tethys

Tethys is Saturn's fifth largest moon. Its irregular shape is 331 miles (533 kilometers) in mean radius, with dimensions 669 x 657 x 654 miles (1076.8 x 1057.4 x 1052.6 kilometers). This cold, airless and heavily scarred body is very similar to sister moons Dione and Rhea except that Tethys is not as heavily cratered as the other two. This may be because its proximity to Saturn causes more tidal warming, and that warming kept Tethys partially molten longer, erasing or dulling more of the early terrain.

Tethys' density is 0.97 times that of liquid water, which suggests that Tethys is composed almost entirely of water ice plus a small amount of rock.

Tethys has a high reflectivity (or visual albedo) of 1.229 in the visual range, again suggesting a composition largely of water ice, which would behave like rock in the Tethyan average temperature of -305 degrees Fahrenheit (-187 degrees Celsius). Many of the crater floors on Tethys are bright, which also suggests an abundance of water ice. Also contributing to the high reflectivity is that Tethys is bombarded by Saturn E-ring water-ice particles generated by geysers on Enceladus.

Tethys appeared as a tiny dot to astronomers until the Voyager (1 and 2) encounters in 1980 and 1981. The Voyager images showed a major impact crater and a great chasm. The Cassini spacecraft has added details including a great variety of colors at small scales suggesting a variety of materials not seen elsewhere.