Neptune's largest moon has an unusual orbit that suggests it formed elsewhere before being captured by the outermost planet's gravity. Triton is the only large moon in the Solar System that moves in the opposite direction to its planet's rotation.
In 1989 the Voyager 2 probe showed scientists close-up views of Triton's ice-covered, relatively crater-free surface.
The English amateur astronomer William Lassell discovered Triton in 1846.
Photo: Triton taken by the Voyager probe (NASA)
Triton's unusual orbit is an important clue about its past.
Backwards Triton falls under Voyager's gaze.
The Voyager 2 probe, launched in 1977, flew by Triton in 1989. The moon's unexpected, miles-high nitrogen geysers proved that geologic activity is possible in the coldest outer reaches of the Solar System.
It is predicted that Saturn will lose its rings.
In one hundred million years it is predicted that Saturn's rings will disappear. In one billion years, Neptune may develop its own rings when its moon Triton spirals from orbit and disintegrates into countless fragments.
Triton is the largest natural satellite of the planet Neptune. It was discovered on October 10, 1846, by English astronomer William Lassell. 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. At 2,700 kilometres (1,700 mi) in diameter, it is the seventh-largest moon in the Solar System. Because of its retrograde orbit and composition similar to Pluto's, Triton is thought to have been a dwarf planet captured from the Kuiper belt. Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust, an icy mantle and a substantial core of rock and metal. The core makes up two-thirds of its total mass. Triton has a mean density of 2.061 g/cm3 and is composed of approximately 15–35% water ice.
Triton is one of the few moons in the Solar System known to be geologically active (the others being Jupiter's Io and Saturn's Enceladus). As a consequence, its surface is relatively young with sparse impact craters, and a complex geological history revealed in intricate cryovolcanic and tectonic terrains. Part of its surface has geysers erupting sublimated nitrogen gas, contributing to a tenuous nitrogen atmosphere less than 1/70,000 the pressure of Earth's atmosphere at sea level.