Triton (astronomy), largest moon of the planet Neptune. Triton orbits Neptune at an average distance of 354,800 km (220,400 mi). Triton rotates once every 5.877 Earth days and completes one orbit in the same amount of time, which means that one side of the moon faces Neptune at all times while the other faces out into space. Triton’s radius is 1,353 km (841 mi), making it one of the ten largest moons in the solar system. Based on the moon’s density, scientists estimate that Triton is about one-quarter ice and three-quarters rock. Triton has a complex and geologically young surface.

Triton follows a retrograde orbit, which means that it moves clockwise as seen from Neptune’s north pole, in the opposite direction that Neptune rotates. All other large bodies in the solar system travel counterclockwise as seen from the north pole of the body they orbit. The planets travel around the Sun in counterclockwise orbits, and the Sun and most of the planets rotate counterclockwise as well. This uniform orbital and rotational motion is a result of the fact that the Sun, planets, and moons condensed from a rotating cloud of material. Triton’s retrograde orbit has led scientists to theorize that the moon formed elsewhere in the solar system and that Neptune captured it from an independent orbit around the Sun. According to this theory, Triton was captured by Neptune’s gravity into an elliptical (oval-shaped) orbit around the planet. The moon probably rotated on its axis more quickly at that time. Over about a billion years, Neptune’s gravity slowed Triton’s rotation so that it rotated only once during each orbit and forced Triton into a circular orbit. This made Triton flex, rubbing together rocks inside the moon and producing frictional heat that reshaped Triton’s surface, erasing many impact craters. As a result, Triton has few craters but many cracks in its surface.

Triton has flat ice plains and volcano-like features that measure up to 200 km (120 mi) across. Triton’s southern hemisphere has a pink polar cap with a blue edge. The pink color is probably caused by compounds formed when frozen methane is exposed to sunlight, and the blue color may indicate fresh deposits of nitrogen ice. Geysers spew dark plumes into Triton’s thin atmosphere. Voyager 2 actually caught one erupting as the spacecraft passed by Neptune in 1989. These geysers may be caused by sunlight vaporizing liquid nitrogen under the surface. Nitrogen ice crystals form clouds 5 to 10 km (3 to 6 mi) above Triton’s surface.

Astronomers have discovered that since 1989, Triton’s temperature has increased from –235°C (–392°F) to –233°C (-389°F). This warming trend is slowly turning frozen nitrogen on Triton into gas, creating a denser atmosphere. Scientists believe that Triton’s warming is part of a natural cycle that repeats itself every few hundred years when the moon’s southern hemisphere points directly into the Sun. Direct sunlight on the southern hemisphere heats the polar ice fields on Triton, causing a gradual global warming.

British astronomer William Lassell discovered Triton in 1846, only weeks after Neptune was discovered. In Greek mythology, Triton was the son of the god Poseidon, whom the Romans named Neptune. Individual features on Triton bear the names of water deities outside of Greek and Roman mythology.