Animation of cryovolcanism of Triton, the largest moon of Neptune. These icy geysers were observed by the Voyager 2 spacecraft. It is thought that solar heating vaporised nitrogen below a translucent surface, until the pressure built up enough to force an eruption. The gas picked up dust and other material, making the plumes visible. They reached a height of some eight kilometres, Triton is an unusual moon in that it orbits Neptune in the opposite direction to the planet's rotation. This indicates that it is a captured Kuiper Belt Object like the dwarf planet Pluto. Triton is larger than Pluto, and has a very similar composition.