Largest Moon of Neptune - Triton (7th Moon outwards from Neptune)

Discovery

Neptune has thirteen known moons. The largest of which is Triton, discovered by William Lassell on October 10, 1846, just 17 days after the discovery of Neptune itself.

Naming
Triton is named after the Greek sea god Triton, the son of Poseidon (the Greek god comparable to the Roman Neptune). The name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire, but it did not come into common use until at least the 1930s.


Voyager 2 photomosaic of Triton's sub-Neptunian hemisphere.



Triton 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.

Stats

Diameter: 2,705 km

Semi-major axis: 354,759 km

Orbital Period: 5.88 days

Triton is the seventh largest moon in the Solar System, and is larger than the dwarf planets Pluto and Eris. Because of its retrograde orbit and composition similar to Pluto's, it is thought to have been captured from the Kuiper belt.

Capture from Kuiper belt

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 orbits of irregular moons and disrupted those of smaller regular moons, dispersing them through gravitational interactions.

Two types of mechanisms have been proposed for Triton's capture.

In order to be gravitationally captured by a planet, a passing body must lose sufficient energy to be slowed down to a speed less than that required to escape. 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).

A more recent and now favored hypothesis suggests that, before its capture, Triton had a massive companion similar to Pluto's moon Charon with which it formed a binary. When the binary encountered Neptune, it interacted in such a way that orbital energy was transferred from Triton to its companion; the latter was expelled, while Triton became bound to Neptune.

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. Events like this may have been common during the formation of Neptune, or later when it migrated outward.

Orbit

Triton takes as long to rotate on its axis as it does to make one orbit of Neptune; and therefore always keeps the same hemisphere pointed to Neptune.

Atmosphere

Triton has a surface of mostly frozen nitrogen, a mostly water ice crust, an icy mantle and a substantial core of rock and metal.

Triton is one of the few moons in the Solar System known to be geologically active. As a consequence, its surface is relatively young and has relatively few impact craters, with a complex geological history revealed in intricate and mysterious cryovolcanic and tectonic terrains. Part of its crust is dotted with geysers believed to erupt nitrogen.

Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and 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 −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.