Triton, a moon of Neptune
Neptune's biggest moon (2,700 kilometres wide) and most interesting one is Triton. It was discovered in 1846, just a month after the planet was discovered, and, up to 1989, was thought to be one of only two moons orbiting Neptune (Nereid was discovered in 1949). A further six moons were discovered in 1989 by Voyager 2 and another 5 in 2002 and 2003. The picture to the left shows a view of Neptune from Triton's surface, created using images from Voyager 2.
All other large moons in the Solar System go from west to east around their planets (they appear to go from left to right in front of the planet, in the same direction of planet's rotation). Triton is the only large moon in the Solar System to orbit its planet in an opposite direction. This suggests that it might have been a planet captured by Neptune's pull of gravity, and now orbits the planet.
The main feature of Triton is that it is a geologically active moon. This means that it is still forming. Earth is geologically active. This is shown by its volcanic eruptions and Earthquakes. Io, a moon of Jupiter, is the most active moon in the Solar System, with volcanoes reforming its surface constantly. Triton is too far away from the Sun to be warm enough for volcanoes. In fact, it has the coldest surface measured in the Solar System (-235°c) although it does have its own eruptions. Nitrogen is a gas on Earth and makes up most of its atmosphere. When it is cold enough, it becomes a liquid. Nitrogen is contained as a frozen liquid in Triton's ice caps, and when the ice caps melt after winter and the temperature increases slightly, eruptions of this nitrogen as a gas takes place. These are seen as dark plumes on Triton's surface, and they show that, although Earth's moon is a dead world, a moon as far away from the Sun as Triton can still be living.
In this picture, the dark plumes are eruptions of Nitrogen gas into Triton's extremely thin atmosphere. The Nitrogen can also cause very small, almost invisible clouds in Triton's atmosphere!