Neptune's moon system is dominated by Triton, the seventh-largest moon in the Solar System, which orbits retrograde — strong evidence it was captured from the Kuiper Belt rather than forming in place. Triton is geologically active despite surface temperatures near 38 K, with nitrogen geysers and a thin atmosphere. Its retrograde orbit is slowly decaying; in a few billion years tidal forces will break it apart. Neptune's other major moons — including Proteus, Larissa, and Nereid — are much smaller and mostly irregular. Nereid has one of the most eccentric orbits of any moon, suggesting past gravitational upheaval during Triton's capture.