Triton [TRY-tun] 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 (-391¡ F). It has an extremely thin atmosphere. Nitrogen ice particles might form thin clouds a few kilometers above the surface. The atmospheric pressure at Triton's surface is about 14 microbars, 1/70,000th the surface pressure on Earth.

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. It also has a density of about 2.066 grams per cubic centimeter (the density of water is 1.0 gram per cubic centimeter). This means Triton contains more rock in its interior than the icy satellites of Saturn and Uranus do. The relatively high density and the retrograde orbit has 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.

With a radius of 1,350 (839 mi), about 22% smaller than Earth's moon, Triton is by far the largest satellite of Neptune. It 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, about -391 degrees Farenheit); 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 constitute a vast south polar cap believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying these pink ices 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. The bluish-green band visible in this image extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas includes what is called the cataloupe terrain, whose origin is unknown, and a set of 'cryovolcanic' landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton's interior.