Neptune's backwards-orbiting captive
Triton
Moon of Neptune · discovered 1846 by William Lassell
Triton is by far the largest of Neptune's moons, holding more than 99 per cent of the mass of everything orbiting the planet. It is the only large moon in the Solar System that travels around its planet in the opposite direction to the planet's spin, a strong sign that it was born elsewhere and later captured.
Triton is also one of the coldest places ever measured, yet it is far from dead. When Voyager 2 flew past in 1989, it spotted dark plumes rising from the frozen surface, making Triton one of very few worlds known to be geologically active today.
Key facts
- Diameter
- 2,707 km78% of Earth's Moon
- Mass
- 2.14 × 10²² kg
- Surface gravity
- 0.779 m/s²0.079 g
- Density
- 2,061 kg/m³
- Distance from Neptune
- 354,759 km≈ 14.3 Neptune radii
- Orbital period
- 5.88 daysRetrograde — against the planet's spin
- Discovered
- 1846William Lassell
- Surface
- Nitrogen, water and carbon dioxide ices; few craters
- Atmosphere
- Thin nitrogen with traces of methane and hazes
- Surface temperature
- About −235 °C, among the coldest measured
- Notable feature
- Nitrogen geysers and 'cantaloupe terrain'
- Named after
- Triton, Greek sea god and son of Poseidon
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Triton's surface is a mix of frozen nitrogen, water ice, carbon dioxide and methane. It has very few impact craters, suggesting the surface is geologically young, perhaps only around 100 million years old or less.
Much of the western hemisphere is covered by 'cantaloupe terrain', a dimpled landscape of rounded depressions and crossing ridges resembling the skin of a melon. Scientists think it may have formed as blobs of less dense ice rose through the crust. Elsewhere are smooth plains and walled basins that some researchers interpret as cryovolcanic features, where icy slush once erupted instead of molten rock.
The southern polar cap, a bright, slightly pinkish expanse of nitrogen frost, is streaked with dark deposits. Voyager 2 saw plumes there rising about 8 kilometres before being blown sideways by winds into trails more than 100 kilometres long. These geysers may be driven by sunlight warming nitrogen ice from below, though an internal heat source has also been proposed.
Interior and composition
Triton is fairly dense for an icy moon, suggesting a large rock and metal core beneath a mantle of water ice and a surface crust of frozen volatiles. Heat from radioactive decay, and especially from tidal forces during and after its capture, may have been enough to keep a layer of liquid water beneath the ice. Whether a subsurface ocean survives today remains an open question.
Orbit and rotation
Triton orbits Neptune every 5.9 days in a retrograde direction, on a path tilted steeply to Neptune's equator, and it is tidally locked. Because of the combined tilt of Triton's orbit and Neptune's axis, its poles take turns facing the Sun over Neptune's 165-year orbit, producing extreme seasons.
Its backward motion means tides from Neptune steadily sap its orbital energy, drawing it slowly closer. Scientists estimate that in roughly 3.6 billion years Triton could cross Neptune's Roche limit and be torn apart, perhaps forming a spectacular ring system, though the exact timing is uncertain.
Origin
Triton almost certainly did not form around Neptune. Its retrograde orbit and its strong similarity to Pluto in size, density and surface ices point to an origin in the Kuiper Belt. One widely discussed idea is that Triton was once part of a binary pair; when the pair swept close to Neptune, its partner was flung away and Triton was captured.
Capture would have been violent for Neptune's original moons, which may have been scattered or destroyed. Tidal heating as Triton's orbit circularised could also have melted much of its interior.
Exploration
Only one spacecraft has ever visited Triton, so much of what we know comes from a single day in 1989, supplemented by telescopes on Earth and in space.
Voyager 2 (1989)
On 25 August 1989, Voyager 2 flew within about 40,000 kilometres of Triton, imaging around 40 per cent of its surface and discovering the geysers, cantaloupe terrain and thin nitrogen atmosphere.
Stellar occultations (1990s–present)
By watching Triton pass in front of distant stars, astronomers have tracked changes in its atmosphere, which appeared to thicken in the years after Voyager's visit.
Space telescopes and mission concepts
Hubble and JWST continue to study Triton's ices. Proposed missions, including NASA's Trident flyby concept, have not yet been selected, so no return visit is currently scheduled.
Surprising facts about Triton
- Triton is the only large moon in the Solar System with a retrograde orbit, travelling against the direction of Neptune's rotation.
- Its surface temperature of about −235 °C is among the coldest ever measured on any body in the Solar System.
- Despite the cold, Voyager 2 saw dark plumes erupting about 8 kilometres high, making Triton one of only a handful of worlds known to be active today.
- Triton contains more than 99.5 per cent of the mass of all of Neptune's known moons combined.
- It is spiralling slowly inwards and, scientists estimate, could be torn apart by Neptune's gravity in roughly 3.6 billion years.