With an average diameter of 2700 kilometres (about 1700 miles), Triton is Neptune’s largest moon, and the seventh largest of all the moons in the Solar System. It orbits its parent planet in a reverse direction, the only large moon to do so (the others that do are tiny, irregular objects). Beneath its icy crust lies a mantle of water, which may be liquid if underground temperatures are high enough. On the surface, however, it is extremely cold. With an average surface temperature of -238°C (-396°F), Triton is one of the coldest worlds in the Solar System.

Cantaloupe terrain

Scientists discovered more about the surface of Triton when the Voyager 2 space probe flew past in 1989 at a distance of about 40,000 kilometres (25,000 miles), sending back detailed images.

​​​​​​​They revealed an icy surface featuring few craters, but covered by a dense network of ridges and valleys. This is described as "cantaloupe" terrain, because of its similarity—when viewed from space—to the skin of a cantaloupe melon. It is probably the result of the ice melting to a slush, then refreezing. Triton’s granite-hard surface is made up of 55% nitrogen ice, with water ice and frozen carbon dioxide mixed in. A thin coating of pure nitrogen ice covers some, perhaps all, of the moon's surface.

Tidal heating

Triton’s warm underground ocean is a leftover from a time when the entire moon was so hot it was in a liquid state (astronomers know this from studies of its internal structure). Where did this heat come from? A clue to its possible origin is Triton's unusual orbit: it orbits Neptune in the opposite direction to the spin of the planet. This suggests that Triton was not formed at the same time as Neptune, but "captured" at a later date from elsewhere—most likely the Kuiper Belt, a vast zone of icy objects surrounding the Solar System. Triton may once have been a dwarf planet resident in this zone, as Pluto, Haumea and Makemake are today.

Like our own Moon, Triton orbits with the same side facing inwards to its parent planet at all times: it is "tidally locked". Soon after its capture, Triton would have taken a wide elliptical path around Neptune, meaning that Neptune's gravitational pull would be constantly stretching and squashing the moon, as it approached then moved further away. The friction of these tidal forces would heat up Triton's interior. This underground heating causes​​​​​​​cryovolcanic activity on the moon, in which liquid water, ammonia and methane erupt through cracks in its crust. The freezing of ices at the surface creates Triton's jagged "icescape".
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