Rhea is Saturn's second-largest moon, after Titan. It takes its name from Rhea, a Titan in Greek mythology who was the brother and husband of Cronus, and the mother of Zeus and his siblings. All of Saturn's moons are named after Titans.
- Physical Characteristics -
Rhea has a diameter of about 1500 kilometres (930 miles), measurably less than the Earth's own Moon but still large enough to have compressed into a spherical shape under the force of its own gravity. From a distance, it appears more or less to resemble Earth's own Moon, with a grey, rocky surface pitted by impact craters left by billions of years of impacts. Beneath the surface, however, the moon is quite different: like many bodies at this distance from the Sun, about three-quarters of its volume consists of ice rather than rock. Like many moons, Rhea is also tidally locked with Saturn, meaning that its period of rotation (i.e. one "day" on Rhea) lasts the same amount of time as its orbital period around the planet. The same process affects our own Moon, but in Rhea's case, its orbit and day last just 4.5 Earth-days.
Cassini's close approaches to Rhea, however, have turned up evidence of something more remarkable: Rhea might have its own ring system. The rings, if they do exist, are so thin and tenuous that they have never been imaged - but some anomalous readings from Voyager 1, and some more detailed readings by Cassini, do indicate some sort of disruption in a ring around Rhea which might be explained by rings. If this is ever confirmed, it would make Rhea the first and so far only moon in the solar system to have a ring system, although one theory suggests that the bizarre equatorial ridge on Iapetus is the remains of a ring system, as well.
- Discovery and Exploration -
Rhea was first discovered by the 17th-century European astronomer Giovanni Cassini, along with several of its companions. However, the name was not given until two centuries later. Like Titan, it is large enough that some studies can be completed by means of telescopes from Earth.
The most striking images of Rhea, however, of course come from 20th-century space probes. The first comparatively close-up images came from Voyager 1, when it flew past Saturn at some distance around 1980. In the past five years, quite a number of highly detailed images have been taken by the Cassini probe (named after the astronomer Cassini). Because Rhea lacks the intriguing features of Titan, it is a lesser subject of research interest, although new images will probably continue to come in. On its closest approach, Cassini zipped by at an almost uncomfortably close 100 kilometres (60 miles).