| Triton |
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The largest natural satellite of Neptune. It was discovered in October 1846 by William Lassell, only 17 days after the discovery of Neptune itself. It circles Neptune every 5.9 days in a retrograde orbit tilted at 23° to the planet's equatorial plane. This unusual orbit has led to speculation that Triton was captured rather than having formed close to Neptune. The Voyager 2 encounter with the Neptune system on 25 August 1989, and its approach to within 4,000 kilometres (2,500 miles) of Triton, revealed a wealth of detail. Triton's diameter was found to be 2,700 kilometres (1,680 miles), slightly less than thought previously. Its gravitational effect on the spacecraft's trajectory suggests that the bright, icy outer crust and mantle must overlie a substantial core of rock (perhaps even metal) containing two-thirds of the satellite's mass. The surface temperature is 38 K, making it the coldest known object in the solar system. Its size, structure and other properties suggest that the planet Pluto may be very similar to Triton. Triton is surrounded by a tenuous atmosphere (the surface pressure is 15 microbars) of nitrogen with a trace of methane. The south polar cap is coated with a bright frost, possibly of nitrogen ice, which is gradually evaporating. (Owing to the orbital characteristics of Neptune and Triton, this region had been in sunlight continuously for nearly 100 years when it was observed. No impact craters were detected in this region.) The equatorial region exhibited a variety of terrains suggestive of complex volcanic activity still continuing, including plume eruptions. Triton's surface is certainly relatively young in astronomical terms. See also: Table 6. |