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Rhea (moon)

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Not to be confused with the asteroid named577 Rhea.
Rhea
Mosaic of Rhea, assembled fromCassiniimagery taken on 26 November 2005
Discovery
Discovered byG. D. Cassini[1]
Discovery dateDecember 23, 1672[1]
Designations
Designation
Saturn V
Pronunciation/ˈriː.ə/[2]
Named after
ῬέᾱRheā
AdjectivesRhean/ˈriː.ən/[3]
Orbital characteristics [4]
527040km[5]
Eccentricity0.001[5]
4.518212d
8.48 km/s[a]
Inclination0.35°[5]
Satellite ofSaturn
Physical characteristics
Dimensions1532.4 × 1525.6 × 1524.4 km [6]
763.5±0.5 km[7]
7325342km2[b]
Mass(2.3064854±0.0000522)×1021kg[7](~3.9×10−4Earths)
1.2372±0.0029 g/cm3[7]
0.26m/s2[c]
0.3911±0.0045[8](disputed/unclear[9])
0.635 km/s
4.518212d
(synchronous)
zero
Albedo0.949±0.003(geometric) [10]
Surfacetemp. min mean max
Kelvin 53K 99 K
10 [11]

Rhea(/ˈriː.ə/) is the second-largestmoonofSaturnand theninth-largest moonin theSolar System,with a surface area that is comparable to the area ofAustralia.It is the smallest body in theSolar Systemfor which precise measurements have confirmed a shape consistent withhydrostatic equilibrium.Rhea has a nearly circular orbit around Saturn, but it is alsotidally locked,like Saturn's other major moons; that is, itrotateswith 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 moonDione,it has high-albedoice 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 byGiovanni Domenico Cassini.Since then, it has been visited by bothVoyager probesand was the subject of close targetedflybysby theCassiniorbiter in 2005, 2007, 2010, 2011, and once more in 2013.

Discovery[edit]

Rhea was discovered byGiovanni Domenico Cassinion 23 December 1672, with a 10.4-metre (34 ft) telescope made byGiuseppe Campani.[1][12]Cassini named the four moons he discovered (Tethys, Dione, Rhea, andIapetus)Sidera Lodoicea(the stars of Louis) to honor KingLouis XIV.[1]Rhea was the secondmoon of Saturnthat Cassini discovered, and the third moon discovered around Saturn overall.[1]

Name[edit]

Rhea is named after theTitanRheaofGreek mythology,the "mother of the gods" and wife ofKronos,theGreek counterpartof the godSaturn.It is also designatedSaturn V(being the fifth major moon going outward from the planet, afterMimas,Enceladus,Tethys,andDione).[13][14]

Astronomers fell into the habit of referring to them andTitanasSaturn IthroughSaturn V.[1]Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended toSaturn VII,and then toSaturn VIIIwith the discovery ofHyperionin 1848.[14]

Rhea was not named until 1847, whenJohn Herschel(son ofWilliam Herschel,discoverer of the planetUranusand two other moons of Saturn,MimasandEnceladus) suggested inResults of Astronomical Observations made at the Cape of Good Hopethat the names of the Titans, sisters and brothers of Kronos (Saturn, in Roman mythology), be used.[15][1]

Orbit[edit]

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

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

Physical characteristics[edit]

Size, mass, and internal structure[edit]

Size comparison ofEarth(right), theMoon(left top), and Rhea (left down)

Rhea is an icy body with adensityof 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 ofIce 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 ofTitan,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 about 7,330,000 square kilometres (2,830,000 sq mi), similar to Australia (7,688,287 km2).[22][b]

Before theCassini-Huygensmission, it was assumed that Rhea had a rocky core.[23]However, measurements taken during a close flyby by theCassiniorbiter in 2005 cast this into doubt. In a paper published in 2007 it was claimed that the axial dimensionlessmoment of inertiacoefficient 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 theCassiniprobe.[25]A year later yet another paper claimed that the moon may not be inhydrostatic 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 theCassiniradio 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]

Thetriaxialshape of Rhea is consistent with a homogeneous body inhydrostatic equilibriumrotating at Rhea's angular velocity.[27]Modelling in 2006 suggested that Rhea could be barely capable of sustaining aninternal liquid-water oceanthrough heating byradioactive decay;such an ocean would have to be at about 176 K, theeutectic temperaturefor 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[edit]

Main article:List of geological features on Rhea

Rhea's features resemble those ofDione,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.

Surface features on Rhea well defined due to the lighting

Rhea has a rather typical heavilycrateredsurface,[29]with the exceptions of a few large Dione-type chasmata or fractures (formerly known aswispy 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, calledTirawa,is roughly comparable in size to the basin Odysseus onTethys.[29]There is a 48 km-diameter impact crater at 112°W that is prominent because of an extended system of brightrays,which extend up to 400 km (250 mi) away from the crater, across most of one hemisphere.[30][32]This crater, calledInktomi,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 byLunar and Planetary Science.[30]Rhea'simpact cratersare more crisply defined than the flatter craters that are pervasive onGanymedeandCallisto;it is theorized that this is due to a much lowersurface gravity(0.26m/s2,compared to Ganymede's 1.428 m/s2and 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]

Closeup showing two craters on Rhea's surface taken in 2013 byCassinispacecraft

Its surface can be divided into two geologically different areas based oncraterdensity; 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 onCallisto,the craters lack the high relief features seen on theMoonandMercury.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 theiralbedo).[36]The extensive dark areas are thought to be depositedtholins,which are a mix of complexorganic compoundsgenerated on the ice bypyrolysisandradiolysisof simple compounds containing carbon, nitrogen and hydrogen.[37]The trailing side of Rhea's surface isirradiated by Saturn's magnetosphere,which may cause chemical-level changes on the surface, includingradiolysis(see§ Atmosphere). Particles from Saturn'sE-ringare 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 andcryovolcanicactivity: there arefaultsystems 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[edit]

The moons of Saturn are thought to have formed throughco-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 byErik AsphaugandAndreas Reuferfor the formation ofTitanmay also shine a new light on the origin of Rhea andIapetus.In this model, Titan was formed in a series ofgiant impactsbetween 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)
    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
    View of Rhea's leading hemisphere with crater Inktomi and its prominentray systemjust below center;impact basinTirawais at upper left

Atmosphere[edit]

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

Possible ring system[edit]

Main article:Rings of Rhea

On March 6, 2008,NASAannounced 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 asCassinipassed by Rhea.[44][45][46]Dust and debris could extend out to Rhea'sHill 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, whenCassinimade 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[edit]

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

There were five close targeted fly-bys by theCassiniorbiter, which was one part of the dual orbiter and landerCassini-Huygensmission. Launched in 1997,Cassini-Huygenswas targeted at the Saturn system; in total it took more than 450 thousand images.[50]Cassinipassed 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]

See also[edit]

Notes[edit]

  1. ^Calculated on the basis of other parameters.
  2. ^abThesurface areacan be estimated, given the radius, with the formula4πr2
  3. ^Surface area derived from the radius (r):4\pi r^2.
  4. ^The moons more massive than Rhea are: theMoon,the fourGalilean 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. SeeJPLSSD.
  5. ^More precisely, 0.3911.[24]
  6. ^More precisely, 0.3721.[25]

References[edit]

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