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-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 byGiovanni Domenico Cassini. Since then, it has been visited by bothVoyager probes and was the subject of close targetedflybys by theCassini orbiter in 2005, 2007, 2010, 2011, and once more in 2013.
Astronomers fell into the habit of referring to them andTitan asSaturn I throughSaturn V.[1] Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended toSaturn VII, and then toSaturn VIII with the discovery ofHyperion in 1848.[14]
Rhea was not named until 1847, whenJohn Herschel (son ofWilliam Herschel, discoverer of the planetUranus and two other moons of Saturn,Mimas andEnceladus) suggested inResults of Astronomical Observations made at the Cape of Good Hope that the names of the Titans, sisters and brothers of Cronus (Saturn, in Roman mythology), be used.[15][1]
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a Greekrho (the initial of Rhea) combined with the crook of the Saturn symbol as the symbol of Rhea (). This symbol is not widely used.[16]
The orbit of Rhea has very loweccentricity (0.001), meaning it is nearly circular. It has a lowinclination of less than a degree, inclined by only 0.35° from Saturn's equatorial plane.[5]
Rhea istidally locked and rotates synchronously; that is, itrotates at 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.[17][18]
Rhea is the second largest moon of Saturn, but with a mean diameter of 1,528 kilometers (949 miles) it is less than a third the radius of Saturn's largest moon,Titan. Rhea is an icy body with adensity 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 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.[19][20][21] Although Rhea is the ninth-largest moon in the Solar System, it is only the tenth-most massive. 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), about the size of Australia (7,688,287 km2).[22][b]
Before theCassini–Huygens mission, it was assumed that Rhea had a rocky core.[23] However, measurements taken during a close flyby by theCassini orbiter in 2005 cast this into doubt. In a paper published in 2007 it was claimed that the axial dimensionlessmoment 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 theCassini probe.[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 theCassini 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]
Thetriaxial shape of Rhea is consistent with a homogeneous body inhydrostatic equilibrium rotating at Rhea's angular velocity.[27] Modelling in 2006 suggested that Rhea could be barely capable of sustaining aninternal liquid-water ocean through heating byradioactive decay; such an ocean would have to be at about 176 K, theeutectic 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]
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 heavilycratered surface,[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 craters are more crisply defined than the flatter craters that are pervasive onGanymede andCallisto; it is theorized that this is due to a much lowersurface gravity (0.26m/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]
Closeup showing two craters on Rhea's surface taken in 2013 byCassini spacecraft
Its surface can be divided into two geologically different areas based oncrater 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 onCallisto, the craters lack the high relief features seen on theMoon andMercury. 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 compounds generated on the ice bypyrolysis andradiolysis of 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-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 andcryovolcanic activity: there arefault 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]
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 Asphaug andAndreas Reufer for the formation ofTitan may also shine a new light on the origin of Rhea andIapetus. In this model, Titan was formed in a series ofgiant 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 prominentray system just below center;impact basinTirawa is at upper left
On November 27, 2010,NASA announced 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 105 to 106 molecules in a cubic centimeter, depending on local temperature. The main source of oxygen isradiolysis of 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 tooxidation of the organics present in ice or tooutgassing of the moon's interior.[38][42][43]
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 asCassini passed 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, whenCassini 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]
The first images of Rhea were obtained byVoyager 1 & 2 spacecraft in 1980–1981.
There were five close targeted fly-bys by theCassini orbiter, which was one part of the dual orbiter and landerCassini–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]
^abThesurface area can be estimated, given the radius, with the formula4πr2
^Surface area derived from the radius (r):4\pi r^2.
^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.
^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.ISBN978-1-4020-9216-9.
^abcAnderson, 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.
^abIess, 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.
^abcdWagner, 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.
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