RHEA

one of the eight large, regular satellites of Saturn. It has a diameter of about 1,530 km (949 miles) and orbits Saturn in a prograde, nearly circular orbit at a mean distance of 527,040 km (326,765 miles). Rhea's density is 1.3 times that of water, and so the satellite is assumed to be composed mostly of water ice. Infrared spectra also show a surface composed mainly of water frost. The reflectivity of Rhea's surface is comparatively high, but large regional variations are apparent. Like most of the other satellites of Saturn, Rhea is locked in a rotational commensurability, keeping the same hemisphere toward the planet. Like that of Dione, another Saturn satellite, the hemisphere that points in the direction of orbital motion is bright and heavily cratered, whereas the trailing hemisphere is darker with bright wispy streaks, a paucity of craters, and indications of resurfacing. In spite of the fact that Rhea's surface is icy rather than rocky, the cratered side of the satellite strongly resembles the extensively cratered highlands of Mercury or of the Moon. Its surface is in fact the most heavily cratered in the Saturn system. Ice apparently has mechanical properties similar to rock under the very low equilibrium temperatures at Saturn's orbit (about 90 K). The bright, linear, polygonal streaks observed on the trailing hemisphere of Rhea may be the result of major resurfacing and the escape of volatiles such as water or methane gas through fissures. Bright patches in craters may also be attributed to the transport of volatiles.