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 is Neptune’s moon, Triton, taken in 1989 by Voyager 2 - the only spacecraft ever to pass it. Triton has fascinating terrain, a thin atmosphere & even evidence for ice volcanoes on this world of...
Triton (moon) - Water, the most abundant volatile in the Solar System, comprises Triton's mantle, which lies over a core of rock and metal. There is enough rock in Triton's interior for radioactive decay to power convection in the mantle. The heat sufficient to maintain a "subterranean ocean" with a layer of liquid water that suggests the possibility of life.
Neptune's moon Triton, as imaged by the New Horizons spacecraft