Neptune's largest moon, Triton, is larger than Pluto. Of all satellites in the Solar System, Triton has the second densest atmosphere, though it is still extremely thin. Its mainly nitrogen atmosphere is a characteristic shared with Earth, Titan, and perhaps Pluto. Triton's density is twice that of water; among the major outer satellites only Io and Europa are denser. The density indicates that Triton is about 75 percent rock. The remaining amount is ice, which Voyager data and chemical studies suggest is mostly nitrogen ice near the surface; ordinary water ice probably forms a "bedrock" not far below. This icy crust may extend to a depth of 300 km, below which it is expected that the composition would change to predominantly rock.With exception of Europa, Triton is the least cratered icy satellite, indicating that the volcanic landscape seen by Voyager is geologically young, being perhaps less than 600 million years in age. Although this seems old, it is less than one-seventh the age of the Solar System and is about the same average age as Earth's surface. Voyager 2 showed an unusual-looking terrain that is unique to Triton-the so-called "Cantaloupe terrain" because of its resemblance to the skin of a cantaloupe. It is believed to result from volcanic and tectonic processes, but the precise mechanism of formation is unknown. Other terrain's show rifts, cones with craters on their summits and flanks, dome-like extrusions, and steep-walled pits resembling terrestrial caldera's, thus appearing like some volcanic landscapes of Earth. Exotic lava's consisting of variations of water-ice-ammonia-methanol mixtures probably erupted to form these features.
Though it is still unclear what drives them, it may be that much of Triton's observed volcanic eruptions derive from the energy exchange caused by Triton's capture into orbit around Neptune, triggering melting and intense geological activity (it has been suggested that Triton, which orbits Neptune in a direction opposite that of all other large satellites in the Solar System, was captured by Neptune from a reservoir of similar objects theorized to exist beyond Neptune). The existence of active geysers and the possibility of ammonia in Triton's mantle suggest that the geologic activity may be more recent, even active. Or perhaps Triton may now be geologically dead, except for the occasional venting of gases, changes caused by wind-blown dust, and the migration of nitrogen frost from the south polar ice cap.