Triton (moon)
| Discovery | |||||||
|---|---|---|---|---|---|---|---|
| Discovered by | William Lassell | ||||||
| Discovered on | October 10, 1846 | ||||||
| Orbital characteristics | |||||||
| Semimajor axis | 354,760 km | ||||||
| Eccentricity | 0.0000 | ||||||
| Orbital period | 5.877 d (retrograde) | ||||||
| Inclination | 156.834° | ||||||
| Is a satellite of | Neptune | ||||||
| Physical characteristics | |||||||
| Mean diameter | 2706.8 km | ||||||
| Mass | 2.147×1022 kg | ||||||
| Mean density | 2.05 g/cm3 | ||||||
| Surface gravity | 0.78 m/s2 | ||||||
| Rotation period | synchronous | ||||||
| Axial tilt | |||||||
| Albedo | 0.7 | ||||||
| Surface temp |
| ||||||
| Atmospheric characteristics | |||||||
| Pressure | 0.001 kPa | ||||||
| Nitrogen | 99.9% | ||||||
| Methane | 0.1% | ||||||
The name "Triton" was proposed by Camille Flammarion in 1880. It was independently proposed by others [1] [1] [1], however in 1909 it was "not in general use". As late as 1939 it was noted that although Triton had a name, the name was "not generally used". [1] In the astronomical literature it was simply referred to as "the satellite of Neptune".
It is perhaps strange that Lassell did not see fit to name his discovery, since names were given a few years later to his subsequent discoveries of an eighth moon (Hyperion) of Saturn and of the third and fourth moons (Ariel and Umbriel) of Uranus.
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.
Perhaps it was the discovery of Nereid, the second moon of Neptune, in 1949 that finally prompted making "Triton" official.
| Table of contents |
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2 Seasons 3 Physical Characteristics 4 See also 5 External links |
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, Pasiphaë and Sinope and Saturn's moon Phoebe also orbit retrograde, 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 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.
Due to its retrograde motion, Triton's already-close orbit is slowly decaying further from tidal interactions, and it is predicted that in about 100 million years, it will be smashed by Neptune's gravitational field and end up as a gigantic ring around its mother planet.
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 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, 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.
Geologists recognise the following types of surface feature on Triton:
Orbit
Seasons
Physical Characteristics
| Neptune |
|---|
| Naiad | Thalassa | Despina | Galatea | Larissa | Proteus | Triton |
| Nereid | S/2002 N 1's group | S/2002 N 2's group | S/2002 N 4's group |
| (For other moons, see: Neptune's natural satellites) |