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|Discovered by||William Lassell|
|Discovered on||October 10, 1846|
|Semimajor axis||354,800 km|
|Orbital period|| 5.877 d|
|Mean diameter||2706.8 km|
|Mean density||2.05 g/cm3|
|Surface gravity||0.78 m/s2|
Triton (trye'-ton, Greek Τρίτων) is the planet Neptune's largest moon, discovered by William Lassell on 1846-10-10 , just 17 days after the planet itself was discovered. Lassell incorrectly believed that he had also seen a ring around Neptune. Triton is also designated Neptune I.
Triton is named after Triton, a sea god from Greek mythology. The name was proposed by Camille Flammarion in 1880. It is perhaps strange that Lassell, the discoverer, did not see fit to name his own discovery, since he gave names a few years later to his subsequent discoveries of an eighth moon of Saturn (Hyperion), and of the third and fourth moons of Uranus (Ariel and Umbriel).
After Flammarion's proposal, the name Triton was also independently proposed by others   , but in 1909 it was reported as "not in general use". As late as 1939 it was noted that although Triton had a name, the name was "not generally used" . In astronomical literature it was simply referred to as "the satellite of Neptune". Oddly enough, most of the references to Triton in the astronomical literature in the late 19th and early 20th centuries are to the name of a supposed Martian canal.
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). Some of the small outer moons of Jupiter and Saturn also have retrograde orbits, but all of them have less than 10% of the diameter of Triton. Moons in retrograde orbit cannot form out of the same region of the solar nebula as the planets they orbit, but must be captured from elsewhere; it is thought that Triton may be a captured Kuiper belt object. The capture of Triton may explain a number of features of the Neptunian system including the extremely eccentric orbit of Neptune's outermost moon Nereid, the paucity of moons as compared to the other gas giants (Triton's orbit would initially have crossed those of many other lighter moons, dispersing them through gravitational interaction), and the evidence of differentiation in 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.
Due to its retrograde motion, Triton's already-close orbit is slowly decaying further from tidal interactions and it is predicted between 1.4 and 3.6 billion years from now, Triton will pass within Neptune's Roche limit . The most likely outcome will be collision with Neptune's atmosphere, although ring formation due to tidal disruption is also a possible outcome.
Another unique feature of Triton's orbit, arising from tidal effects on such a large moon so close to its primary, is that it is for all practical purposes a perfect circle with an eccentricity of 0 to sixteen decimal places.
Triton's axis of rotation is also unusual, tilted 157° with respect to Neptune's axis, which is in turn inclined 30° 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 twice per Neptunian year, 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 g/cm³, 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 K, even colder than Pluto's average temperature of 44 K. Surprisingly, however, Triton is geologically active; its surface is fresh and sparsely cratered, and the Voyager 2 probe observed numerous icy 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, probably the result of freezing/thawing cycles. Triton's surface area is 23 million km².
Potential for life
Because of its geological activity and possible tidal heating (eons ago, when it was captured by Neptune; there is no tidal heating now), it has sometimes been suggested that Triton may harbour aquatic life-forms in a liquid stratum under its surface (similar suggestions have been made about Jupiter's moon Europa). If Triton is a candidate for extra-terrestrial life, it would not be like life on Earth, owing to the extreme cold, the nitrogen and methane environment, and the fact that the moon lies within Neptune's dangerous magnetosphere.
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