n.
Largest of Neptune 's known moons.
Its diameter is about 1,680 mi (2,700 km), somewhat less than that of Earth's Moon. Triton moves in a retrograde orbit, opposite the direction of Neptune's rotation, with a period of 5.9 Earth days, while keeping the same face toward Neptune. It has a very thin atmosphere of nitrogen and methane and a surface temperature of -400 °F (-240 °C). Its surface is covered with enormous expanses of ice sculpted with fissures, puckers, and ridge-crossed depressions and pitted by what appear to be a few meteorite craters. Plumes of gas observed by the Voyager 2 spacecraft may be gas venting through fissures when the surface is warmed by sunlight.