The Solar System's large moons tend towards the weird and wonderful, and Triton is no exception. It's one of the few Solar System bodies that show geological activity, and the only large moon to orbit in the opposite direction to its planet's rotation. Neptune has fourteen known moons and Triton's mass accounts for about 99.7% of their total mass.
How Triton got its name
The planet Neptune was discovered in 1846 due to its gravitational effect on Uranus. When he received news of the discovery, British astronomer John Herschel suggested that his friend William Lassell look for Neptunian moons. Just over a week later – seventeen days after Neptune's discovery – Lassell had discovered the moon we now call Triton.
It took some time to get agreement on a name for the new planet, but eventually Neptune, the name of the Roman sea god, was accepted. They just called the moon Neptune's satellite. In 1880 French astronomer Camille Flammarion suggested the name Triton for it. Triton was Neptune's son and messenger. He was represented as a merman, and could calm the waves by blowing through his conch shell horn.
The moon didn't actually need a name until 1949 when Gerard Kuiper discovered a second Neptunian moon. He named it Nereid after the sea nymphs who were Neptune's attendants. Following this discovery, the largest moon was also given a name.
How Triton compares with other moons
Triton is the seventh largest Solar System moon. With a radius of about 1350 km, it's closest in size to Jupiter's moon Europa which has a radius of about 1560 km. Although they have similar radii, Europa is 50% more dense than Triton.
Our Moon is about 384,400 km from Earth. Triton is somewhat closer than that to Neptune at 354,800 km. For these two moons – and almost every other moon we know of – a day is equal to a month. They rotate on their axes once during the time they orbit their planets. But Triton moves much faster than the Moon, circling Neptune in less than six Earth days.
Triton's orbit is almost perfectly circular, but it's highly tilted to Neptune's orbit. Its axis of rotation is also tilted so much compared to Neptune's axis that its polar and equatorial regions are alternately pointed at the Sun. This means that seasons on Triton show extreme changes.
Perhaps the most surprising feature of Triton's orbit is that it is retrograde - it orbits in the opposite direction to Neptune's spin. Since the Solar System formed from a revolving debris disk around the Sun, it's normal for objects to move in the same direction.
Geology and surface features
Triton is a reddish color, probably the result of reactions of methane ice with ultraviolet radiation. There are polar icecaps, but they aren't made of frozen water. They consist of frozen nitrogen and methane. Over half of Triton's surface is frozen nitrogen. The rest is water ice and frozen carbon dioxide with traces of methane and carbon monoxide.
Data from Voyager 2's fly-by of Triton shows a geologically young surface with few impact craters. A highly reflective surface reflects most of the sunlight that it receives.
A unique feature is the extensive “cantaloupe terrain” of Triton's western hemisphere. It was given this name because it looks rather like the skin of a cantaloupe melon. It's made up of of smoothly curved depressions 30-40 km in diameter.
We know that Triton is geologically active because Voyager 2 observed eruption plumes of nitrogen gas and dust. These aren't volcanoes of molten rock, but rather cryovolcanoes, ice volcanoes like those on Saturn's Enceladus.
Based on the data and models of planetary geology, scientists think that Triton must have a layered interior like a planet. There should be a solid core, a mantle and a crust. The core is probably rock and metal, while the mantle is water. There is enough rock for radioactive decay to occur, probably heating the mantle, creating convection currents. This happens on Earth, although the mantle is semi-liquid rock, not water.
Triton has a very tenuous nitrogen atmosphere with traces of methane and carbon monoxide.
Its high reflectivity, combined with its distance from the Sun and lack of blanketing atmosphere, make Triton the coldest body in the main part of the Solar System. Even Pluto isn't quite as cold. Triton's surface temperature averages only -235oC (-391oF).
The thin atmosphere varies seasonally, getting thicker when warmed. Obviously, even at the height of summer, Triton is not warm. However there is enough heat from the Sun for some of the frozen surface nitrogen, methane and carbon monoxide to sublimate. In sublimation, instead of melting, a solid changes directly into a gas. You may have seen dry ice (frozen carbon dioxide) do this on Earth.
Triton's retrograde orbit shows that it didn't form as part of a Neptunian system. It is a captured object. But where did it form?
Interestingly, although Triton is very unlike the other moons, it's very similar to Pluto. Triton and Pluto are almost the same size, though Triton is slightly larger. They're the same reddish color. Both bodies have a density of just over 2 g/cm