Neptune over Triton. The planet Neptune seen from its largest moon, Triton. Triton was one of the highlights of the Voyager 2 mission to the outer planets. With a radius of 1350 km it is by far the largest of Neptune's moons, and has a retrograde, synchronous orbit. Its terrain is extensively faulted, and the polar caps are covered with a pinkish mixture of nitrogen and methane ice. It also has a very weak methane atmosphere. The surface temperature is about 37 Kelvin (-236 Celsius).