Triton
The largest of Neptune's 13 known satellites, Triton was discovered in 1846 by British astronomer William Lassell. Triton circles Neptune in a tilted, circular, retrograde orbit, completing an orbit in 5.875 days at an average distance of 205,000 mi (330,000 km) above the planet's cloud tops.
Triton shows evidence of a remarkable geologic history, and Voyager 2 images show active geyser-like eruptions spewing invisible nitrogen gas and dark dust particles 1 to 5 mi (2 to 8 km) into space.
Triton is about three-quarters the size of Earth's Moon and has a diameter of about 1,680 mi (2,705 km) and a mean density of about 2.066 grams per cubic centimeter. (The density of water is 1.0 grams per cubic centimeter.) This means that Triton contains more rock in its interior than the icy satellites of Saturn and Uranus.
The relatively high density and the retrograde orbit offer strong evidence that Triton did not originate near Neptune, but is a captured object.
An extremely thin atmosphere extends as much as 500 mi (800 km) above the satellite's surface. Tiny nitrogen ice particles may form thin clouds a few kilometers above the surface. Triton is very bright, reflecting 60% to 95% of the sunlight that strikes it. (By comparison, Earth's Moon reflects only 11%.)
The atmospheric pressure at Triton's surface is about 14 microbars, a mere 1/70,000th the surface pressure on Earth. Temperature at the surface is about 38°K (–391°F), making it the coldest surface of any body yet visited in the solar system.
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