Triton

Moon· orbits Neptune

Triton is Neptune's largest moon and the only large moon orbiting backwards (retrograde) — a captured Kuiper-belt world with a young nitrogen-ice surface and active nitrogen geysers, seen by Voyager 2 in 1989.

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Key facts

Mass

2.14 × 10²² kg

0.291 Moon masses

Radius

1,353 km

78% of our Moon's radius

Surface gravity

0.78 m/s²

8% of Earth's gravity

Day length

5.9 days

Orbit around Neptune

5.88 Earth days

Science

Physical

Mass: 2.14 × 10²² kg · 0.291 Moon masses; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 1,353 km · 78% of our Moon's radius; JPL SSD Planetary Satellite Physical Parameters (mean radius)
Density: 2,064 kg/m³ · 2.1× water; Computed from mass and radius
Surface gravity: 0.78 m/s² · 8% of Earth's gravity; Computed from mass and radius
Escape velocity: 1.45 km/s · 13% of Earth's escape velocity; Computed from mass and radius

Orbit around Neptune

Distance from Neptune: 354,800 km; From JPL orbital elements
Orbital period: 5.9 days; From JPL orbital elements
Eccentricity: 0; From JPL orbital elements
Inclination to Neptune's Laplace plane: 157.30 °; From JPL orbital elements · The Laplace plane is the one a moon's orbit precesses about: close in it is the planet's equatorial plane, the plane of the rings, and further out it tilts toward the planet's own orbit.

Rotation

Rotation period: 5.9 days
Tidal locking: Tidally locked to its parent

Atmosphere

Composition: Tenuous N₂ with traces of CH₄ and CO
Surface pressure: 1.4 Pa

Temperature

Surface: 38 K (-235 °C)

Composition

Bulk composition: A nitrogen-ice surface (with CO₂, CO and methane ices) over a water-ice crust and a large rocky core; one of the coldest measured surfaces in the Solar System.

Radiation & temperature

Insolation: 0.00112 S⊕ · 1/893 of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Bond albedo: 0.76 · reflects 76% of incoming sunlight
Equilibrium temperature: 36 K (-238 °C); Computed from albedo + distance from the Sun (airless black body)
Surface temperature: 38 K (-235 °C)

Discovery & history

Discovered: October 10, 1846
By: William Lassell
Method: Visual discovery (telescopic)
Name: Triton, a sea-god son of Poseidon (Neptune) in Greek myth.

William Lassell discovered Triton just 17 days after Neptune itself was found in 1846. It orbits backwards — the only large moon to do so — marking it as a captured Kuiper Belt object. Voyager 2 found it geologically active, with nitrogen geysers erupting from one of the coldest surfaces ever measured.