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Triton, a picture taken by Voyager 2 in 1989Triton, a picture taken by Voyager 2 in 1989 At 2700 kilometres (about 1700 miles) in diameter, Triton is Neptune’s largest moon, and the seventh largest of all the moons in the Solar System. It orbits its parent planet in a reverse direction, the only large moon to do so (the others that do are tiny, irregular objects). Beneath its icy crust lies a mantle of water, which may be liquid if underground temperatures are high enough. On the surface, however, it is extremely cold. With an average surface temperature of -238°C (-396°F), Triton is the coldest world in the Solar System.

A close-up view of the surface of TritonA close-up view of the surface of TritonPart of Triton's southern hemisphere, showing geysersPart of Triton's southern hemisphere, showing geysers

Surface features

Scientists gained knowledge of the surface of Triton when the Voyager 2 space probe flew past Triton in 1989 at a distance of 40,000 kilometres (25,000 miles), sending back detailed images.

Triton’s granite-hard surface is made chiefly of frozen nitrogen—a gas on Earth. The rest is made of frozen water and carbon dioxide. For an unknown reason, the ice sometimes melts to a slush, then refreezes. This results in an “icescape” of ridges and cracks. Nitrogen gas builds up under the ice crust, then bursts out through weak points in giant “geysers”.

There are very few craters on Triton’s surface. This is because the crust is constantly being smoothed over by water and ammonia oozing to the surface. Astronomers think Triton may be internally heated. There could even be a layer of liquid water beneath its crust: a "subterranean ocean" similar to that thought to exist beneath the surface of Europa, with—just possibly—life forms inside it.


Triton comprises more than 99.5% of the total mass of objects that orbit around Neptune.

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