Triton (moon of the planet Neptune)
Distance from Neptune: 354,760
Triton is the largest moon of Neptune, with a diameter of 2,700 kilometers (1,680 miles). It was discovered by William Lassell, a British astronomer, in 1846 scarcely a month after Neptune was discovered. Triton is colder than any other measured object in the Solar System with a surface temperature of -235° C. It has an extremely thin atmosphere. The atmospheric pressure at Triton's surface is about 14 microbars, 1/70,000th the surface pressure on Earth. It is speculated that nitrogen ice particles might form thin clouds a few kilometers above the surface.
Triton is the only large satellite in the Solar System to circle a planet in a retrograde direction -- in a direction opposite to the rotation of the planet. Triton contains more rock in its interior than the icy satellites of Saturn and Uranus. Its relatively high density and retrograde orbit have led some scientists to suggest that Triton may have been captured by Neptune as it traveled through space several billion years ago. If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the satellite might even have been liquid for as long as one billion years after its capture by Neptune. Triton is scarred by enormous cracks. Voyager 2 images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometers into the atmosphere.
Triton is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon, Titan). Triton has the coldest surface known anywhere in the Solar System (38 K); it is so cold that most of Triton's nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. The pinkish deposits that cover a vast portion of the south polar cap is believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying this pink ice are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the Voyager 2 flyby. There is a bluish-green band that extends all the way around Triton near the equator; and may consist of relatively fresh nitrogen frost deposits.