Triton, Neptune’s Large Moon

Triton’s distinction among the jovian moons is a retrograde (backward) orbit—in the reverse direction of the other moons. Moreover, Triton is inclined on its axis about 20 degrees and is the only large jovian moon that does not orbit in the equatorial plane of its planet. Many astronomers believe that these peculiarities are the result of some violent event, perhaps a collision. Others suggest that Triton did not form as part of the Neptunian system of moons, but was captured later by the planet’s gravitational field.
Triton’s atmosphere is so thin that Voyager 2 had no trouble imaging the moon’s surface, finding vast lakes of water ice or water-ammonia mixtures there. Nitrogen frost, found at the polar caps, appears to retreat and reforms seasonally.