Triton

Triton is the largest moon of Neptune, by far. Its diameter of 1680 miles (2700 km) makes it slightly smaller than the Earth's Moon. Triton orbits Neptune at a distance of 220,000 miles (355,000 km) with a period of 5.88 days. Triton orbits in the opposite direction (retrograde) from the direction of Neptune's rotation; it is the only large moon in the solar system which does this.

Images from the Voyager 2 spacecraft showed Triton to be a geologically active moon, with a young and complex surface lacking many impact craters. Triton's surface is mostly covered by nitrogen frost, mixed with traces of condensed methane, carbon dioxide, and carbon monoxide; there are also polar caps. Triton has a very high albedo; this high reflectivity makes its surface one of the coldest in the solar system. Voyager 2 measured the surface temperature at a very low -240°C K (-400°F). Triton has a tenuous but windy nitrogen atmosphere. Voyager 2 observed nitrogen geysers erupting from Triton's surface, being blown horizontally by the winds, and then depositing dark material on the surface.

Triton's retrograde orbit makes it is unlikely that Triton could have formed at the same time and place as Neptune itself. Triton is also similar in size, density, and chemical composition to Pluto - the best-known Kuiper Belt object - and for these reasons, there is strong speculation that Triton was once a Kuiper Belt object that has been captured into orbit around Neptune.