via Discovery Space.
Neptune’s moon Triton wasn’t always thus. At one time, the icy world, with a diameter 40% larger than Pluto at 2,700km, orbited in the Kuiper belt before being captured by Neptune’s gravity. The major clue to this origin is that the moon orbits the planet in the opposite way to the one expected.
The surface of this frigid world is covered with ices, water and carbon dioxide cover the surface in an even layer. However ices belonging to materials that can become gases at temperatures experienced by parts of the moon’s surface at certain times of its day and Neptune’s year tend to move about.
Nitrogen, Carbon monoxide and Methane all spend part of their time frozen to the surface and part of the time in the atmosphere, much like the ice caps on Mars or even here on Earth. As they’re all able to sublimate and move around, it is expected that the three ices will do so on a similar timescale and in similar ways. This is true for Nitrogen and CO, but Methane tends to do its own thing. Whereas Nitrogen and Carbon Dioxide prefer to spend time as ices facing Neptune, Methane hides on the other side. Researchers believe that the peculiar behaviour comes from Methane’s freezing point being closer to the surface temperature of Triton, leading it to live a halfway house between the water/carbon dioxide ice distributions and the nitrogen/carbon monoxide ice distributions.