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 models 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).Dione is very similar to Tethys, though it lacks a large impact crater and its canyons are not so immense. Any geologic activity that may have occurred on Rhea occurred long ago and evidence has been obliterated by impact cratering.