But the Voyager 2 spacecraft, which flew past Triton in 1989, revealed quite a lot about the icy moon. Voyager photographed giant geysers that blasted dark plumes of gas and dust up to five miles high. It found that Triton's surface is mainly frozen nitrogen and methane on top of frozen water that's as hard as granite. Triton's surface reflects much of the sunlight that reaches it. Combined with Triton's great distance from the Sun, this makes the moon very cold.
Triton orbits Neptune in the opposite direction from all the other satellites.
Because of its strange orbit, astronomers think Triton may have been captured by Neptune instead of forming with the planet itself. Many millions of years from now, Triton's orbit will carry it too close to Neptune, and the planet's gravity will pull the moon apart. Triton's remains will scatter along its orbital path, forming rings as bright as Saturn's.