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Triton is located at roughly ~30 AU and is the largest moon of Neptune that was discovered by William Lassell in 1846. Voyager 2 reached the Neptunian system in 1989 which gathered Triton’s first images, revealing its thin atmosphere, “cantaloupe” terrain, polar caps, cryovolcanism and other geologic features. Additional photos revealed that the surface composition is mainly water ice, and a thin atmosphere of nitrogen, carbon dioxide, methane, and other trace volatiles. Triton has a young surface (towards the possibility there is a sub-surface, NH₃-rich, liquid ocean underneath.
Triton’s H₂O ice shell based on the surface geologic features and its history of obliquity tides and radiogenic heating. Due to the -23° angle of its tilt, Triton’s obliquity tides are still in effect to this day which continue to heat its interior. Triton’s orbital history claims it was a dwarf planet from the Kuiper Belt that ended up being captured in Neptune’s orbit after being expelled from a three-body system of planetary bodies with Pluto and Charon.
Its expulsion from that system explains how it has a retrograde orbit around Neptune and possibly its tilt. There are no future missions planned at the moment other than a concept designed by ESA that could possibly be funded.
Not much was known about the largest satellite of Neptune, Triton, until Voyager 2’s flyby in 1989, which took 12 years to reach it from when it had launched from Earth. Scientists have only scratched the surface of getting to know more of how Triton formed its geologic features and its origin.
From the pictures scientists have gathered from Voyager 2, they show that the surface of Triton is very complex. To delve more into the details of Triton, scientists must discuss certain topics, which are as follows: its orbital dynamics, unique geologic characteristics, debates regarding Triton, and the future work scientists have planned. There is a possibility that Triton was a Kuiper Belt Object that had wandered into Neptune’s orbit, as well as, the presence of a sub-surface ocean.
There are many characteristics that make Triton a very unique satellite in our Solar System. It has a retrograde rotation similar to Venus. Its inclined and circular orbit lies between a group of small inner prograde satellites and a number of exterior irregular satellites with both prograde and retrograde orbits. This unusual configuration has led to the belief that Triton originally orbited the Sun before orbiting around Neptune. Immediately after Neptune’s capture, Triton’s orbit was highly elliptical, leading to large tides. These tides in turn caused heating, reducing Triton’s effective rigidity, and thus further increasing the amplitude of tidal deformation and radiogenic heating.
This positive feedback probably led to a brief, intense period of heating, up to ~ 1 Gyr after capture, after which Triton’s orbit was essentially circular. Neptune’s other moons have a much closer orbit and distance as opposed to Triton. Triton has a more elliptical orbit than most of the other moons, such as Naiad, Despina, and Larissa. Neptune’s moons have a typical orbit in the plane of Neptune’s equator, except for Triton’s orbit that is tilted at -23̊. Triton’s odd orbital configuration makes heating by obliquity tides unusually effective.
Triton’s density is 2.06 g/cm³, which is very close to Pluto’s density, they are very similar in size, and both geologically active. There is no presence of mountains or huge trenches on Triton meaning that it has a fairly thin crust. Voyager 2 only captured images of about 40% of Triton’s surface. The terrain from the Voyager images displayed a “cantaloupe-like” pattern, volcanic fields of geysers and ice volcanoes, and its polar caps. Triton’s unique surface showed evidence for temporal changes in the distribution of surface ice, revealed a hazy atmosphere, and showed vertical plumes of material rising kilometers above the surface where it was caught in a sublimation wind flowing from the South Polar Region. The “cantaloupe” terrain of the Bubembe Regio (western region) may have formed via diapiric (dome-like) overturn of a layered crust ~ 20 km thick.
It has dark, wind-blown spots that have been strewn across its landscape from cryovolcanism. The ice volcanoes on its surface have umbrella-type eruptions, due to the presence of a thin atmosphere. The Raz Fossae, two prominent, roughly 15 km wide, troughs en echelon is among the most outstanding tectonic features of Triton because they are interpreted as grabens. These grabens have about a 60-degree slope formed as a result of lithospheric heat flow. Normal faulting in water ice is favorable at roughly 60 degrees. Triton has a young , which shows evidence that there are not very many impact craters. Based on its young apparent age, we assume that Triton’s icy surface is being deformed by convection, similar to young surfaces on Europa and Enceladus.
The surface gas compositions are predominantly H₂O with trace amounts of volatiles: N₄, CO, CO₂, and CH₄. About half of Triton’s surface is composed of N₂ in solid solution with a minute amount of CO and CH₄. The other half is comprised of sections of either CO₂ or H₂O. Triton’s interior geology is unique amongst the other Jovian planetary systems. Triton’s radius is approximately 1353 km, with an average density of approximately 2065 kg m³. From limited observations, it is difficult to ascertain the structure of the icy portion of Triton. Triton’s interior structure is comprised of an inner and outer core of silicate with a radius of 950 km, overlain by an ocean of liquid NH₃ and H₂O, a freezing front, and a 403 km thick H₂O ice shell. The base of the ice shell is preferentially warmed by tidal heating. Since the shell–ocean interface is held at a constant temperature, higher dissipation at the base of the shell reduces the thermal gradient. As a result, higher orbital eccentricities lead to lower basal heat fluxes.
Presence of a Sub-surface Ocean There is a possibility that Triton has a presence of a sub-surface ocean. If Triton’s ice mantle contains significant amounts of ammonia (a potent antifreeze), there could be an extensive liquid layer of ammonia-water inside this satellite. Due to the viscosity of water ice (the same is true for ammonia dehydrate ice), the onset of convection in the outer layers of icy satellites is much more difficult, allowing outer shells stable against convection, and possible internal oceans to escape freezing. As the ocean cools and crystallization progresses, composition of the liquid H₂O evolves along the liquidus curves becomes more NH₃-rich. The NH₃ liquid becomes denser than the crystallized H₂O ice, which also reduces the viscosity of any remaining H₂O liquid.
The crystallized H₂O ice creates a layer covering the NH₃ liquid, known as the freezing front. With sufficient tidal blanketing, it is possible to create and retain an NH₃-rich fluid layer at the base of the H₂O ice shell. It is still unclear whether or not an ocean is present within Triton. If the ice shell is convective, heat will escape preventing an ocean from developing. The heat sources available to prevent Triton’s ocean from freezing relies on the amount of H₂O ice that is protecting the NH₃ liquid. It is uncertain how thick the H₂O ice shell is, which would affect the amount of convection that is present. There is more debate of the possibility of an ocean rather than any debate arguing against this possibility. Moon vs. Dwarf Planet There is a possibility that Triton was a Kuiper Belt Object that had wandered into Neptune’s orbit.
The retrograde orbit of Neptune’s largest satellite, Triton, argues strongly for a capture origin. The highly eccentric orbit of Neptune’s outer moon Nereid may have been produced by Triton during the capture epoch. Triton was once a member of a binary system with a range of plausible characteristics, including ones similar to the Pluto–Charon pair. One possible outcome of gravitational encounters between a binary system and a planet is an exchange reaction, where one member of the binary is expelled and its place taken by the planet. Thus, Triton was more likely a dwarf planet from the Kuiper Belt that was part of a three-body system of planetary bodies.
With Pluto and Charon, Pluto’s moon, which was expelled and ended up in Neptune’s gravity, all while maintaining its retrograde orbit. Triton likely formed as a dwarf planet in the Kuiper belt (like Pluto) before being captured by Neptune. This would prove that Triton did not form from a collision with Neptune. It is postulated that Neptune formed at around 12–15 AU via planetesimal accumulation, before migrating to its present orbit at ~30 AU through a process of angular momentum exchange with a disk of planetesimals that initially extended out to 30–35 AU, interacting with the planets via gravitational scattering. This scenario explains the dynamical structure of the Kuiper Belt. In summary, Triton was not a moon during its early history but it became one much later.
There have not been any missions to Triton since Voyager 2, but there is a concept that has been prepared if the proper funding were to be approved in the future. The European Space Agency has already set up a concept of the details of how an orbiter would reach Triton but this concept has not been approved yet so it is just an example. This is only one possible mission concept that places a spacecraft in orbit around Neptune, which makes multiple flybys of Triton. The ESA has envisioned that an orbiter would take a 15-year interplanetary cruise that would involve two Earth gravity assists and a single Jupiter gravity assist.
Following Neptune Orbital Insertion, the orbiter will last for two years and includes 55 Triton flybys (providing global surface coverage). The mission, if launched from Kourou, will be starting with an interplanetary transfer arrival conditions given by the first stage of this mission analysis. At the beginning of the mission, the spacecraft will fly between the inner rings and executes NOI at 3000 km altitude. During the three phases of the mission, there are inclined Neptune orbits, orbits in Triton’s orbital plane, and 55 Triton flybys that cover the full range of Triton orbital locations, with altitudes between 150 and 1000 km. There are currently no planned future missions that have been funded and have a launch date.
Triton continues to amaze scientists and anyone who it captures interest in. It is unique in its own way and has proven that by its characteristics compared to the other planetary bodies of our Solar System, by its fascinating geology, and its orbital history. The future orbital visits will help scientists retrieve more details about the surface of Triton and possibly help answer more unanswered questions. The images the orbital will retrieve will display images we may not have been able to see before that the Voyager 2 mission wasn’t able to capture.
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