map of Triton All detailed knowledge of the surface of Triton was acquired from a distance of 40,000 km by the
Voyager 2 spacecraft during a single encounter in 1989. Analysis of crater density and distribution has suggested that in geological terms, Triton's surface is extremely young, with regions varying from an estimated 50 million years old to just an estimated 6 million years old. The surface also has deposits of
tholins, a dark, tarry slurry of various organic chemical compounds.
Cryovolcanism One of the largest
cryovolcanic features found on Triton is
Leviathan Patera, a caldera-like feature roughly 100 km in diameter seen near the equator. Surrounding this caldera is a massive cryovolcanic plain, Cipango Planum, which is at least 490,000 km2 in area; assuming Leviathan Patera is the primary vent, Leviathan Patera is one of the largest volcanic or cryovolcanic constructs in the Solar System. This feature is also connected to two enormous cryolava lakes seen northwest of the caldera. Because the cryolava on Triton is believed to be primarily water ice with some ammonia, these lakes would qualify as stable bodies of surface liquid water while they were molten. This is the first place such bodies have been found apart from Earth, and Triton is the only icy body known to feature cryolava lakes, although similar cryomagmatic extrusions can be seen on
Ariel,
Ganymede,
Charon, and
Titan.
Plumes The
Voyager 2 probe in 1989 observed a handful of
geyser-like eruptions of nitrogen gas or water and
entrained dust from beneath the surface of Triton in plumes up to 8 km high. one hypothesis is that Triton's plumes are driven by solar heating underneath a transparent or
translucent layer of nitrogen ice, creating a sort of "solid
greenhouse effect". As solar radiation warms the darker material beneath, this causes a rapid increase in pressure as the nitrogen begins to
sublimate until enough pressure accumulates for it to erupt through the translucent layer. This model is largely supported by the observation that Triton was near peak southern summer at the time of
Voyager 2s flyby, ensuring its southern polar cap was receiving prolonged sunlight.
CO2 geysers on Mars are thought to erupt from its
south polar cap each spring in the same way. The significant geological activity on Triton has led to alternative proposals that the plumes may be cryovolcanic in nature, rather than driven by solar radiation. A cryovolcanic origin better explains the estimated output of Triton's plumes, which possibly exceeds . This is similar to that which is estimated for Enceladus's cryovolcanic plumes at . If Triton's plumes are cryovolcanically driven, it remains to be explained why they predominantly appear over its southern polar cap. Triton's high surface heat flux may directly melt or vaporize nitrogen ice at the base of its polar caps, creating 'hot spots' which break through the ice or move to the ice caps' margins, before erupting explosively. Though only observed up close once by the
Voyager 2 spacecraft, it is estimated that a plume eruption on Triton may last up to a year. File:Leviathan Patera Volcanic Dome.gif|Close up of the volcanic province of
Leviathan Patera, the caldera in the center of the image.
Kraken Catena and
Set Catena extend radially from the caldera to the right and upper-right of the image, while
Ruach Planitia is seen to the upper left. Just off-screen to the lower left is a fault zone aligned radially with the caldera, indicating a close connection between the tectonics and volcanology of this geologic unit. File:Voyager 2 Triton 14bg r90ccw colorized.jpg|Dark streaks across Triton's south polar cap surface, thought to be dust deposits left by eruptions of
nitrogen geysers File:Cryolava-lake-triton.jpg|Two large
cryolava lakes on Triton, seen west of
Leviathan Patera. Combined, they are nearly the size of
Kraken Mare on
Titan. These features are unusually crater free, indicating they are young and were recently molten.
Polar cap, plains and ridges Triton's south polar region is covered by a highly reflective cap of frozen nitrogen and methane sprinkled by impact craters and openings of geysers. Little is known about the north pole because it was on the night side during the
Voyager 2 encounter, but it is thought that Triton must also have a north polar ice cap. The high plains found on Triton's eastern hemisphere, such as Cipango Planum, cover over and blot out older features, and are therefore almost certainly the result of icy lava washing over the previous landscape. The plains are dotted with pits, such as
Leviathan Patera, which are probably the vents from which this lava emerged. The composition of the lava is unknown, although a mixture of ammonia and water is suspected. Four roughly circular "walled plains" have been identified on Triton. They are the flattest regions so far discovered, with a variance in altitude of less than 200 m. They are thought to have formed from the eruption of icy lava. The plains near Triton's eastern limb are dotted with black spots, the
maculae. Some maculae are simple dark spots with diffuse boundaries, and others comprise a dark central patch surrounded by a white halo with sharp boundaries. The maculae typically have diameters of about 100 km and widths of the halos of between 20 and 30 km. There are extensive ridges and valleys in complex patterns across Triton's surface, probably the result of freeze–thaw cycles. Many also appear to be tectonic and may result from an extension or
strike-slip faulting. There are long double ridges of ice with central troughs bearing a strong resemblance to
Europan lineae (although they have a larger scale), and which may have a similar origin, possibly shear heating from strike-slip motion along faults caused by diurnal tidal stresses experienced before Triton's orbit was fully circularized. These faults with parallel ridges expelled from the interior cross complex terrain with valleys in the equatorial region. The ridges and furrows, or
sulci, such as
Yasu Sulci,
Ho Sulci, and
Lo Sulci, are thought to be of intermediate age in Triton's geological history, and in many cases to have formed concurrently. They tend to be clustered in groups or "packets".
Cantaloupe terrain '', with crosscutting
Europa-like double ridges. Slidr Sulci (vertical) and Tano Sulci form the prominent "X". Triton's western hemisphere consists of a strange series of fissures and depressions known as "cantaloupe terrain" because it resembles the skin of a
cantaloupe melon. Although it has few craters, it is thought that this is the oldest terrain on Triton. It probably covers much of Triton's western half. Cantaloupe terrain, which is mostly dirty water ice, is known to exist only on Triton. It contains depressions in diameter. The depressions (
cavi) are probably not impact craters because they are all of the similar size and have smooth curves. The leading hypothesis for their formation is
diapirism, the rising of "lumps" of less dense material through a stratum of denser material. Alternative hypotheses include formation by collapses, or by flooding caused by
cryovolcanism.
Impact craters "walled plains". The paucity of craters is evidence of extensive, relatively recent, geologic activity. Due to constant erasure and modification by ongoing geological activity,
impact craters on Triton's surface are relatively rare. A census of Triton's craters imaged by
Voyager 2 found only 179 that were incontestably of impact origin, compared with 835 observed for
Uranus's moon
Miranda, which has only three percent of Triton's
surface area. The largest crater observed on Triton thought to have been created by an impact is a feature called
Mazomba. Although larger craters have been observed, they are generally thought to be volcanic. The few impact craters on Triton are almost all concentrated in the leading hemisphere—that facing the direction of the orbital motion—with the majority concentrated around the equator between 30° and 70° longitude, resulting from material swept up from orbit around Neptune. Because it orbits with one side permanently facing the planet, astronomers expect that Triton should have fewer impacts on its trailing hemisphere, due to impacts on the leading hemisphere being more frequent and more violent.
Voyager 2 imaged only 40% of Triton's surface, so this remains uncertain. However, the observed cratering asymmetry exceeds what can be explained based on the impactor populations, and implies a younger surface age for the crater-free regions (≤ 6 million years old) than for the cratered regions (≤ 50 million years old). == Observation and exploration ==