(See description below)
Neptune's moon Triton
Triton has been visited by only one spacecraft, Voyager 2 on Aug 25 1989, and almost everything we know about it comes from this encounter. The light area at the bottom of the image is the southern polar cap. With a radius of 1,350 (839 mi), about 22% smaller than Earth's moon, Triton is by far the largest satellite of Neptune. It is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon, Titan). Triton has the coldest surface known anywhere in the Solar System (38 °K, about -391 °F); it is so cold that most of Triton's nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. The vast south polar cap is believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying these pink ices are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the Voyager 2 flyby. The rest of the surface consists of a so-called cantaloupe terrain, whose origin is unknown, and a set of 'cryovolcanic' landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton's interior.
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'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
(0.7 - 0.8) which means that little of the Sun's meager light is
absorbed. At this temperature methane, nitrogen and carbon
dioxide all freeze solid.
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.
There are extensive ridges and valleys in complex patterns all
over Triton's surface. These are probably the result of
freezing/thawing cycles.
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"