(mostly written in 2004, before the Cassini spacecraft reached Saturn)
 
Left: Leading side of Rhea viewed with Voyager 1 in 1980
Right: Trailing side of Rhea, showing wisps similar to those on Dione
Rhea is the second largest of Saturn's moons, approximately 950 miles in diameter. Although somewhat larger than Dione, it is otherwise very similar. Its density is slightly lower, but still greater than that of most Saturnian moons, indicating that it is made mostly of ice, but has a substantial minority of heavier materials. In addition, it has the same sort of two-sided appearance as Dione, with a heavily cratered leading edge, and a trailing edge with wispy features thought to be related to some sort of ancient ice vulcanism.
In the close-up of the leading edge, below, we can see not only huge numbers of larger craters, but also the suggestion of a streaky alignment of the smaller craters. This is probably the result of multiple impacts of moderate-sized objects ejected from larger impact craters, in a way similar to the formation of "rays" on our own Moon. Similar features probably exist on many other moons in the Solar System, but in most cases we don't have close-up images of the moons, and such small detail is not visible.
Close-up of leading side of Rhea, showing intense cratering, taken with the Voyager spacecraft in 1980. Note the streaky alignment of smaller craters at the "top" of the picture, suggesting multiple collisions by fragments of individual objects, such as ejecta from larger collisions, or fragile comets. (Voyager, NASA, Calvin J. Hamilton, apod030608) |
A portion of the side of Rhea which always faces Saturn, taken by the Cassini spacecraft during a close (60,000 miles distance) approach in March of 2006. An intensely cratered surface, which can have hardly changed in more than a billion years, save by additional cratering so frequent that the older craters no longer appear round, having been shattered by more recent cratering. About 1000 miles in diameter, Rhea is Saturn's second largest moon, although much smaller than Titan, which is four times its diameter, and sixty times its volume. Primarily made of water ice, Rhea is presumed to have a small rocky core, as mixtures of ice and rock this large should have generated some heat during their formation, allowing heavier materials to sink to the center. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod060530) |
Data for Rhea
Discovered by Giovanni Cassini in 1672
Named after one of the daughters of Uranus and Gaea
Orbital size 527,000 km (approximately 328,000 miles)
Orbital eccentricity 0%
Orbital inclination 0.3 degrees
Orbital period 4.5175 days
Rotation period 4.5175 days (synchronous rotation, one side permanently facing Saturn)
Diameter 1530 km (about 950 miles)
Mass 1/2400 of Earth, 1/30 of Earth's Moon
Surface gravity 3% of Earth, 1/6 of Earth's Moon
Density 1.3 times density of water (Composition probably mostly ice, some rock)
Albedo (reflectivity) 70%
Surface temperature 280 to 360 degrees below zero Fahrenheit (day to night variation)
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