Triton is, with a diameter of 2,700 km, the largest of Neptune’s 13 moons. It is the only large moon in the Solar System with a retrograde orbit (an orbit in the opposite direction to its planet’s rotation), which cannot have formed out of the same region as Neptune, so it must have been captured from elsewhere. Because of its retrograde orbit and composition similar to Pluto’s, Triton is thought to have been captured from the Kuiper belt.
Triton is locked in synchronous rotation with Neptune (one side faces the planet at all times), but because of its unusual orbital inclination both polar regions take turns facing the Sun.
It has a sparsely cratered surface with smooth volcanic plains, mounds and round pits formed by icy lava flows. It consists of a crust of frozen nitrogen and water ice over an icy mantle believed to cover a core of rock and metal. The core makes up two-thirds of its total mass and is composed of approximately 15–35% water ice. Its high density implies that Triton contains more rock in its interior than the icy satellites of Saturn and Uranus.
Triton, Io and Venus are the only bodies in the solar system besides Earth that are known to be volcanically active at the present time. As a consequence, Triton’s surface is extremely young. Its entire surface is cut by complex valleys and ridges, probably the result of tectonics and icy volcanism. Part of its crust is dotted with geysers believed to erupt nitrogen.
The southern polar region of Triton is covered by a highly reflective cap of frozen nitrogen and methane sprinkled by impact craters and openings of geysers. Triton is one of the coolest objects in our solar system. It is so cold that most of its nitrogen is condensed as frost, giving its surface an icy sheen that reflects 70 percent of the sunlight that hits it.
Triton has a thin nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near the surface. The atmosphere of Triton most likely originates from its volcanic activity.
It is named after the Greek sea god Triton, the son of Poseidon (the Greek god comparable to the Roman Neptune).