Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System.
Rhea is an icy body with a density of about 1.233 g/cm³. It is made of ~25% rock (density ~3.25 g/cm³) and ~75% water ice (density ~0.93 g/cm³), with an icy core. While Rhea is the ninth largest moon, it is only the tenth most massive moon.
Rhea’s 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 covered with craters, including several large impact basins such as Tirawa. It also 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 20 miles in diameter, whereas the second area, in parts of the polar and equatorial regions, has only 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.