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Orbital period: 141 hours
Distance to Earth: 4,338,000,000 km
Orbit: 354,760 km from Neptune
Diameter: 2700 km
Orbital period: 6 days

Discovered by Lassell in 1846 only a   few weeks after the discovery of Neptune itself.

Triton has been visited by only one spacecraft, Voyager  2 on Aug 25 1989.  Almost everything we know about it comes from this  encounter.

Triton’s orbit is retrograde. It is the  only large moon to orbit “backwards”, the only other moons with retrograde orbits are Jupiter’s   moons Ananke,  Carme, Pasiphae and Sinope and Saturn’s Phoebe all of which are less than 1/10 the diameter  of Triton.  Triton could not have condensed from the primordial  Solar Nebula in       this configuration; it must have  formed elsewhere (perhaps in the Kuiper Belt)  and later been captured  by Neptune (perhaps involving a collision with  another now shattered Neptunian moon).  A capture scenario could account  not only for Triton’s orbit but also  for the unusual orbit of  Nereid and provide the energy needed to melt and   differentiate Triton’s interior.

Because of its retrograde orbit, tidal interactions between Neptune and  Triton remove energy from Triton thus lowering its orbit.  At some very  distant future time it will  either break up (perhaps forming a ring) or  crash into Neptune.

The unusual nature of Triton’s orbit, the similarity of bulk properties  between Pluto and Triton, and the highly  eccentric,   Neptune-crossing nature of Pluto’s orbit suggest some historical connection between them. Exactly what this might be is purely   conjecture at this time however.

Triton, a moon of Neptune

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). This adds up to an orientation  with respect to the   Sun somewhat like Uranus’s  with polar and equatorial regions alternately pointing toward the Sun.  This probably results 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.

Triton’s density (2.0) is slightly greater than that of Saturn’s icy moons  (e.g. Rhea). Triton is probably only about 25% water   ice with remainder rocky material.

Voyager found that Triton has an atmosphere, albeit a very tenuous one (about  0.01 millibar), composed mostly of  nitrogen  with a small amount of methane. A thin haze extends up 5-10 km.

The temperature at the surface of Triton is only 34.5 K (-235 C, -391 F),   as cold as Pluto. This is due in part to its high        albedo (.7 – .8) which means that little of the  Sun’s meager light is absorbed.  At this temperature methane,  nitrogen and carbon dioxide all freeze solid.

Triton ice cap

There are  very few craters visible; the surface is  relatively young. Almost the entire  southern hemisphere is covered with an “ice cap” of frozen nitrogen and  methane (right).

There are extensive ridges and valleys in complex patterns all over Triton’s surface. These are probably the result of freezing/thawing cycles.

Triton surface

The most interesting (and totally unexpected)  features of this unusually interesting world are the   ice volcanoes. The eruptive material is probably  liquid nitrogen,  dust, or methane compounds from beneath the surface.  One of Voyager’s  images shows an actual plume rising 8 km  above the surface and extending  140 km “downwind” (left).

Triton, Io and Venus  are  the only bodies in the solar system besides  Earth that are known to be volcanically active at   the present time (though Mars clearly was in the past).  It’s also interesting to note that very different volcanic processes occur  in the outer solar system.       Earth’s and Venus’ (and Mars’) eruptions  are of rocky material and  are driven by internal heat.  Io’s  eruptions are probably sulfur or sulfur compounds  driven by tidal  interactions with Jupiter.  Triton’s eruptions are of very volatile  compounds like nitrogen or methane driven by seasonal heating from the Sun.



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