Moons · Space Codex
Triton
The only large moon that orbits against its planet's spin, almost certainly a Kuiper belt object captured long ago.
A captured world orbiting backwards
At a glance
Key points
At −235 °C it is one of the coldest surfaces measured, yet Voyager 2 saw geysers erupting.
- The only large moon with a retrograde orbit
- Geysers erupt at −235 °C
- Probably a captured Kuiper belt object
Origin
Triton orbits Neptune backwards and steeply tilted, which no moon formed alongside its planet should do. The best explanation is capture: Triton was once a dwarf planet in the Kuiper belt, like Pluto, and Neptune's gravity snared it. Tidal heating during capture would have melted much of its interior.
Surface
A pinkish cap of nitrogen and methane ice covers the south pole, streaked with dark deposits blown downwind from geysers. Near the equator lies 'cantaloupe terrain', dimpled ground found nowhere else. Few craters means the surface is young.
Fate
Because its orbit runs backwards, tides are slowly drawing Triton inward. In a few billion years it should cross the distance where Neptune's gravity will tear it apart, possibly leaving a ring.
What we still don't know
What drives the plumes, whether an ocean persists beneath the ice, and what the northern hemisphere looks like: Voyager 2 saw mostly the south during its 1989 flyby.
Missions
- 1977–
The only spacecraft to visit Uranus (1986) and Neptune (1989); interstellar since 5 Nov 2018.
First-hand · our 3D model
How we render it
Triton is a sphere wrapped in a global map built from USGS and NASA spacecraft mosaics. Triton's northern hemisphere was never imaged and is a procedural fill.
It is drawn at about 12% of Neptune's radius, enlarged from its true 5.5% so it stays visible. Its orbit is retrograde: inclined 157.3° to Neptune's equator, it travels against the planet's spin, with its 5.88-day period kept in true ratio and one face always towards Neptune.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
Sources
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
- NASA NSSDC Neptunian Satellite Fact Sheet (opens in a new tab)
- NASA Science: Triton (opens in a new tab)
- Soderblom et al. 1990, Science 250:410 (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.