Triton

The icy world that orbits backwards

Triton is Neptune's biggest moon, and it orbits the opposite way to everyone else! Scientists think Triton was once a dwarf planet wandering far from Neptune, until Neptune captured it. Triton is very cold — one of the coldest places in the Solar System. Yet nitrogen geysers shoot 8 km high there!

Triton (radius 1,353 km) is Neptune's largest moon and the only large moon in the Solar System with a retrograde orbit: it circles against Neptune's rotation every 5.9 days. This suggests it is a captured dwarf planet from the Kuiper Belt (a cousin of Pluto). Its surface is −235 °C — one of the coldest bodies ever measured — made of nitrogen, methane and water ice. In 1989 Voyager 2 saw active nitrogen geysers (8 km high) and 'cantaloupe' terrain. Triton has a thin nitrogen atmosphere. Tidal forces are pulling it toward Neptune: in ~3.6 billion years it will break apart and form a ring bigger than Saturn's.

Fun facts

• Triton was discovered just 17 days after Neptune itself — in 1846.
• Triton's geysers spray nitrogen gas, not water — it's that cold.
• One day Triton will break up and give Neptune a ring bigger than Saturn's.