Triton is Neptune’s largest moon by size, and overall is the solar system’s seventh-largest. Like Neptune, it is relatively poorly understood because of the lack of dedicated space probes to planets beyond Saturn, but it is believed to be a captured Kuiper Belt object. If this theory is eventually confirmed through further study, it would mean that Neptune is essentially similar to Pluto, a dwarf planet which also belongs to the Kuiper population.
Neptune was the first planet discovered by mathematicians, and the discovery of Triton, in 1846, followed just days after the confirmation of the existence of the planet itself. The name was drawn from the son of Poseidon, the Greek god of the sea (whose name in the Roman pantheon, Neptune, was used for the newly discovered, giant blue planet). Their discoverer, brewer and amateur astronomer William Lassell, actually did not suggest the name himself before moving on to discover moons around Uranus. It was not until a second moon was discovered orbiting Neptune, much later, did it become necessary to actually give a proper name to Lassell’s moon.
Triton is a large moon with a radius of about 1350 kilometres, about three-quarters the size of Earth’s own moon. Its orbit, at 350,000 kilometres, is also about the same as the Moon’s distance from Earth. Because Neptune is not graced with the massive moon systems orbiting Saturn or Jupiter, this also means that Triton accounts for virtually all of the mass orbiting its planet – both the twelve other moons, as well as the tenuous ring system. It has a thin nitrogen atmosphere, possibly fueled through the eruption of geysers on the frigid surface.
Triton also, however, has one bizarre feature – which in turn points to what astronomers now believe is a very unusual history. Triton has what is known as a retrograde orbit, meaning it rotates around Neptune in the opposite direction from Neptune’s own rotation around the Sun. No other large moons in the solar system orbit in this manner, although Pluto does have a retrograde orbit around the Sun. Many of the small outer moons of Jupiter, Saturn, and Uranus have retrograde orbits, however – and all of these, it is believed, used to orbit the Sun directly before being captured by the gravity of the gas giants. If the same is true of Triton, then it must have been tugged by Neptune out of the distant population of icy asteroids and comets known as the Kuiper Belt. Pluto, which still orbits the Sun directly, belongs to the same population of objects, although almost all of them are very small in comparison to either Pluto or Triton. Triton was presumably captured when it passed at the right angle through Neptune’s existing moon system; in order to decelerate into its current orbit, it might have collided with a former Neptunian moon, or even (although the odds of this happening at the right moment seem extraordinarily slim) another independently orbiting Kuiper Belt object.
Unfortunately, much less is known about Triton than about the major moons of Saturn or Jupiter. Only Voyager 2 has ever approached Neptune, and at this time neither NASA nor any other space agency has serious plans to build a new space probe specifically to explore either Neptune or any of its moons, including Triton. As a result, the best data source for Triton is still the roughly one-half of the surface imaged by Voyager 2, which turned up potentially intriguing features like water-ammonia volcanoes, nitrogen geysers, and polar ice caps. As much as half of the moon’s surface consists of a unique dimpled or lumpy terrain named “canteloupe” terrain, after its resemblance to the skin of that fruit. How such terrain could have developed is still unknown.