|Rhea, taken by Cassini (NASA). The large crater near the top of the image is Tirawa.|
|Discovered by||Giovanni Domenico Cassini|
|Discovered in||December 23, 1672|
|Orbital characteristics |
|Semimajor axis||527,108 km|
|Revolution period||4.518212 d|
|Inclination||0.345° (to Saturn's equator)|
|Is a satellite of||Saturn|
|Mean diameter||1528.8 km (1535.2×1525×1526.4) (0.1198 Earths)|
|Surface area||7,300,000 km²|
|Mass||2.3166×1021 kg(3.878×10-4 Earths)|
|Mean density||1.24 g/cm3|
|Surface gravity||0.27 m/s2|
|Escape velocity||0.64 km/s|
|Rotation period|| 4.518212 d|
Rhea is named after the titan Rhea of Greek mythology. It is also designated Saturn V.
Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV. 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.
The names of all seven satellites of Saturn then known come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope (), wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.
Physical characteristicsRhea is an icy body with a density of about 1,240 kg/m3. This low density indicates that it has a rocky core taking up less than one-third of the moon's mass with the rest composed of water-ice. Rhean features resemble those of Dione, with dissimilar 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.
Rhea is heavily cratered and has bright wispy markings on its surface. Its surface can be divided into two geologically different areas based on crater 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 craters under that size. This suggests that a major resurfacing event occurred some time during its formation.
On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It has been thought that these bright swaths may be material ejected from ice volcanoes early in Rhea's history when it was still liquid inside. However, recent observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, show that the streaks are in fact ice cliffs, and it is plausible to assume that the bright streaks on the Rhean surface are also ice cliffs.The January 17, 2006 distant flyby by the Cassini spacecraft yielded images of the wispy hemisphere at better resolution and a lower sun angle than previous observations. While scientific analysis is still pending, raw images from the flyby seem to show that Rhea's streaks in fact are ice cliffs similar to those of Dione.
|Janus' group | Mimas | Enceladus | Tethys | Dione | Rhea | Titan | Hyperion | Iapetus | Inuit group | Gallic group | Norse group|
|See also: Pronunciation key | Rings of Saturn|