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Solar System Moons Series

Triton

The backwards moon: retrograde orbit, nitrogen geysers, and the most extreme cold measured.

Author: Juan Carlos Euyoque Aguilera · October 7, 2026 · CC BY 4.0

Image of Triton (NASA/JPL, public domain)
Triton (Voyager 2, NASA/JPL, public domain). Not to scale.
2,706.8 km
Diameter
354,760 km
Distance from Neptune
38 K
Temperature
5.877 days
Orbital period

Triton orbits Neptune at 354,760 km, but backwards: it is the only large moon in the solar system with a retrograde orbit, opposite to the planet's rotation. The favored explanation is that it was not born there: it would be a Kuiper Belt object captured by Neptune.

Voyager 2 visited it in 1989 —the only visit— and found active nitrogen geysers and a 38 K surface, among the coldest ever measured. Half of its world remains unmapped.

Measured data [DATO]

DataValueStatusSource
Diameter2,706.8 km[DATO] observadoNASA Planetary Fact Sheets
Mass2.14×10²² kg[DATO] observadoNASA Planetary Fact Sheets
Mean density2,060 kg/m³[DATO] observadoNASA Planetary Fact Sheets
Surface gravity0.779 m/s²[DATO] observadoNASA Planetary Fact Sheets
Orbital period5.877 days (retrograde)[DATO] observadoNASA Planetary Fact Sheets
Rotation periodsynchronous retrograde[DATO] observadoNASA Planetary Fact Sheets
Distance from Neptune354,760 km[DATO] observadoNASA Planetary Fact Sheets
Temperature38 K (among the coldest measured)[DATO] observadoNASA Science
AtmosphereTenuous: nitrogen + methane[DATO] observadoNASA Science
Notable featureOnly large retrograde moon; nitrogen geysers (Voyager 2)[DATO] observadoNASA Science

Truths [DATO]

  • [DATO] Triton is the only large moon with a retrograde orbit: it circles Neptune opposite to the planet's rotation — a classic capture signature.
  • [DATO] Voyager 2 photographed nitrogen plumes ~8 km high in 1989: active cryovolcanic geysers, driven by sunlight warming nitrogen ice beneath the surface.
  • [DATO] At 38 K, its surface is among the coldest measured in the solar system; nitrogen, methane and carbon monoxide lie frozen.
  • [DATO] Its composition and size resemble Pluto more than the neighboring moons: supporting the captured-Kuiper-Belt-object hypothesis.

Labeled myths [MITO]

[MITO] Triton was always Neptune's moon.

Unconfirmed: the leading hypothesis is capture of a Kuiper Belt object (retrograde orbit + Pluto-like composition), but the exact capture mechanism remains open.

Fuente: NASA Science

[MITO] Triton is a dead, frozen world with no activity.

False: Voyager 2 found active geysers and young terrain; extreme cold does not mean geological stillness.

Fuente: NASA Science

Open questions [PREGUNTA]

  • [PREGUNTA] How exactly did Neptune capture Triton?
  • [PREGUNTA] Is cryovolcanism still active today?
  • [PREGUNTA] What lies in the hemisphere Voyager 2 could not see?

The chain in brief

  1. [MODEL] 1. A captured outsider: It orbits Neptune at 354,760 km, but backwards: it is the only large moon with a retrograde orbit, and the favored explanation is that it was not born there, but is a Kuiper Belt object captured by Neptune.
  2. [DATA] 2. More like Pluto than its neighbors: Its composition and size resemble Pluto more than the neighboring moons: a classic signature of capture.
  3. [DATA] 3. The only visit, in 1989: Voyager 2 visited it in 1989 — the only visit — and photographed nitrogen plumes ~8 km high: active cryovolcanic geysers, driven by sunlight warming nitrogen ice beneath the surface.
  4. [DATA] 4. The most extreme cold measured: Its surface is at 38 K, among the coldest ever measured, with nitrogen, methane, and carbon monoxide frozen.
  5. [DISPUTED] 5. Half a world remains unmapped: What lies in the hemisphere Voyager 2 could not see, and whether cryovolcanism is still active today, remain unchecked.