All of Saturn's larger satellites are made mostly of water ice with some lesser rock component. With the exception of Enceladus, whose surface appears to be virtually pure ice, these moons' surfaces are made of dirty ice, with model suggesting increasing amount of rocky material toward their centers. Enceladus, Tethys, and Dione all exhibit widely varied crater densities and large canyons, indicating that they have all been substantially affected by geologic activity. A likely geologic process on these icy bodies is cryovolcanism, which involves partial melting of an icy satellite's interior and resultant outpourings of icy liquids. Ammonia-water cryovolcanism is widely suspected on all three objects, because the amount of heat available to drive geologic activity there is very meager, and ammonia-water ices (an expected component of Saturnian satellites) have a very low melting point, 176 K (-97 oC).Tethys has a huge canyon system, Ithaca Chasma, that is 3 km deep, up to 100 km wide, and extends three-quarters of the way around the satellite. This canyon system is similar in size to Valles Marineris on Mars but occurs on a body only one-sixth the diameter. The formation of the enormous impact crater Odysseus may be related to the origin of the canyon. The moderately to heavily cratered surface indicates all this activity took place early in Tethys' history, and little has happened since. The moderately cratered plains apparently were produced by cryovolcanism similar to that postulated for Enceladus.