Triton (satellite)
Triton is a satellite of Neptune located 4,500 million kilometers from the Sun. It is one of the coldest stars in the solar system (-235 °C). Discovered by William Lassell on October 10, 1846, just 17 days after the planet's own discovery, it owes its name to the god Triton of Greek mythology. With a diameter of 2,707 km, Triton is Neptune's largest satellite and the seventh in the solar system, as well as being the only large moon that has a retrograde orbit, that is, an orbit whose direction is opposite to the rotation of the Sun. planet (something exceptional in a body of such size). Because of its retrograde orbit and its composition, similar to that of Pluto, Triton is considered to have been captured from the Kuiper belt by Neptune's gravitational pull.
Triton is composed of a crust of frozen nitrogen over a mantle of ice, which is believed to cover a solid core of rock and metal. Triton has an average density of 2.061 g/cm³ and is composed of approximately 15 -35% ice water.
Triton is one of the few satellites in the solar system known to be geologically active. Due to this activity, its surface is relatively young, and reveals a complex geological history from mysterious and intricate cryovolcanic and tectonic terrains. After the passage of Voyager 2 through its vicinity, enigmatic images revealed what appeared to be geysers of liquid nitrogen emanating from its icy surface. This discovery changed the classical concept of volcanism since, until then, it was assumed that icy bodies should not be geologically active. Triton demonstrated that for there to be geological activity it is enough for a fluid medium to be molten rock, nitrogen or water. Triton has a tenuous nitrogen atmosphere whose pressure is less than 1/70,000 the pressure of Earth's atmosphere at sea level.
Due to its close proximity to Neptune, it may break up and eventually yield to the planet's gravity to become a new ring system brighter than Saturn via the Roche limit.
Discovery and name
Triton was discovered by British astronomer William Lassell on October 10, 1846, just 17 days after the planet Neptune (the planet around which Triton orbits) had been discovered by German astronomers Johann Gottfried Galle and Heinrich Louis d'Arrest, who found him following the coordinates given by the French astronomer and mathematician Urbain Le Verrier.
Lassel, originally in the beer trade, began his path in astronomy making lenses for his own beginner's telescope around 1820. When John Herschel received news of the discovery of Neptune, he wrote to Lassell to urge him to look for possible moons around the newly discovered star. Just 8 days later Lassel would find Triton in the sky. Lassel also claimed to have discovered Neptune's rings. However, and despite the fact that the existence of these rings would later be confirmed, their visibility is so terrible that truly powerful instruments would be needed, which leads to questioning the veracity of Lassel's testimony.
The name Triton (Greek Τρίτων) comes from the name of the god of the sea, son of Poseidon (Neptune), in Greek mythology. This name was proposed by Camille Flammarion in his 1880 work Astronomie Populaire. The name Triton was also proposed by others, but it would not begin to be used generically until 1949, when the second moon was discovered. Neptunian Nereid. Previously in the scientific literature it was only referred to as "the satellite of Neptune". Strangely, references to Triton in the late 19th century and early 20th century are to the name of a supposed canal on Mars.
Although it was not Lassell who gave his own discovery a name, he would name his later discoveries: the satellite Hyperion on Saturn, and the third and fourth moons of Uranus, Ariel and Umbriel.
Observation
After its discovery, little was known about what Triton would have to reveal and in the first photograph that was taken, it appeared with a yellowish-pink color. Already in the XIX century its orbital properties were defined with great precision, the retrogradation of its orbit at a very acute angle was ascertained with respect to the orbit of Neptune. It was not until 1930 that the first detailed observations of the satellite could be made and from then little was known about it until the arrival of Voyager 2 at the end of the century XX.
Before the arrival of Voyager 2, astronomers suspected that Triton might have seas of liquid nitrogen as well as a nitrogen/methane atmosphere 30% more dense than Earth's. But, like the famous overestimates of the density of the Martian atmosphere, this was false. As with Mars, a thicker atmosphere is assumed in the early history of the planet, that is, in the time immediately after its creation.
The first attempt to measure the diameter of Triton is attributed to Gerard Kuiper in 1954, who estimated it at 3,400 km. Later measurement attempts reached values between 2,500 and 6,000 km, or slightly smaller than our Moon, about half the diameter of Earth.
The data collected by Voyager 2 after passing through Neptune on August 25, 1989, made it possible to know more precisely the diameter of Triton (2706 km). In the 1990s, different observations were made from Earth to Triton. These observations showed a denser atmosphere than during the passage of Voyager 2.
Orbit and rotation
Triton is unique from the other large moons in the solar system in its retrograde rotation around Neptune (i.e., it orbits in the opposite direction to the planet's rotation). Most of the irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of the moons of Uranus. However, these moons are much further away from their parent planets, and quite small in comparison; the largest of them (Phoebe) barely represents 8% of the diameter (and 0.03% of the mass) of Triton.
The orbit of this satellite is really weird. It has an inclination of 157.340º with respect to Neptune's equator, which produces the retrogradation of the translation of the satellite. This extreme inclination is probably due to the fact that Neptune captured it by the effect of gravity (something similar probably happened with Pluto, hence the inclination and eccentricity of this body), and also its axis of rotation is inclined 30º with respect to the plane of the orbit of Neptune, so that during the Neptunian year each pole points to the Sun, similar to what happens with Uranus. As Neptune orbits the Sun, Triton's polar regions alternate facing it, probably as a result of the radical seasonal changes that occur when one pole, and then the other, receives sunlight.
Also, it is a practically circular orbit, with an eccentricity of almost zero. Unlike the Moon with the Earth, where the effect of the tides causes a distance between both bodies and slows down our planet, the conservation of angular momentum is bringing Neptune and Triton closer together, and accelerating the rotation of the former. This probably derives in the collision of both bodies or in the rupture of this moon within 3.6 billion years, when Triton will pass the Roche Limit of Neptune, resulting in a ring system similar to that of Saturn in both cases..
Capture
Moons with retrograde orbits cannot have formed from the same solar nebula in which the planets they orbit were created, but must have been captured from elsewhere. Therefore Triton is suspected to have originally been an Edgeworth-Kuiper belt body with an independent orbit around the Sun, that is, it is thought to have been something like another Pluto.) The Kuiper belt is a ring composed of small icy bodies that extends from the very center of Neptune's orbit to approximately a distance of 55 AU from the Sun. It is believed to be the point of origin of most short-path comets observed from Earth, as well as home to several large planet-like bodies, including the dwarf planet Pluto, which has been recognized as the largest of a group of Kuiper belt objects (the plutinos), which are found in orbital resonance with Neptune. Triton is slightly larger than Pluto and the composition of both is similar, which leads to the hypothesis that both share the same origin.
This would explain the relative poverty of Neptune's moon system and the high orbital eccentricity of Nereid. A highly eccentric orbit of Triton immediately after its capture would cause Triton, with its gravity, to dislocate the orbits of any satellites Neptune might have had before Triton's arrival.
The circularization of Triton's orbit would have been carried out due to the tidal forces exerted by Neptune, which would liquefy this moon for billions of years, causing a differentiation in its interior layers.
The hypotheses that explain the capture of Triton consider two possibilities. For a moving body to be captured by the gravitational force of a planet, the body in question must lose enough energy so that its speed is reduced in such a way that it is impossible to escape. A first theory about how Triton could have been slowed down in such a way was based on the fact that it collided with another object, either one that passed by the approaches of Neptune (which is highly unlikely), or a moon or protomoon (which is more likely). Another hypothesis suggests that before being captured, Triton possessed a very massive moon similar to Pluto's moon, Charon. When Triton met Neptune, the gravitational pull of Neptune stripped it of its companion and caused it to achieve an orbit around the planet. This hypothesis is reinforced by the large number of Kuiper belt objects with satellites. Capture would occur smoothly and briefly, saving Triton from collision. Events like this could have been very common during the formation process of Neptune, or later when it would migrate outward.
Physical characteristics
Triton is similar in size and composition to Pluto, and by checking Pluto's eccentric orbit traversing Neptune's, we can see clues to Triton's possible origin as a planet similar to Pluto and captured by Neptune. It is the only satellite of Neptune that is spherical in shape.
The gravitational effect of Triton on the path of Voyager 2 suggests that bright ice and mantle must cover a substantial core of rock (likely to contain metal). The core accounts for two-thirds of Triton's total mass (65% to 75%), which is more than any other moon in the Solar System except Io and Europa. The differentiation may have been efficient due to the gravitational effect of Neptune during the capture of Triton. Triton has an average density of 2.05 g/cm³, and is composed of about 25% water ice, essentially located in the mantle.
The surface is composed mainly of nitrogen ice, but also dry ice (carbon dioxide ice), water ice, and carbon monoxide and methane ice. It is thought that ammonia-rich ice could exist on the surface, but it was not detected.
General topography
The total surface area corresponds to 15.5% of the surface area on Earth, or 4.5% of the total area). Triton's dimension suggests that there should be regions of different densities, varying between 2.07 and 2.3 grams per cubic centimeter. There are areas that have rocky exposures, and they are slippery areas, due to icy substances, especially icy methane, that covers part of the surface.
Triton's south polar region is covered in a layer of icy nitrogen and methane peppered with shocking craters and geysers. The icy layer is highly reflective of the little solar energy. It is unknown what the North Pole will be like, since it was in penumbra when Voyager 2 visited Triton. However, it is thought that, like the South Pole, it should have a polar cap.
In the equatorial region, long faults with parallel ridges of ice expelled from the interior cut complex terrain with imperfect valleys. Yasu Sulci, Ho Sulci and Lo Sulci are some of these systems known as "Sulci", a term that means 'grooves'. To the east of these grooves lie the Ryugu and Cipagu plains and the Cipango highlands.
The flat areas of Sipagu Planitia and Abatus Planum in the Southern Hemisphere are surrounded by black dots - the "maculae". Two groups of maculae, Acupara Maculae and Zin Maculae stand out to the east of Abatus Planum. These marks appear to be deposits on the surface left by ice that evaporated, but it is not known for sure what they are composed of and their origin.
Near Sipagu and Abatus Planum there is still a large crater, 27 km in diameter, called Mozamba. Continuing northwest, two other smaller craters (Kurma and Llomba) follow Mozamba crater almost in a straight line. Most of the pools and rough terrain are caused by melting ice, unlike on other moons where shock craters dominate the surface. However, Voyager 2 photographed a striking crater with a diameter of 500 km, which was extensively modified by repeated flooding, melting and faulting.
Melon Peel Land
Tano Sulci is one of the long faults that run through the bizarre Bubembe region of Triton, a region also known as "melon-peel terrain," due to its melon-peel appearance, a of the strangest regions of the Solar System. The origin of this terrane is unknown, but it may have been caused by the rise and fall of nitrogen ice, collapse, and flooding caused by cryovolcanism. Despite being a terrain with few craters, it is believed that it could be the oldest surface on Triton. This terrain could cover most of the Northern Hemisphere.
These unique melon-shell terranes exist only on Triton and comprise depressions 30 to 50 km in diameter, probably not related to meteorite impact because they are too regular, evenly spaced, separated by ridges curved. These peaks could originate from viscous ice eruptions between the ring fractures, and can be up to 1 km high.
Cryovolcanism
Surprisingly, Triton is geologically active; its surface is recent and with few craters. Valleys and ridges exist in a complex pattern across the surface, probably resulting from freezing and warming cycles and from volcanoes. The Voyager 2 probe observed icy volcanoes (the plumes) that vertically spewed liquid nitrogen, dust or methane compounds, coming from below the surface, in smoke reaching 8 km in height. Probably, this volcanic activity is due to azonal heating caused by the Sun, and not like the heating of the volcanoes recorded on Io.
Hili and Mahilani are the observed Tritonian cryovolcanoes, both named after water spirits from African mythologies. Triton is like Earth, Io, perhaps Venus, Europa, Enceladus and Titan, one of the few bodies in the solar system that currently has volcanic activity.
Atmosphere and climate
Triton has a tenuous atmosphere composed of nitrogen (99.9%) with small amounts of methane (0.01%). Tritonian atmospheric pressure is only 14 microbars.
The Voyager 2 probe was able to observe a thin layer of clouds in an image it made of the outline of this moon. These clouds form at the poles and are composed of nitrogen ice; there is also photochemical fog up to a height of 30 km that is composed of various hydrocarbons, similar to those found on Titan, however none of these hydrocarbons were detected. Hydrocarbons are thought to contribute to the pink appearance of the surface.
The surface temperature is about -235 degrees Celsius, still lower than Pluto's mean temperature (about -229 °C), the lowest temperature ever measured in the solar system. At 800 km from the surface, the temperature is -180 °C.
The seasons of the year
Triton's axis of rotation is unusual, tilted 157° relative to Neptune's axis, and 130° relative to Neptune's orbit, exposing one pole to the Sun at a time. As Neptune orbits the Sun, Triton's polar regions swap positions every 82 years, likely resulting in radical seasonal changes each time one pole moves toward the Sun. Given its orbit and axial tilt, Triton presents a cycle of mild and extreme seasons. The most extreme seasons occur at intervals of 700 years. The last great summer in Triton was in 2007.
During the Voyager 2 encounter, Triton's south pole was tilted toward the Sun, which has been the case since Triton was discovered. And, almost the entire southern hemisphere was covered in a cap of frozen nitrogen and methane. That methane may slowly evaporate.
The change from the solid state to the gas state and back to the solid state of the polar cap produces a sudden change in the atmosphere. More recent observations of Triton's atmosphere, from occultation of stars, showed that from 1989 (the date of the Voyager 2 encounter) to 1998 the atmospheric pressure on Triton had doubled. Most models predict that the volatile ice evaporates and increases the pressure of the atmosphere. However, other models predict that the volatile ice found in the South Pole can migrate towards the equator and, thus, do not disappear from the atmosphere, but change location, thus leaving doubts about what could cause the increase in seasonal pressure..
Life on Triton
Triton is one of the coldest places in the solar system. This moon has an unusual orbit, it is retrograde, which is strange orbital behavior. In particular, the interaction with Neptune's other moons could cause internal heating in Triton. With the passage of Voyager 2 in 1989, it was discovered that it had volcanic activity, but of a type of icy volcanism that consists of the melting of water ice and nitrogen and perhaps methane and ammonia.
The atmosphere is made up of nitrogen and methane, these are the same compounds that exist on Saturn's large moon Titan. Nitrogen is also the main compound of the Earth's atmosphere, and methane on Earth is normally associated with life, being a by-product of its activity. But like Titan, Triton is extremely cold. If that were not the case, these two components of the atmosphere would be signs of life.
However, due to geological activity and possible internal heating, it has been suggested that Triton could harbor primitive life forms in liquid water below the surface, much like what has been suggested for Jupiter's moon Europa. Triton and Titan are thus worlds that despite being physically extreme are capable of supporting exotic forms of life unknown on Earth.
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