Triton (moon)

Triton is the largest natural satellite of the planet Neptune. It is special because it is the only large moon in the Solar System that orbits in a retrograde orbit, meaning it moves in the opposite direction to Neptune's rotation. Scientists believe Triton may have once been a dwarf planet from the Kuiper belt before it was captured by Neptune's strong gravity.

Triton is the seventh-largest moon in the Solar System and is bigger than all known dwarf planets. It is made up of about 30–45% water ice and the rest is rock and metal. Triton has a thin, hazy atmosphere and a surface covered in frozen nitrogen. It is also one of the most geologically active worlds in our Solar System, with young surfaces and features formed by cryovolcanic and tectonic activity.

Triton was first seen on October 10, 1846, by the English astronomer William Lassell. The only close-up visit to Triton so far was by the Voyager 2 spacecraft in 1989. Because Voyager 2 could only study about 40% of Triton's surface, scientists have proposed new missions, such as the Trident and Triton Ocean Worlds Surveyor, to learn more about this fascinating moon.

Discovery and naming

Triton was discovered by British astronomer William Lassell on October 10, 1846, just 17 days after the discovery of Neptune. When John Herschel heard about Neptune, he suggested Lassell look for moons, and Lassell found Triton eight days later. Lassell used a telescope he built himself to make this discovery.

Triton is named after the Greek sea god Triton, the son of Poseidon, who is the Greek equivalent of the Roman god Neptune. The name was first suggested in a book in 1880 and was officially chosen many years later. Before another moon called Nereid was found in 1949, Triton was simply known as the moon of Neptune.

Orbit and rotation

Triton is special because it has a retrograde orbit around Neptune, meaning it moves in the opposite direction to Neptune's rotation. It is the only large moon in the Solar System with this kind of orbit. Triton’s orbit is nearly a perfect circle and it is tidally locked, so one side always faces Neptune.

As Triton orbits Neptune, its polar regions experience seasonal changes, with one pole and then the other moving into sunlight. Scientists predict that in the distant future, Triton may come close enough to Neptune to either collide with the planet or break apart, forming a new ring system.

Capture

Main article: Capture of Triton

Triton, Neptune's largest moon, likely did not form around Neptune. Instead, it was captured from elsewhere in the Solar System. Scientists think it may have come from the Kuiper belt, a region filled with icy objects beyond Neptune's orbit. This area is also where Pluto, a dwarf planet similar in size and composition to Triton, is located.

After Triton was pulled into Neptune's orbit, its path was stretched and unusual. This capture might explain why Neptune has fewer moons than other giant planets and why some of its moons have unusual orbits. Over time, Triton's orbit settled into its current shape.

Physical characteristics

Triton is the largest moon of the planet Neptune and makes up more than 99.5% of all the mass that orbits Neptune. It is also the seventh-largest moon in the Solar System and is slightly larger than the dwarf planet Pluto. Triton’s surface is covered with a thin layer of frozen nitrogen, similar to ice, and also contains water ice and dry ice.

Triton has a very high reflectivity, meaning it reflects most of the sunlight that reaches it, which keeps it extremely cold. Scientists think Triton may have a subsurface ocean of water, which could potentially support some forms of life. The moon’s surface shows signs of recent geological activity, likely caused by heat from tides and radioactive decay inside the moon.

Atmosphere

Main articles: Atmosphere of Triton and Climate of Triton

Triton has a very thin atmosphere made mostly of nitrogen, with small amounts of carbon monoxide and methane. This atmosphere forms when nitrogen from Triton's surface turns into gas. Triton's surface is extremely cold, about -237 °C, which keeps the nitrogen in a special icy form.

Triton's atmosphere has layers, like Earth's, but it behaves differently. Winds on Triton can move tiny particles across its surface. The upper part of Triton's atmosphere is warmer than its surface because it absorbs heat from the Sun and Neptune's magnetic field. There are also hazy layers and clouds of frozen nitrogen in Triton's atmosphere.

Surface features

Main article: Geology of Triton

All the details we know about Triton's surface come from the Voyager 2 spacecraft's flyby in 1989. It showed Triton's surface has many different features, including ridges, valleys, plains, and a few craters. Triton is quite flat, with its surface height never changing more than a kilometer. The surface is mostly made of frozen nitrogen, with some water ice and a smaller amount of frozen carbon dioxide.

One special feature is Leviathan Patera, a large, caldera-like area about 100 kilometers wide. Near it are two big lakes of cryolava, made of water ice and ammonia. Triton is also known for its nitrogen geysers, which shoot gas and dust up to 8 kilometers high. These geysers might be caused by the Sun's heat warming the ice below, or they could be cryovolcanic eruptions.

Observation and exploration

The orbital properties of Triton were determined with high accuracy in the 19th century. The first detailed observations of Triton were not made until 1930. Little was known about the satellite until Voyager 2 flew by in 1989.

Before the flyby of Voyager 2, astronomers suspected that Triton might have liquid nitrogen seas. The first attempt to measure the diameter of Triton was made by Gerard Kuiper in 1954. Data from the approach of Voyager 2 to Neptune on August 25, 1989, led to a more accurate estimate of Triton's diameter (2,706 km).

Many concepts for missions to the Neptune system have been proposed, with Triton as a prime target. A proposed lander mission called Triton Hopper would mine nitrogen ice from Triton's surface. Another concept, Trident, was proposed in 2019. Neptune Odyssey is a mission concept studying a Neptune orbiter focused on Triton, possibly launching in 2033. Two lower-cost mission concepts were developed for the New Frontiers program: Triton Ocean World Surveyor, launching in 2031, and Nautilus, launching in 2042.

Maps

Triton is the largest moon of the planet Neptune and is special because it orbits in the opposite direction to Neptune's rotation. This makes it the only large moon in our Solar System with such a retrograde orbit. Maps of Triton show its surface features, helping scientists understand more about this unique moon.

Enhanced-color map; leading hemisphere is on right

Enhanced-color polar maps; south is right