Rhea () is the second-largest natural satellite of Saturn and the ninth-largest moon in the Solar System, with a diameter of 1,528 kilometres (949 mi). Rhea is the smallest body in the Solar System that is confirmed to be in hydrostatic equilibrium. It has a nearly circular orbit around Saturn, but it is also tidally locked, like Saturn's other major moons. It rotates with the same period it revolves or orbits. Thus, one hemisphere always faces towards the planet. The moon 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 (−281.2 °F) and −220 °C (−364.0 °F). 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. Cassini named the four moons he discovered (Tethys, Dione, Rhea, and Iapetus) Sidera Lodoicea (the stars of Louis) to honor King Louis XIV. Rhea was the second moon of Saturn that Cassini discovered, and the third moon discovered around Saturn overall.
Name Rhea is named after the Titan Rhea of Greek mythology, the mother of the first generation of Olympian gods and wife of Cronus, 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). Astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V. 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. 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 Cronus (Saturn, in Roman mythology), be used. Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek rho (the initial of Rhea) combined with the crook of the Saturn symbol as the symbol of Rhea (). This symbol is not widely used.
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. 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.
Physical characteristics
Size, mass, and internal structure
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 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. 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. The surface area of the moon can be estimated at 7,330,000 square kilometres (2,830,000 mi2), about the size of Australia (7,688,287 km2).
Before the Cassini–Huygens mission, it was assumed that Rhea had a rocky core. 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. 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. In the same year, another paper claimed the moment of inertia was about 0.37. Rhea being either partially or fully differentiated would be consistent with the observations of the Cassini probe. 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. 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. The triaxial shape of Rhea is consistent with a homogeneous body in hydrostatic equilibrium rotating at Rhea's angular velocity. 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. More recent indications are that Rhea has a homogeneous interior and therefore this ocean does not exist.
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.
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