Triton (moon)

Triton is the planet Neptune's largest moon, discovered by William Lassell[?] in 1846 just 17 days after the planet itself was discovered. It is named after Triton, from Greek mythology.

Triton is unique among all large moons in the solar system for its retrograde orbit around the planet (i.e., it orbits in a direction opposite to the planet's rotation). Jupiter's moons Ananke, Carme, Pasiphae and Sinope and Saturn's moon Phoebe also orbit retrograde, but all of them are less than 1/10 the diameter of Triton. Moons in retrograde orbit cannot form out of the same solar nebula as the planets they orbit, they must be captured from elsewhere; it is thought that Triton may be a captured Kuiper belt object. The capture of Triton may also explain the extremely eccentric orbit of Neptune's outermost moon Nereid, as well as having provided the heat necessary to melt and differentiate Triton's interior (tidal heating resulting from an eccentric post-capture orbit being circularized could have kept Triton liquid for a billion years). Its similarity in size and composition to Pluto, as well as to Pluto's eccentric Neptune-crossing orbit, provides further tantalizing hints to Triton's possible origin as a Pluto-like planetary body.

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 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. During the Voyager 2 encounter, Triton's south pole was facing the Sun. Almost the entire southern hemisphere is covered with an "ice cap" of frozen nitrogen and methane.

Triton has a density of 2.0, and is probably about 25% water ice with the remainder being rocky material. It has a tenuous nitrogen atmosphere with small amounts of methane. Its atmospheric pressure is only about 0.01 millibar. Triton's surface temperature is only 34.5 Kelvin, as cold as Pluto's. Surprisingly, however, Triton is geologically active; its surface is fresh and sparsely cratered, and the Voyager 2 probe observed numerous volcanoes erupting liquid nitrogen, dust, or methane compounds from beneath the surface in plumes up to 8 km high. This volcanic activity is thought to be driven by seasonal heating from the Sun, unlike the tidal heating responsible for the volcanoes of Io. There are extensive ridges and valleys in complex patterns all over Triton's surface. These are probably the result of freezing/thawing cycles. Triton's surface area is 23 million km².

Due to its retrograde motion, Triton's already-close orbit is slowly decaying further from tidal interactions. One day it will approach Neptune's Roche limit and it will be torn apart by tidal forces, forming a spectacular planetary ring system much like Saturn's.

This color mosaic of Triton was taken on a flyby of the Voyager 2 probe in 1989. Color was synthesized by combining high-resolution images taken through orange, violet, and ultraviolet filters; these images were displayed as red, green, and blue images and combined to create this color version. NASA (full size version)