Rhea [pronounced REE-uh; adjective: Rhean] is the second largest moon of Saturn, but with a diameter of 1,528 km (949 miles) it is less than a third the size of Saturn’s largest moon, Titan. Rhea is a small, cold, airless body that is very similar to sister moons Dione and Tethys. As with the other two moons, Rhea is tidally locked in phase with its parent — one side always faces toward Saturn. Rhea’s surface temperatures are also similar to Dione and Tethys, being roughly as warm as -281 degrees Fahrenheit (-174 degrees Celsius) in sunlit areas and ranging down to -364 degrees Fahrenheit (-220 degrees Celsius) in shaded areas. Also like Dione and Tethys, Rhea has a high reflectivity (or geometric albedo) suggesting a composition largely of water ice, which behaves like rock in Rhea’s temperature range.
Rhea’s density of 1.233 times that of liquid water suggests that Rhea is three quarters ice and one quarter rock. Cassini spacecraft measurements from a close encounter showed a moment of inertia about its axis (a measure of how difficult it is to change its angular motion) of a higher value than what would be expected if Rhea has a rocky core. Thus, it is thought that Rhea is composed of a homogenous mixture of ice and rock — a frozen dirty snowball.
Rhea, at a distance of 527,040 km (327,490 miles), is farther away from Saturn than Dione and Tethys, and because of this Rhea does not receive ample tidal attraction from Saturn to cause internal heating. This has an important effect. Both Dione and Tethys have more areas of smooth plains than Rhea. Such plains are probably areas where liquid water reached the surface and ponded in depressions such as craters, forming flat surfaces before refreezing and thus erasing existing craters. The lesser internal warmth at Rhea could have resulted in fewer erasures, or there could have been more bombardment on Rhea. Whatever the reason, Rhea is more heavily cratered than Dione and Tethys.
Rhea appeared as a tiny dot to astronomers until the Voyager (1 and 2) encounters in 1980 and 1981. The Voyager images showed that Rhea’s features could be divided into two regions: the first being heavily cratered (bright) terrain with craters larger than 40 km (25 miles) across and a second type of area in parts of the polar and equatorial region with craters less than 40 km across. This difference may indicate there was a major resurfacing event some time in Rhea’s history. However, it would have been long ago because there are few young craters with rays extending away from them (as on Earth’s Moon), and the average age of the plains is thought to be around four billion years old.
The Voyager images also showed mysterious linear “wispy” lines with lengths of tens to hundreds of kilometers, often cutting through plains and craters. In 2006, Cassini spacecraft images showed that the wispy areas are subsidence fractures that make canyons (some of them several hundred meters high). The walls of those canyons are bright because darker material falls off them, exposing fresh bright water ice. These fracture cliffs show Rhea may have been tectonically active in its past. This type of surface feature also occurs on Dione and Tethys.
Giovanni Cassini discovered Rhea on 23 December 1672.
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“Except for Titan, Rhea is the largest of the saturnian satellites. Like Dione, its trailing hemisphere is dark and has bright wispy markings, whereas the leading hemisphere is uniformly bright. Parts of the surface of Rhea are dominated by large, degraded craters.” The Compact NASA Atlas of the Solar System by Ronald Greeley and Raymond Batson