Triton is the largest moon of Neptune and a body unlike any other large moon in the Solar System. It orbits Neptune in a retrograde direction on a highly inclined orbit — the only large moon to do so — and it has a surface and composition almost indistinguishable from the dwarf planet Pluto. Together these facts strongly imply that Triton formed in the Kuiper Belt and was captured by Neptune early in the Solar System’s history, almost certainly disrupting Neptune’s original moon system in the process.

Triton’s surface, imaged up-close only by Voyager 2 in August 1989, shows nitrogen-ice plains, “cantaloupe terrain” of hummocky pits, and — spectacularly — active nitrogen geysers erupting 8 km into a tenuous atmosphere. The geysers are driven by seasonal subsurface warming and are one of only four places in the Solar System where active volcanism has been seen in progress (the others being Earth, Io, and Enceladus).