Rhea (moon)
Rhea is the second largest moon of Saturn. It is particularly heavily cratered. It was discovered in 1672 by Giovanni Cassini. Rhea is an icy body with a density of 1.33 gm/cm3. 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. Rhea's features resemble those of Dione, with dissimilar leading and trailing hemispheres, suggesting similar composition and histories. The temperature on Rhea is -174°C in direct sunlight and between -200°C and -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 kilometers in diameter, whereas the second area, in parts of the polar and equatorial regions, has craters under 40 kilometers in diameter. This suggests that a major resurfacing event occurred some time during its formation.
The leading hemisphere is heavily cratered and uniformly bright. Like 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 is thought that these bright swaths may be material ejected from ice volcanoes early in Rhea's history when it was still liquid inside.