![]() The fact that Tethys and other Saturnian moons have such objects implies that their respective Lagrangian points have been stable against outside influences for millions of years. These three moons complete their orbits around Saturn moving as a unit, spaced out at 60-degree intervals first Telesto, then Tethys, followed by Calypso. These smaller moons are held in Lagrangian points (L4 and L5, respectively), where objects are stable with the larger controlling body. Tethys has gravitationally locked two smaller moons into its own subsystem - Telesto and Calypso. As with all but two of the major Saturnian moons, Tethys is tidally locked in phase with its parent planet - the same side always faces toward Saturn. Tethys orbits 183,000 miles (295,000 kilometers) from Saturn, taking 45.3 hours to circle the planet. ![]() The Cassini spacecraft has added details including a great variety of colors at small scales suggesting a variety of materials not seen elsewhere. ![]() The Voyager images showed a major impact crater and a great chasm. Tethys appeared as a tiny dot to astronomers until the Voyager (1 and 2) encounters in 19. Also contributing to the high reflectivity is that Tethys is bombarded by Saturn E-ring water-ice particles generated by geysers on Enceladus. Many of the crater floors on Tethys are bright, which also suggests an abundance of water ice. 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). ![]() 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. 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. 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. 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). Giovanni Cassini discovered Tethys on March 21, 1684. ![]()
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