Artist's representation of the capture of Triton (Mark Garlick/Science Photo Library)

The Solar System's big moons are weird and wonderful. Neptune's Triton has ice volcanoes, a strange “cantaloupe terrain”, and crazy seasons. Of Neptune's fourteen known moons, Triton's mass accounts for 99.7% of the total. Orbiting in the opposite direction to Neptune's rotation, it didn't form near Neptune. But where did it come from?

Naming Triton
Neptune was discovered in 1846 through its gravitational effect on Uranus. When British astronomer John Herschel received news of the discovery, he suggested to his friend William Lassell that he look for Neptunian moons. Just over a week later Lassell had discovered the moon we call Triton.

It wasn't easy to get agreement on a name for the new planet, but astronomers finally settled on Neptune, the Roman sea god. The moon was just Neptune's satellite. In 1880 French astronomer Camille Flammarion suggested naming it Triton. Neptune's son and messenger, Triton was represented as a merman who could blow through his conch shell horn to calm the waves. The suggestion was generally ignored.

In fact, no name was needed 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 finally named.

Comparing Triton with other moons
Triton is the seventh largest Solar System moon. A diameter of 2700 km (1680 mi) makes it almost as large as Jupiter's moon Europa, though Europa is 50% denser.

Triton is around the same distance from Neptune as our Moon is from Earth. As with other moons, Triton always keeps the same side to the planet, making a day the same length as a month. However, Triton takes less than six days to whiz around Neptune. This is because of the stronger gravity of massive Neptune. If Triton had moved as slowly as our Moon, it would have been broken apart or pulled into its planet long ago.

Triton's orbit is bizarre for such a large object. Although it's almost perfectly circular, it's highly tilted to Neptune's orbit. Its axis of rotation is also tilted so much that the polar and equatorial regions point alternately at the Sun, causing extreme seasonal changes.

Yet the most surprising feature of Triton's orbit is that it's retrograde – it orbits in the opposite direction to Neptune's spin. Since the Solar System formed from a debris disk revolving around the Sun, the planets and their major moons tend to orbit in the same direction as the rotation of the Sun. But sometime in its history, Triton came crashing into the Neptunian system from elsewhere, disrupting the existing moons, and ending up in its peculiar orbit as a captured object.

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're made of frozen nitrogen and methane, not water. 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.

A unique feature is the extensive cantaloupe terrain of Triton's western hemisphere. Made up of of smoothly curved depressions 30-40 km in diameter, it resembles the skin of a cantaloupe melon.

Data from Voyager 2's fly-by of Triton show a geologically young surface with few impact craters, suggesting that Triton is still geologically active. That was confirmed when eruption plumes of nitrogen gas and dust were observed. The eruptions aren't from volcanoes like the ones we know on Earth that spew molten rock. They are cryovolcanoes, ice volcanoes like those on Saturn's Enceladus.

Planetary 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, heating the mantle and creating convection currents. This happens on Earth where the mantle is semi-liquid rock, not water.

Atmosphere
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 -235°C (-391°F).

The thin atmosphere varies seasonally, getting thicker when warmed. Obviously, even at the height of summer, Triton isn't balmy. However, there is enough heat from the Sun for some sublimation of the frozen surface nitrogen, methane and carbon monoxide. In sublimation, instead of melting, a solid changes directly into a gas. You may have seen dry ice (frozen carbon dioxide) do this.

Pluto's cousin?
Triton's retrograde orbit shows that it didn't form as part of the Neptunian system. But where did this captured object come from?

Although Triton is very unlike the other moons, it's very similar to Pluto. Triton is only slightly larger than Pluto, and they're the same reddish color. Both bodies have a density of just over 2 grams per cubic centimeter and a similar composition. Triton was almost certainly born in the Kuiper belt.