The moon probably rotated on its axis. Over about a billion years, Neptune's gravity slowed Triton's rotation so that it rotated only once during each orbit and forced Triton into a circular orbit. This made Triton flex, rubbing together rocks inside and producing frictional heat that reshaped Triton's surface. Triton has few craters, but many cracks in its surface. Triton's southern hemisphere has a pink polar cap with a blue edge.
Geysers spew dark plumes into Triton's thin atmosphere.
These may be caused by sunlight vaporizing liquid nitrogen under the surface. This warming trend is slowly turning frozen nitrogen on Triton into gas, creating a denser atmosphere. Scientists believe that Triton's warming is part of a natural cycle that repeats itself every few hundred years when the moon's southern hemisphere points directly into the Sun. Direct sunlight on the southern hemisphere heats the polar ice fields on Triton, causing a gradual global warming. British astronomer William Lassell discovered Triton in 1846, only weeks after Neptune was discovered.