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Triton is the largest moon of the planet Neptune, discovered on October 10, 1846 by William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2700 km in diameter, it is the seventh-largest moon in the Solar System. Triton comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and twelve other known moons. It is also more massive than all the Solar System's 159 known smaller moons combined.

Because of its retrograde orbit (unique for an object of its size) and similar composition to Pluto, Triton is thought to have been captured from the Kuiper belt. Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a substantial core of rock and metal. The core makes up two-thirds of its total mass. Triton has a mean density of 2.061 g/cm³ and is composed of approximately 15–35% water ice.

Triton is one of the few moons in the Solar System known to be geologically active. Its crust is dotted with geysers believed to erupt nitrogen. As a consequence, its surface is relatively young, with a complex geological history revealed in intricate and mysterious tectonic terrains. Triton has a tenuous nitrogen atmosphere less than 1/70 000^th the pressure of Earth's atmosphere at sea level.

The moon was discovered by British astronomer William Lassell on October 10, 1846, just 17 days after Neptune itself was discovered by German astronomers Johann Gottfried Galle and Heinrich Louis d'Arrest.

A brewer by trade, Lassell began making mirrors for his amateur telescope in 1820. When John Herschel received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell did so and discovered Triton just eight days later. Lassell also claimed to have discovered rings. However, although Neptune does have rings, they are so faint and dark that it is unlikely he actually observed them.

Triton is named after the Greek sea god Triton (Τρίτων), the son of Poseidon (the Greek god comparable to the Roman Neptune).

Orbit and rotation

Triton is unique among all large moons in the Solar System for its retrograde orbit around its planet (i.e., it orbits in a direction opposite to the planet's rotation). Most of the outer irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus' outer moons. However, these moons are all quite small in comparison; the largest of them (Phoebe) has only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton orbits in synchronous rotation about Neptune; it keeps one face oriented toward the planet at all times. Triton's axis of rotation is also unusual, tilted 157 degrees with respect to Neptune's axis, which is in turn inclined 30 degrees from the plane of Neptune's orbit. The net result of these two axial tilts is that Triton's rotational axis lies close to the plane of Neptune's orbit, and hence during Neptune's year each pole points almost directly toward the Sun, much like Uranus'. As Neptune orbits the Sun, Triton's polar regions take turns facing the sun, probably resulting in radical seasonal changes as one pole then the other moves into the sunlight. The proximity of such a large moon to Neptune has resulted in tidal deceleration, which has altered the moon's orbit and rotation through gravitational interaction.

Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity of almost zero. However, viscoelastic damping from tides alone are not believed to be capable of circularizing Triton's orbit in the time since the origin of the system, and gas drag from a prograde debris disc is likely to have played a substantial role. Tidal interactions have also meant that Triton's already close orbit to Neptune is slowly decaying further, and predictions are that, some 3.6 billion years from now, Triton will pass within Neptune's Roche limit. This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a ring system similar to that found around Saturn.

Capture

Moons in retrograde orbits cannot have formed out of the same region of the solar nebula as the planets they orbit, but must have been captured from elsewhere. Triton is therefore suspected of being captured from the Kuiper belt. The Kuiper belt is a ring of small icy objects extending outward from just inside the orbit of Neptune to about 55 AU from the Sun. Believed to be the point of origin for the majority of short-period comets observed from Earth, it is also home to several large, planet-like bodies including Pluto, which is now recognized as the largest in a population of Kuiper belt objects (the Plutinos) locked in orbital step with Neptune. Triton is only slightly larger than Pluto and nearly identical in composition, which has led to the hypothesis that the two share a common origin.

The proposed capture of Triton may explain several features of the Neptunian system including the extremely eccentric orbit of Neptune's moon Nereid and the scarcity of moons as compared to the other gas giants. Triton's initially eccentric orbit would have intersected irregular moons and disrupted those of smaller natural moons, dispersing them through gravitational interactions.

The circularization of Triton's eccentric post-capture orbit would have resulted in heating of the moon by tidal forces. These would have kept Triton liquid for a billion years, which is supported by evidence of differentiation in the moon's interior.

There are two ways which Triton's capture may have occurred. In order to be gravitationally captured by a planet, a moon must be slowed down by losing energy. An early theory of how Triton may have been slowed was by collision with another object, either one that happened to be passing by Neptune (which is unlikely), or a moon or proto-moon in orbit around Neptune (which is more likely). Another hypothesis suggests that, before its capture, Triton may have had a massive companion similar to Pluto's moon Charon with which it formed a binary. When the binary encountered Neptune, Triton's companion was expelled, providing the required mechanism to capture Triton in an orbit around the planet. This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects. The event was brief but gentle, saving Triton from collisional disruption; and events like this may have been common during the formation of Neptune, or later when it migrated outward.

Physical characteristics

Triton has a similar size, density (2.061 g/cm³), temperature and chemical composition to that of Pluto. As with Pluto, 55% of Triton's surface is covered with frozen nitrogen, with water ice comprising 15–35% and dry ice (frozen carbon dioxide) forming the remaining 10–20%. Traces include 0.1% methane and 0.05% carbon monoxide ice. There could be ammonia on the surface that resulted from possible ammonia dihydrate in the lithosphere. This density means Triton is probably about 30–45% water ice, with the remainder being rocky material. Triton's surface area is 23 million km², which is 4.5% of Earth, or 15.5% of Earth's land area. Triton is very bright, reflecting 60–95% of the sunlight that reaches it. By comparison, Earth's moon reflects only 11%. Triton's reddish colour is believed to be the result of methane ice which reduces to carbon under bombardment from ultraviolet radiation.

Because Triton's surface indicates a long history of melting, models of its interior posit that Triton is differentiated, like Earth, into a solid core, a mantle and a crust. Water, the most abundant volatile in the Solar System, comprises the moon's mantle, which lies over a core of rock and metal. There is enough rock in Triton's interior for solid-state convection to be occurring within its mantle, powered by radioactive decay. The heat may even be sufficient to maintain a "subterranean ocean" similar to that which is hypothesized to exist underneath the surface of Europa. The possible presence of a layer of liquid water suggests the possibility, if unlikely, of life.

Triton has a tenuous nitrogen atmosphere with small amounts of methane near the surface. Like Pluto's atmosphere, the atmosphere of Triton is believed to have resulted from evaporation of nitrogen from the moon's surface. The surface temperature is at least 35.6 K (−237.6 °C) because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature. An upper limit with kelvins in the low 40s can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. This temperature range is colder than Pluto's average equilibrium temperature of 44 K (−229 °C). Triton's surface atmospheric pressure is only about 1.4–1.9 pascal (0.014–0.019 millibar).

Turbulence at Triton's surface creates a troposphere (a "weather region") rising to an altitude of 8 km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometre in size. Unlike other atmospheres, Triton's has no stratosphere, and instead consists of a thermosphere from 8 to 950 km above the surface, and an exosphere above that. The temperature of Triton's upper atmosphere, at 95 ± 5 kelvins, is higher than the temperature at the surface due to heat deposited from space. A haze permeates most of Triton's troposphere, believed to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also possesses clouds of condensed nitrogen that lie between 1 and 3 km from the surface.

In the 1990s, observations from Earth were made of Triton's limb as the moon passed in front of stars. These observations indicated the presence of a denser atmosphere than was thought from Voyager 2 data. Other observations have shown an increase in temperature by 5% from 1989 to 1998. These observations indicate Triton is approaching an unusually warm summer season that only happens once every few hundred years. Theories for this warming include a change of frost patterns on Triton's surface and a change in ice albedo, which would allow more heat to be absorbed. Another theory argues the changes in temperature are a result of deposition of dark, red material from geological processes on the moon. Because Triton's Bond albedo is among the highest within the Solar System, it is sensitive to small variations in spectral albedo.

Surface features

All detailed knowledge of the surface of Triton was acquired in a single encounter by the Voyager 2 spacecraft in 1989. The 40% of Triton's surface imaged by Voyager revealed rocky outcrops, canyons and icy melt, mainly frozen methane. Triton is relatively flat; its observed topography never varies beyond a kilometer. There are relatively few impact craters on Triton. Recent analysis of crater density and distribution has suggested that in geological terms, Triton's surface is extremely young, with regions varying from 50 million years old to just 6 million years old.

Cryovolcanism

Triton is geologically active; its surface is young and has relatively few impact craters. When the Voyager 2 probe studied Triton, it observed numerous icy volcanoes or geysers erupting liquid nitrogen, dust, or methane compounds from beneath the surface in plumes up to 8 km high. Although Triton is made of various ices, its subsurface processes are similar to those that produce volcanoes and rift valleys on Earth, but with water and ammonia lavas as opposed to liquid rock.

The volcanic activity is thought to be driven by seasonal heating from the Sun, unlike the tidal heating responsible for the volcanoes of Io. Solar heat is trapped under surface nitrogen ice, which creates a form of "solid greenhouse effect", slowly heating the subsurface until nitrogen beneath evaporates and erupts through the crust. Between 1977 and the Voyager flyby in 1989, Triton shifted from a reddish colour, similar to Pluto, to a far paler hue, suggesting that cryovolcanism in the intervening decade had covered older reddish material with lighter nitrogen frosts.

Triton's entire surface is cut by complex valleys and ridges, probably the result of tectonics and icy volcanism. The vast majority of surface features on Triton are endogenic—the result of internal geological processes rather than external processes such as impacts. Most are volcanic and extrusive in nature, rather than tectonic. Hili and Mahilani are two candidate cryovolcanoes that have been observed on Triton. (They are named after a Zulu water sprite and a Tongan sea spirit, respectively.) Triton thus joins the Earth, Io, and Enceladus as one of the few worlds of the Solar System with current known volcanic activity. (Venus, Mars, Titan, and Dione may also be volcanically active.) The eruption of volatiles from Triton's equator and its deposition at the poles may redistribute enough mass over the course of 10 000 years to cause polar wander.