatorio ARVAL">
{{cite web
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<ref name="Lassell1848">
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<ref name="Anderson2003">
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<ref name="Anderson_2007">
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<ref name="Anderson2008">
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<ref name="Thomas2007">
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<ref name="MooreSchenk_2004">
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<ref name="Wagner2008">
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<ref name="Schenk2009">
{{cite journal
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<ref name="atmosphere">{{cite web
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<ref name="rings">{{Cite web
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|archiveurl=https://web.archive.org/web/20121022041030/http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html
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<ref name="Jones2008">
{{cite journal
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<ref name="LakdawallaE">
{{cite web
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<ref name="Lakdawalla2">{{cite web
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{{cite journal
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<ref name="norings">
{{cite journal
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{{cite web
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}}
</ref>

}}

== External links ==
{{Spoken Wikipedia|En-Rhea_(moon).ogg|date=2011-10-29}}
{{Commons category|Rhea (moon)|Rhea}}
* [https://web.archive.org/web/20070801204714/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Rhea Rhea Profile] at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
* [https://web.archive.org/web/20060831203603/http://www.planetary.org/explore/topics/saturn/rhea.html The Planetary Society: Rhea]
* [http://ciclops.org/search.php?x=20&y=7&search=Rhea ''Cassini'' images of Rhea] {{Webarchive|url=https://web.archive.org/web/20110813151222/http://ciclops.org/search.php?x=20&y=7&search=Rhea |date=2011-08-13 }}
* [http://photojournal.jpl.nasa.gov/target/Rhea Images of Rhea at JPL's Planetary Photojournal]
* Movie of [https://web.archive.org/web/20100601171509/http://sos.noaa.gov/videos/Rhea.mov Rhea's rotation] from the National Oceanic and Atmospheric Administration site
* Rhea [http://www.ciclops.org/view/7313/Map_of_Rhea_-_March_2012 global] and [http://www.ciclops.org/view/7314/Rhea_Polar_Maps_-_March_2012 polar] basemaps (March 2012) from Cassini images
* [http://www.ciclops.org/view/6626/The_Rhea_Atlas Rhea altlas] {{Webarchive|url=https://web.archive.org/web/20130424012008/http://www.ciclops.org/view/6626/The_Rhea_Atlas |date=2013-04-24 }} (released December 2010) from Cassini images
* [http://planetarynames.wr.usgs.gov/Page/RHEA/target Rhea nomenclature] and [http://planetarynames.wr.usgs.gov/images/rhea_comp.pdf Rhea map with feature names] from the [http://planetarynames.wr.usgs.gov USGS planetary nomenclature page]
* [https://www.google.com/maps/space/rhea/@3.4035608,-102.4187081,6475987m/data=!3m1!1e3 Google Rhea 3D], interactive map of the moon
* [https://www.bbc.co.uk/news/science-environment-11834954 Saturn's moon Rhea has thin atmosphere]
* [https://royalsocietypublishing.org/doi/epdf/10.1098/rstl.1686.0013 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]

{{Rhea|state=uncollapsed}}
{{Moons of Saturn}}
{{Solar System moons (compact)}}
{{Saturn}}
{{Atmospheres}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{Authority control}}

[[Category:Rhea (moon)| ]]
[[Category:Astronomical objects discovered in 1672|16721223]]
[[Category:Discoveries by Giovanni Domenico Cassini]]
[[Category:Moons of Saturn]]
[[Category:Moons with a prograde orbit]]