Triton

A moon that orbits backwards, and one of the coldest places ever measured.

Triton is by far the largest of Neptune’s 16 known moons, and it holds more than 99.5% of all the mass orbiting the planet. It circles Neptune in the opposite direction to the planet’s spin, which strongly suggests it was captured from the Kuiper belt rather than formed in place. When Voyager 2 flew past in 1989, it found a young, frozen surface and dark plumes rising from it.

How big is Triton?

Triton is about 2,705 km across, roughly 78% of the diameter of our Moon and 21% of Earth’s. It is the seventh largest moon in the solar system, and slightly bigger than the dwarf planet Pluto.

Its mass is about 2.14 × 10²² kg, around 29% of the Moon’s mass and 0.36% of Earth’s. With a density of 2.06 g/cm³, Triton is probably about two thirds rock and metal and one third water ice, much like Pluto. Surface gravity is about 0.78 m/s², around 8% of what you feel on Earth.

Why does Triton orbit backwards?

Triton orbits Neptune at about 354,800 km, a little closer than our Moon is to Earth, and takes 5.88 days per lap. It is tidally locked, so its day is also 5.88 days long. Its orbit is tilted about 157° to Neptune’s equator, which means it travels in a retrograde direction: backwards compared with Neptune’s rotation.

Moons that form from the disc around a young planet orbit in the same direction as the planet spins. A large retrograde moon is very hard to explain that way, so most scientists think Triton began as an independent Kuiper belt object, like Pluto, and was captured by Neptune. One proposed route is that Triton was half of a binary pair and lost its partner during a close pass by Neptune.

Capture would have thrown Neptune’s original moons into disarray, which may explain the strange, stretched orbit of the moon Nereid. Tides heated Triton strongly as its orbit settled into the near-perfect circle seen today. Those same tides are now slowly pulling it inwards. In perhaps 3.6 billion years it may come close enough to be torn apart and form a ring, though that estimate is uncertain.

What is Triton’s surface like?

Triton is about 4.5 billion km from the Sun, so sunlight there is roughly 900 times weaker than on Earth. Its icy surface reflects most of what does arrive. Voyager 2 measured a surface temperature of about −235 °C (38 K), one of the coldest natural surfaces measured in the solar system.

The surface is mostly frozen nitrogen, with water ice, carbon dioxide, methane and carbon monoxide ices. A large polar cap of nitrogen frost covered much of the southern hemisphere in 1989. Elsewhere Voyager saw “cantaloupe terrain”, a wrinkled pattern of dimples and ridges found nowhere else, along with smooth plains that look like frozen lava lakes of water and ammonia.

There are few impact craters, which means the surface is geologically young. Estimates range from about 10 to 100 million years old, very recent compared with the 4.5-billion-year age of the solar system.

Geysers and a thin atmosphere

Voyager 2 spotted dark plumes rising about 8 km above the surface, then drifting sideways for more than 100 km in the wind. One leading idea is that weak sunlight shining through clear nitrogen ice warms dark material underneath, until pressurised gas bursts out, carrying dust with it. Other researchers think internal heat plays a part. The mechanism is not settled.

Triton has a thin atmosphere of nitrogen with small amounts of methane and carbon monoxide. In 1989 its surface pressure was about 1.4 pascals, about 1/70,000 of Earth’s. Even so, it is thick enough to carry haze and thin clouds. Neptune takes 165 years to orbit the Sun, so Triton’s seasons last decades. Observations from Earth since 1989, made by watching stars pass behind Triton, show the pressure has changed as frost moves between the poles.

Discovery and exploration

British astronomer William Lassell discovered Triton on 10 October 1846, just 17 days after Neptune itself was found. The name, from the son of the sea god Poseidon in Greek mythology, came into use only in the twentieth century.

Only one spacecraft has visited. Voyager 2 made its closest approach on 25 August 1989, passing about 40,000 km from Triton and imaging around 40% of its surface. NASA studied a low-cost flyby mission called Trident in 2020 and 2021, but it was not selected, so no mission to Triton is currently approved. A gravity-assist route, explained in our lesson on gravity assists, would be needed to get there in a reasonable time.

Can you see Triton from Earth?

Not with your eyes or binoculars. Neptune itself, at about magnitude 7.8, needs binoculars, and Triton is roughly 200 times fainter at about magnitude 13.5. With a 200 mm or larger telescope, a dark sky and a good star chart, you can pick it out as a faint point that shifts around Neptune over its 5.88-day orbit. Use the orrery to see where Neptune is this month.

Data sources

• JPL SSD: Planetary Satellite Physical Parameters
• NASA Science: Triton
• NASA NSSDCA Neptunian Satellite Fact Sheet
• NASA Science: Voyager mission