In astronomy, largest of Neptune's moons. It has a diameter of 2,700 km/1,680 mi, and orbits Neptune every 5.88 days in a retrograde (east to west) direction at a distance of 354,000 km/220,000 mi. It takes the same time to rotate about its own axis as it does to make one revolution of Neptune.
Triton is slightly larger than the dwarf planet Pluto, which it is thought to resemble in composition and appearance. Probably Triton was formerly a separate body like Pluto but was captured by Neptune. Triton was discovered in 1846 by English astronomer William Lassell (1799–1880) only weeks after the discovery of Neptune. Triton's surface, as revealed by the Voyager 2 space probe, has a temperature of 38 K (−235°C/−391°F), making it the coldest known place in the Solar System. It is covered with frozen nitrogen and methane, some of which evaporates to form a tenuous atmosphere with a pressure only 0.00001 that of the Earth at sea level. Triton has a pink south polar cap, probably coloured by the effects of solar radiation on methane ice. Dark streaks on Triton are thought to be formed by geysers of liquid nitrogen. The surface has few impact craters (the largest is Mazomba, with a diameter of 27 km/17 mi), indicating that many of the earlier craters have been erased by the erupting and freezing of water (cryovulcanism).
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