Most of the satellites in the solar system fall into one of three categories: regular satellites that formed about their parent planets, collisional fragments of regular satellites, and captured asteroids. The Moon, Triton, and Charon do not fit into any of these three categories.
Nearly all of the satellites of the outer planets have densities less than 2000 kg/m3. This is much lower than the density of rocky material, 3500 kg/m3, so ices must make up a large fraction of the material of most satellites. Most of the ice in the outer solar system is probably frozen water.
There are three sources of internal heat for satellites: the conversion of gravitational energy into heat during formation, radioactive heating, and tidal heating. The thermal history of a satellite depends not only on these sources of energy, but also on how easily heat is transported to its surface by convection and conduction.
Phobos and Deimos, the satellites of Mars, are believed to be captured asteroids. Their surfaces are pocked by craters. Cracks on Phobos radiate from the crater Stickney, which was produced by an impact nearly violent enough to destroy Phobos.
The intense tidal heating of Io makes it the most volcanically active body in the solar system. Volcanic material is deposited so rapidly on Io's surface that all evidence of impact cratering has been covered up. Sulfur and its compounds provide the yellow and red colors of Io's surface and may also power the plumes that propel material hundreds of kilometers above Io's surface.
Europa has a very smooth, icy surface even though it is made mostly of rock. Europa's surface has been smoothed by glacierlike flows and, probably, by flows of water from the interior. A thick ocean of water probably exists below its icy crust.
Ganymede's surface shows evidence of past internal activity and crustal movement. Callisto's heavily cratered surface shows no evidence of any internal activity. Both Ganymede and Callisto may have oceans of water far beneath the surface.
Saturn's icy satellites show evidence of a wide range of internal activity. At one extreme is Rhea, which has a heavily cratered surface, while at the other extreme is Enceladus, which has been extensively resurfaced. Tidal heating may explain why Rhea seems to have experienced little or no internal activity while Enceladus seems to have experienced so much.
Saturn's largest satellite is Titan. Its atmosphere contains mostly nitrogen and is thicker than the Earth's. Titan is cold enough that atmospheric ethane can condense to liquid form. In fact, Titan may have ethane rain and lakes or oceans of ethane.
All of the larger satellites of Uranus have been more active than the Saturnian satellites of comparable size. Tidal heating may have powered this internal activity.
Uranus's satellite, Miranda, is very unusual in appearance. Its surface is a jumbled mixture of different types of terrain. The way in which Miranda evolved to its present form is not known.
Triton orbits Neptune in a retrograde direction and is slowly spiraling inward. Triton probably formed elsewhere and was captured into a retrograde orbit. The surface of Triton shows evidence of considerable resurfacing and internal activity. Plumes of dark material erupt from beneath the surface of Triton and are carried far downwind by its thin atmosphere.
