Title: Triton (moon) Content: thumb|274x274px|William Lassell, the discoverer of Triton thumb |180px |Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black). thumb|Triton compared to the Earth and the Moon. Triton, or Neptune I, is the largest moon of the planet Neptune. It is the seventh-largest moon in the Solar System. Triton is slightly smaller than Earth's Moon. Triton has a complicated geological history: it is thought to have a comparatively young surface compared to the age of the Solar System itself. It was discovered by the British astronomer William Lassell on 10 October 1846, just 17 days after Neptune itself was discovered by the German astronomer Johann Gottfried Galle and Heinrich Louis d'Arrest. Triton is believed to be a captured Kuiper Belt object, and is the coldest-known body in the Solar System. The surface temperature of Triton was recorded by Voyager 2 as -235 °C (-391 °F). Triton has its own magnetic field and has a faint trace of an atmosphere. == Orbit == Drag and tidal interaction cause Triton to fall slowly into lowered orbits. This process is immensely slow. 3.6 billion years from now, Triton will pass within Neptune's Roche limit.Chyba C.F; Jankowski D.G. & Nicholson P.D. (1989). "Tidal evolution in the Neptune-Triton system". Astronomy and Astrophysics 219 (1–2): L23–L26. It will either collide with Neptune's atmosphere or break up, forming a ring system similar to that found around the planet Saturn. The reason why Triton is thought to be a captured object is its orbit, which is unique in the Solar System. It is both retrograde (see diagram) and highly tilted. Although there are other satellites with retrograde orbits, they are much smaller than Triton, and much more distant from their host planets (their "primaries"). Triton, like our Moon, shows only one face to the planet: it rotates in synchrony to Neptune. thumb|Neptune and Triton with their sizes and distance from each other to scale. == Surface == All we know about the surface of Triton comes from one fly-by of Voyager 2 in 1989. There are few impact craters. This suggests the surface is quite young in astronomical terms: estimates are six to 50 million years.Schenk, Paul M. & Zahnle, Kevin (2007). "On the negligible surface age of Triton". Icarus 192 (1): 135–49. Despite its very cold temperature, the surface of Triton is geologically active. There are events which look like geysers, volcanos and earthquakes. All the materials involved are quite different from those on Earth. Most of the gases are frozen most of the time. Gases like nitrogen are frozen until something occurs to melt them, then it turns back into a gas. A temperature increase of just 4 K  is enough to make this happen. == Past formation and capture == Like most TNOs, Triton is mostly icy. While Triton's orbit does give us insight on its past, it is mostly unknown. It is speculated that Triton formed in a binary system, like Pluto, and was moving at extremely high speeds. However, when it passed through Neptune's Roche sphere the other body will have been flung out, reducing Triton's speed and allowing it to be captured. Currently, Triton is the most massive TNO to ever have existed, and its effects on Neptune's moon system were cataclysmic. All large moons were destroyed, flung out of orbit or crashed into Neptune itself.