Rhea

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Rhea 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 -281degrees 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.

                                    Another Icy World

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Rhea is an icy body very similar to Dione, although slightly larger in size. It has a very low density, which indicates that it is composed mainly of water ice with rocky material making up less than one third of its total mass. The temperature on Rhea ranges from -281° F (-174° C) in the sunlight to -364° F (-220° C in the shade.

Rhea's density of 1.233 times that of liquid water suggests that Rhea is three quarters ice and one quarter rock. 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.

                                        Features of Rhea

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Rhea is one of the most heavily cratered satellites in the Solar System. The surface features of this moon can be divided into two different geological areas based on the density of the craters. The first area contains craters that are larger than 25 miles (40 km) in diameter. The largest of these craters is Izanagi. It has a diameter of 140 miles (225 km). Craters in the second area are smaller than 25 miles in diameter. The difference in these areas indicates that some great event served to resurface parts of this moon at some point in the past. Similar differences in crater size can be found on Mimas and Dione.

Like Jupiter's moon Callisto, Rhea's craters lack the high surface features found in craters on Mercury and the Moon. Like Dione, Rhea's orbit around Saturn is synchronous, meaning that it always keeps the same face toward Saturn. On the trailing hemisphere of Rhea can be found several bright, wispy lines. These linear features are believed to be fractures, and may indicate that Rhea has expanded and contracted due to internal heating and cooling. Similar linear features are found on Dione. Rhea has no detectable atmosphere
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                                Statistics for Rhea

Discovered by                            -    Giovanni Cassini
Year of Discovery                       -    1684
Diameter                                  -    950 miles (1, 529 km)
Mean Distance from Saturn          -    327,586 miles (527,040 km)
Rotational Period                       -    4.53 Days
Orbital Period                            -    4.52 Days
Orbital Eccentricity                     -    0.001
Orbital Inclination                       -    0.35 degrees
Mean Surface Temperature          -    -323 F (-197 C)
Main Atmospheric Component       -    None
Apparent Magnitude                    -    9.7