Neptune's largest moon, Triton, is the most exotic moon in that planet’s satellite system. Triton was discovered by British astronomer William Lassell in 1846, just 17 days after the discovery of Neptune. It is named after the Greek god of the sea. Triton is strange in several ways. It moves around Neptune in a retrograde direction, opposite to that of all other large satellites. Its orbit is also highly inclined. This unusual motion suggests that Triton may have originated as an interplanetary body, and Neptune’s gravity captured it into its present orbit. At a diameter of 2,705 kilometers, it’s about 18% bigger than Pluto, which orbits around the Sun in the same region as Neptune (the orbits of the two planets actually cross). Triton and Pluto may be two surviving examples of a whole group of bodies that formed near Neptune's orbit, called Kuiper Belt Objects. Triton was captured, and Pluto was left behind. Other objects may have crashed into Neptune long ago, or been gravitationally perturbed into orbits beyond Pluto.
Neptune's moon Triton as seen by Voyager 2. Click here for original source URL
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Click here for original source URL.
Another unusual trait of Triton is its sparse atmosphere. The composition resembles that of Saturn's satellite, Titan: mostly nitrogen with some methane. However, there’s only a very small amount of gas surrounding Triton; the surface pressure is tiny, barely a thousandth that of Mars. The surface is coated with what appears to be ice, but it's not water ice. Instead, it’s probably made of frozen nitrogen and methane. This may explain the thin atmosphere: it’s so cold that atmospheric gases freeze out onto the surface.
Voyager 2, the only spacecraft to visit the Neptune system, returned surprising images of smoking volcanic vents on Triton. The photos show plumes of dark smoke rising vertically 8 kilometers (26,000 feet) off the surface, then shearing off in high-altitude winds. These geysers are thought to form when sunlight melts part of the ice-covered surface, releasing material trapped beneath the ice. The plumes may be made of some combination of dust, methane, or liquid nitrogen. The near-absence of impact craters also indicates that the surface has been actively resurfaced in current geological time.
What is the source of the energy that drives the eruptions on Triton? Triton is the coldest body ever studied at close range, with measured surface temperature of 38 K (-391 °F) on the sunlit side! Theorists have pointed out that if Triton had been originally captured into an elliptical orbit around Neptune, tidal forces would have altered its orbit to the present circular shape, while heating the interior at the same time. This internal tidal heating was probably the source of the heat that resurfaced Triton with fresh ice and left smoking vents on its surface. Neptune’s tidal forces are also slowly robbing energy from Triton’s orbit, bringing it closer to the planet. Eventually, Triton will either impact Neptune, or perhaps break up due to tidal forces, forming a new ring around the planet