Triton

Triton (Neptune I) is Neptune's largest moon, much larger than Neptune's other moons, and for a long time, Neptune's only known moon. Its radius is 840 miles, about 1/5 Earth radius, and its mass is on the order of 1/275 of Earth's. Cryovolcanism has been detected on it. Triton is in a retrograde orbit and because of this as well as its Pluto-like composition, it is thought to be a captured dwarf planet. Recent study of such captures suggest Triton might have previously been a minor planet with a binary companion: simulations show captures involving just two bodies require very particular circumstances, but three-body interactions involving a binary object with a third object can more easily result in such an exchange. Triton and Neptune have been closely-observed during just one flyby, that of Voyager 2 in 1989. Among its discoveries were dark plumes emitted from the surface, which remain a mystery.