Triton is Neptune’s giant moon and the only large moon in the Solar System that orbits backwards, against its planet’s spin. Voyager 2 found a young, cantaloupe-textured landscape of nitrogen and methane ices, with dark geysers erupting through the frost even at -235 °C - the coldest surface ever measured on a Solar System body.
At -235 °C its surface is the coldest ever measured in the Solar System - yet geysers still erupt through the ice.
William Lassell discovered it just 17 days after Neptune itself was found in 1846.
Its backwards orbit is slowly decaying: in roughly 3.6 billion years Neptune’s tides will tear it apart into a ring.
A captured Kuiper Belt object
Moons that form alongside their planet orbit in the direction the planet spins - Triton goes the other way. That retrograde orbit is the fingerprint of capture: Triton almost certainly formed in the Kuiper Belt, like Pluto, wandered too close, and was snared by Neptune’s gravity - a violent event that would have wrecked any moons Neptune had before. Its size, density and surface ices are strikingly Pluto-like: Triton may be Pluto’s captured twin.
Physical characteristics
Diameter
2,707 km
Mass
2.140e+22 kg
Surface gravity
0.78 m/s²
Density
2.06 g/cm³
Tidally locked
Yes
Mean temperature
-235 °C
Geometric albedo
0.76
Atmosphere and composition
Atmosphere
A thin nitrogen atmosphere with a trace of methane - surface pressure about 1/70,000 of Earth’s.
Composition
A dense rock-and-metal core making up about two-thirds of its mass, under a mantle of water ice frosted with nitrogen, methane and carbon-dioxide ices.
Discovery
Discovered by
William Lassell
Year
1846
How
24-inch reflector, 17 days after Neptune’s discovery
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.