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Detail of Triton's Surface

Detail of Triton's Surface

Neptune’s largest moon, Triton, was discovered by the British astronomer William Lassell, on October 10, 1846. Triton was discovered only a little over two weeks after Neptune itself was found. Despite its early discovery, the moon did not receive its current name for many years. The name Triton was suggested in 1880; Triton is the son of the sea god Poseidon, the equivalent of the Roman god Neptune, in Greek mythology. Other Neptune moons were also given names of mythological figures associated with water.

With a diameter of 2,700 kilometers, Triton is the seventh largest moon in the Solar System and the sixteenth largest object in the Solar System. The satellite is larger than the dwarf planets Eris and Pluto. Triton is considered to be one of the coldest places in the Solar System. The moon is -235°C while Pluto averages about -229°C. Scientists say that Pluto may drop as low as -240°C at the furthest point from the Sun in its orbit, but it also gets much warmer closer to the Sun, giving it a higher average temperature than Triton.

Triton is unusual for several reasons, but one of the biggest is its retrograde orbit. The moon orbits Neptune backwards compared to the orbit of all the other moons. This means that Triton didn’t form in orbit around Neptune, but formed somewhere else and was captured later on by Neptune’s gravity. It’s possible that Triton was a Kuiper Belt Object that fell into Neptune’s gravity field. Or maybe Neptune stole it from another planet.

It is also one of the few moons that are geologically active, which means that its surface is relatively young due to its changing nature. Volcanoes have been discovered on the planet, but these eruptions are ammonia and water lava rather than the liquid rock found on Earth. Because of the geological activity which is constantly changing the moon’s surface, there are very few impact craters on Triton. Additionally, Triton has a slight atmosphere comprised mostly of nitrogen with some methane. Scientists believe the atmosphere is the result of nitrogen evaporating from the moon’s surface.

The moon has a reddish tint, which is probably due to the methane ice turning to carbon due to exposure to ultraviolet radiation. Over half of the moon’s surface is covered with frozen nitrogen while frozen carbon dioxide and frozen water cover most of the remaining surface. There are also traces of other substances such as methane and carbon monoxide on the moon’s surface. Underneath the surface is a mantle comprised of ice, rock, and metal. This core is what makes the moon so dense.

Triton consists of a crust of frozen methane over an icy mantle that contains rock and metal. In fact, the heavy core makes up two-thirds of its mass, and helps give the moon a density above 2 grams per cubic centimeter. While other icy objects are much closer to water’s 1 gram/cubic centimeter.

Another surprising feature of Triton is the fact that it’s geologically active. When NASA’s Voyager 2 made a flyby of the Moon in 1989, it saw geysers of liquid nitrogen blasting out of two distinct features on the surface of Triton. Compared to other objects in the Solar System, the surface of Triton is very young. This means that some process on the moon is constantly resurfacing it. These nitrogen geysers can send plumes of liquid nitrogen 8 km above the surface of the moon, and probably account for its young appearance.

Triton is doomed. The moon is tidally locked to Neptune, keeping one face turned towards the planet at all times. But its orbit is decaying. In approximately 3.6 billion years from now, it will pass below Neptune’s Roche limit and be torn apart. After that, Neptune will have a huge ring like Saturn, until those particles crash into the planet as well. It would be something to watch.


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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