Rhea (moon)
Second-largest moon of Saturn, smallest hydrostatic body.
Rhea, Saturn's second-largest moon and the ninth-largest in the Solar System, measures 1,528 kilometers (949 miles) across. It holds the distinction of being the smallest known body in the Solar System that has settled into a rounded, hydrostatic equilibrium shape. The moon follows a nearly circular path around Saturn and is tidally locked, so the same hemisphere perpetually faces the planet, rotating once per orbit.
With a low density of about 1.236 g/cm³, Rhea is composed of roughly three-quarters water ice and one-quarter rock. Its surface is heavily cratered and shows distinct differences between its leading and trailing hemispheres. Bright, wispy streaks—high-albedo ice cliffs similar to those on Saturn's moon Dione—are visible from space. Surface temperatures range from −174 °C (−281.2 °F) to −220 °C (−364.0 °F).
Giovanni Domenico Cassini discovered Rhea on 23 December 1672 using a 10.4-meter (34-foot) telescope built by Giuseppe Campani. He named it, along with Tethys, Dione, and Iapetus, the *Sidera Lodoicea* (stars of Louis) in honor of King Louis XIV. Rhea was Cassini's second Saturnian moon discovery and the third moon ever found orbiting Saturn.
The moon is named after the Titan Rhea from Greek mythology, mother of the first Olympian gods and wife of Cronus (the Greek equivalent of Saturn). It is also designated Saturn V, as the fifth major moon outward from the planet after Mimas, Enceladus, Tethys, and Dione. Astronomers originally referred to these moons and Titan as Saturn I through Saturn V. After Mimas and Enceladus were found in 1789, the numbering extended to Saturn VII, and to Saturn VIII with Hyperion's discovery in 1848. Rhea itself was not named until 1847, when John Herschel proposed using the names of the Titans—siblings and sisters of Cronus—in his *Results of Astronomical Observations made at the Cape of Good Hope*. A proposed symbol for Rhea, a Greek rho combined with the crook of Saturn's symbol, was designed by Denis Moskowitz but is not widely used.
Rhea's orbit is nearly circular, with an eccentricity of just 0.001, and is inclined only 0.35° from Saturn's equatorial plane. Because it is tidally locked, one hemisphere always faces Saturn (the near pole), while the leading hemisphere always points forward in its orbit and the trailing hemisphere faces backward.
Quick Facts
- Mpc Name
- Saturn V
- Pronounced
- ˈ · r · eɪ · . · ə
- Named After
- Ῥέᾱ Rheā
- Adjectives
- Rhean ˈ · r · eɪ · . · ən
- Discoverer
- G. D. Cassini
- Discovered
- December 23, 1672
- Periapsis
- 526 · 512 km
- Apoapsis
- 527 · 567 km
- Eccentricity
- 0.001
- Inclination
- 0.35°
- Satellite Of
- Saturn
Facts from the source article.
Lore & Background
Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672 using a 10.4-metre 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. The moon was not named until 1847, when John Herschel suggested using the names of the Titans, sisters and brothers of Cronus (Saturn). Rhea is named after the Titan Rhea of Greek mythology, the mother of the first generation of Olympian gods and wife of Cronus. It is also designated Saturn V, being the fifth major moon going outward from the planet.
Rhea has a low density of about 1.236 g/cm³, indicating it is composed of roughly three-quarters ice and one-quarter rock. The surface is heavily cratered, with distinct leading and trailing hemispheres. Like Dione, it has high-albedo ice cliffs that appear as bright wispy streaks. The surface temperature varies between −174 °C and −220 °C. Rhea has two very large impact basins on its hemisphere facing away from Saturn, about 400 and 500 km across. The crater Inktomi, nicknamed 'The Splat', is 48 km in diameter and may be one of the youngest craters on the inner moons of Saturn. The moon's interior is debated: measurements from the Cassini orbiter in 2005 suggested a homogeneous interior with a moment of inertia of about 0.4, but other analyses have disputed this, and some claim Rhea may not be in hydrostatic equilibrium. Modeling in 2006 suggested a possible internal liquid-water ocean, but more recent indications are that Rhea has a homogeneous interior and therefore this ocean does not exist.
Rhea 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. Its surface can be divided into two geologically different areas based on crater density, suggesting a major resurfacing event occurred some time during its formation. The leading hemisphere is heavily cratered and uniformly bright, while the trailing hemisphere has a network of bright swaths on a dark background, believed to be tectonic features such as depressions and troughs with ice-covered cliff sides.
Reader's Guide
Rhea holds significance as the second-largest moon of Saturn and the smallest body in the Solar System confirmed to be in hydrostatic equilibrium. Its discovery by Giovanni Domenico Cassini in 1672 contributed to the early mapping of Saturn's satellite system. The moon's composition—roughly three-quarters ice and one-quarter rock—and its heavily cratered surface provide clues about the history of the Saturnian system. The debate over Rhea's interior structure, with measurements from the Cassini orbiter suggesting a homogeneous interior but other analyses disputing this, highlights the challenges of interpreting remote-sensing data. The presence of bright wispy streaks and large impact basins like Tirawa and Inktomi ('The Splat') offer insights into the moon's geological history. Rhea's nearly circular orbit and tidal locking are typical of Saturn's major moons. The moon's legacy includes being visited by both Voyager probes and multiple targeted flybys by the Cassini orbiter, which provided detailed data on its surface and interior. The naming convention established by John Herschel in 1847, using Titans from Greek mythology, set a precedent for naming Saturn's moons.
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