The moon may have been shattered and reassembled in a primordial collision. Note the odd markings on the surface, especially the division between the polar cap and the rest of the planet. 8 known moons, mostly a few tens to hundreds of km in diameter. Orbit is retrograde, however, so it is possible Triton is a captured body. May be rock mixed with a lot of ice.
Surface looks very smooth, with high reflectivity.
Frost on surface is probably solid Nitrogen and methane. Surface temperature around 40K, lowest in solar system. Polar cap in southern hemisphere. May be frozen nitrogen. Plumes rising from the surface observed by the Voyager suggest that there may be geysers of liquid N 2 released when sunlight warms the poles.