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).Titan is the second largest satellite in the Solar System and the only satellite with an atmosphere thicker than Earth's; it has long interested planetologists. Like Earth's atmosphere, Titan's is mostly nitrogen but also includes methane, carbon monoxide, and ethane. Rainfall of liquid methane, ethane, and nitrogen is possible at the 93 K (-180 oC) surface temperature, leading to the belief that erosion may be a significant process at work there. Our only opportunity so far to see Titan's surface close up-when Voyager 1 passed by it in 1980-was thwarted by the thick orange smog. We have since discovered that the smog is transparent at certain infrared wavelengths, and so the Hubble Space Telescope was recently used to image Titan's surface, allowing us to see detail as small as 300 km. The overall reflectance of the surface suggests that it could be composed of dirty water ice. Brightness variations suggest that some sort of meteorologic and (or) geologic processes have modified the reflectivity of various parts of the surface.
The origin and geologic histories of the Saturnian satellites should be more fully understood after the future arrival of spacecraft such as the Cassini mission. In 2004, Cassini will enter into orbit around Saturn and drop a probe onto Titan's surface.