Triton (moon)

Triton is the largest natural satellite of the planet Neptune. It is special because it orbits in the opposite direction to Neptune's rotation. This is called a retrograde orbit. Scientists think Triton may have been a dwarf planet from the Kuiper belt before Neptune's strong gravity pulled it in.

Triton is the seventh-largest moon in the Solar System. It is bigger than any known dwarf planets. Triton is made of about 30–45% water ice, with the rest being rock and metal. It has a thin, hazy atmosphere and a surface covered in frozen nitrogen. Triton is very active, with young surfaces created 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 was by the Voyager 2 spacecraft in 1989. Because Voyager 2 could only study part of Triton's surface, scientists want to send new missions to learn more about this interesting 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. This means 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. This means one side always faces Neptune.

As Triton orbits Neptune, its polar regions change with seasons. One pole moves into sunlight, then the other. Scientists think that in the far future, Triton might come close enough to Neptune to either hit the planet or break apart, forming a new ring system.

Capture

Main article: Capture of Triton

Triton, Neptune's largest moon, probably did not form with Neptune. Instead, it was captured from somewhere else in the Solar System. Scientists think it may have come from the Kuiper belt, a region with icy objects beyond Neptune. This area is also where Pluto, a dwarf planet similar to Triton, is found.

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

Physical characteristics

Triton is the largest moon of the planet 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. It also contains water ice and dry ice.

Triton reflects most of the sunlight that reaches it, which keeps it very cold. Scientists think Triton may have a subsurface ocean of water. 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

We learned about Triton's surface from the Voyager 2 spacecraft in 1989. Triton's surface has many different features, like ridges, valleys, plains, and a few craters. Triton is very flat, with its surface height changing by no more than a kilometer.

The surface is mostly made of frozen nitrogen, with some water ice and a little frozen carbon dioxide.

One special feature is Leviathan Patera, a large area about 100 kilometers wide. Near it are two big lakes of cryolava, made of water ice and ammonia. Triton also has nitrogen geysers that shoot gas and dust up high. These geysers might be caused by the Sun's heat or cryovolcanic activity.

Observation and exploration

The orbit of Triton was measured well in the 1800s. We did not learn much until 1930. We learned more when Voyager 2 flew past in 1989.

Before Voyager 2 flew by, some thought Triton might have seas of liquid nitrogen. Gerard Kuiper tried to measure Triton's size in 1954. Data from Voyager 2 helped us learn that Triton is about 2,706 km wide.

Many ideas for trips to the Neptune system have been suggested, with Triton as a main goal. One idea is a lander called the Triton Hopper to collect nitrogen ice. Another idea is Trident, suggested in 2019. Neptune Odyssey is a plan to study Triton from orbit, maybe starting in 2033. Two smaller plans were made for the New Frontiers program: Triton Ocean World Surveyor, starting in 2031, and Nautilus, starting in 2042.

Maps

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

Enhanced-color map; leading hemisphere is on right

Enhanced-color polar maps; south is right