Triton, the largest moon of Neptune. Because Triton has a retrograde orbit (opposite that of it's planet), it was likely captured by Neptune's gravitational pull from the Kuiper Belt. It's about the same size and has a very similar makeup to Pluto, another Kuiper Belt object. Triton's orbit is slowly shrinking, and it is expected that in about 3.6 billion years, it will either collide with Neptune or be destroyed, forming a Saturn-like ring system.
This view of the volcanic plains of Neptune's moon Triton was produced using topographic maps derived from images acquired by NASA's Voyager spacecraft during its August 1989 flyby, 20 years ago this week.
NASA's Voyager 2 spacecraft gave humanity its first glimpse of Neptune and its moon Triton in the summer of 1989. The images were taken on Aug. 20, 1989, at a range of 4.4 million miles from the planet, 4 days and 20 hours before closest approach on Aug. 25. NASA