Rhea

There is also an asteroid named 577 Rhea.

Rhea

Rhea, taken by Cassini (NASA). The large crater near the top of the image is Tirawa.
Discovery
Discovered by	Giovanni Domenico Cassini
Discovered in	December 23, 1672
Orbital characteristics [1]
Semimajor axis	527,108 km
Eccentricity	0.0012583
Revolution period	4.518212 d
Inclination	0.345° (to Saturn's equator)
Is a satellite of	Saturn
Physical characteristics
Mean diameter	1528.8 km (1535.2×1525×1526.4) [2](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 (synchronous)
Axial tilt	zero
Albedo	0.65
Surface temp.	min mean max 53 K 99 K
Atmosphere	none

Rhea (ree'-a, IPA /ˈriːə/, Greek Ῥέᾱ) is the second largest moon of Saturn and was discovered in 1672 by Giovanni Domenico Cassini. It might have some rings around it.

Name

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 ([3]), wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.

Physical characteristics

Cassini color image of Rhea, showing the wispy trailing hemisphere

Rhea 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.

The leading hemisphere is heavily cratered and uniformly bright. As on Callisto, the craters lack the high relief features seen on the Moon and Mercury.

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.

Higher-resolution image of the wispy hemisphere, showing 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.

See also:

• List of geological features on Rhea

The Cassini orbiter performed a targeted flyby of Rhea on November 25, 2005 with an altitude of 500 kilometers.

External links

• The Planetary Society: Rhea

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