Saturn: Moons: Rhea
Rhea [pronounced REE-uh; adjective: Rhean] is the second largest moon of Saturn, but with a diameter of 1,528 kilometers (949 miles) it is less than a third the size of the 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 -174 degrees Celsius (-281 degrees Fahrenheit) in sunlit areas and ranging down to -220 degrees Celsius (-364 degrees Fahrenheit) in shaded areas. Also like them, it has a high reflectivity (or geometric albedo) suggesting a composition largely of water ice, which behaves like rock in Rhea's temperature range.
At 527,040 kilometers (327,490 miles), Rhea is farther away from Saturn than these other two moons, so there is much less tidal attraction from parent Saturn to cause internal heating on Rhea. This has an important effect. The other two moons 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 I and Voyager II 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 kilometers (25 miles) across and a second type of area in parts of the polar and equatorial region with craters less than 40 kilometers 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.
Rhea's "Wispy" Lines
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, and again this type of surface feature also occurs on Dione and Tethys.
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.
Discovery
Giovanni Cassini discovered Rhea in 1672. John Herschel suggested that the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus, known to the Romans as Saturn.
Origin of Names
The name Rhea comes from the Greek goddess (or titan) Rhea, who was the daughter of Uranus and Gaea. Her husband was Cronus (the Roman Saturn). Rhea was also called the mother of the gods because she gave birth to several of the gods of Mount Olympus, including Zeus (the Roman Jupiter).
Cassini referred to Rhea as one of the Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three were Tethys, Dione, and Iapetus). Astronomers also refer to Rhea as Saturn V denoting the fifth moon in distance from Saturn. Geological features on Rhea generally get their names from people and places from creation myths. The International Astronomical Union now controls naming of astronomical bodies.
