Discovered1846William Lassell, 17 days after Neptune[1]
About the image. Drawn as a pale, pinkish ball of nitrogen ice with dark streaks left by its geysers. Voyager 2 photographed part of Triton once, in 1989; the rest of its surface has never been seen, and its details here are imagined.
Seventeen days after the planet Neptune was discovered in 1846, the English astronomer William Lassell found a moon beside it. Triton, 2 707 kilometres across, turned out to be one of the strangest moons in the Solar System. It circles Neptune backwards, against the direction the planet spins, and it is the only large moon anywhere in the Solar System that does so.
A captured world
Moons that form around a planet orbit the same way it spins, so a backward orbit points to an outsider. Triton almost certainly began as a body orbiting the Sun beyond Neptune, a world of the same family as Pluto, but about 40% more massive. In 2006 Craig Agnor and Douglas Hamilton showed how Neptune could have caught it: if Triton was one half of a pair of bodies orbiting each other, a close pass by Neptune could pull one of them in and throw the other away. Triton was the one that stayed.
Voyager 2’s only visit
Only one spacecraft has ever been to Triton. Voyager 2 flew past on 25 August 1989 and photographed part of its surface, somewhere between 40% and two-thirds of it in different accounts. What it saw was young: almost no craters, so the surface is probably less than 100 million years old. Part of it is covered by what scientists call cantaloupe terrain, a dimpled pattern of pits 30 to 40 kilometres across that looks like the skin of a melon and is probably made by blobs of warmer ice rising from below.
The biggest surprise was activity. Voyager found at least four geysers erupting from the surface. The two best seen rose as narrow columns about 8 kilometres high, and at the top the wind carried their dark material sideways for more than 100 kilometres, leaving long dark streaks on the ice below.
One of the coldest surfaces
Triton is covered mostly in nitrogen ice, with water ice and frozen carbon dioxide. Spectra of that ice put the surface temperature at about 38 K (-235 °C), one of the coldest ever measured on any surface, although shadowed craters on our own Moon have measured colder still. Nitrogen that would be gas on Earth lies frozen on the ground here.
Like many icy worlds, Triton may hide an ocean under its crust, kept liquid by heat from radioactive elements in its rock. This has not been confirmed.
A slow ending
The same backward orbit that reveals Triton’s past also decides its future. Tides between Triton and Neptune are slowly drawing it inward. One day it will reach the distance at which Neptune’s gravity tears a moon apart, and it will break up or crash into the planet, but estimates of when range from about 3.6 billion to about 28 billion years. A mission called Trident was proposed to fly past Triton in 2038; it was a NASA finalist in 2020 but was not selected, and no spacecraft is now on its way.
How hot is it?
Earth (average)288 K · 15 °C
Water boils373 K · 100 °C
Venus surface737 K · 464 °C
Iron melts1 811 K · 1 540 °C
Sun's surface5 772 K · 5 500 °C
Triton38 K · -235 °C
Triton: surface temperature. Logarithmic scale from 100 K to 10 000 K. Reference values: NASA, IAU.
Gallery
A rough, pale pink plain of frost and scattered ice blocks under a black, starry sky, a thin grey plume rising from beyond low hills and a small blue crescent of Neptune, an impression of a geyser on Triton. AI-generated painting.Triton, 3D artist's impression, rendered from the interactive model.Triton, 3D artist's impression, from another angle, rendered from the interactive model.Triton, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
1846William Lassell discovers Triton, 17 days after Neptune.
1989Voyager 2 flies past on 25 August, the only visit so far.
1990Triton's geyser plumes are described from Voyager's images.
2006Craig Agnor and Douglas Hamilton show how Neptune could have captured it.
2020The Trident flyby mission becomes a NASA finalist; it is not selected in 2021.
Questions people ask
Why does Triton orbit backwards?
Because it did not form around Neptune. It once orbited the Sun beyond Neptune and was captured, most likely when Neptune broke up a pair of bodies orbiting each other.
How cold is Triton?
About 38 K (-235 °C), one of the coldest surfaces ever measured, though shadowed craters on the Moon have measured colder.
Will Triton crash into Neptune?
Eventually, probably. Its orbit is slowly shrinking, and it will break up or hit the planet, but estimates of when range from about 3.6 to 28 billion years.