Triton isNeptune(Neptune) LargestNatural satelliteIt is also the first Neptune satellite discovered.On October 10, 1846, British astronomerWilliam Lassell William Lassell discovered Triton.It is the only solar system withRetrograde orbitIts orbit is opposite to the rotation direction of the planet.[1-2]Triton has a diameter of 2706.8 kilometers,[3]It is the seventh largest satellite in the solar system and the only Neptune satellite with enough massHydrostatic balanceAnd is the second largest satellite in terms of mass ratio with the main planet, only second toMoonAndearthMass ratio of.Because its retrograde orbit is similar toPlutoThe material composition of Triton is believed to be derived fromKuiper beltCaptureddwarf planet。[4]
Triton's surface is mostly frozen nitrogen, and its shell is mostly composed of water ice,[5]It has an ice mantle and a core mainly composed of rocks and metals.The core accounts for two-thirds of its total quality.The average density is 2.061 g/cmthree，[3]Explain that it contains about 15 – 35% water ice.[6]
During the flyby of Triton in 1989,Voyager 2It is found that the surface temperature is 38 K (- 235 ° C), and the activeGeyser。Only Voyager 2 of the spacecraft has visited Triton.[7]Triton is one of the few known satellites with active geological activities in the solar system (others areIo、Europa、Enceladus、titan）。Geologic activities lead to the relatively young surface of Triton, almost no obviousImpact crater。The complex low-temperature volcanoes and tectonic topography show the complex geological history.Some parts of Triton's surface have geysers that emit sublimated nitrogen, thus forming a weaknitrogenThe atmosphere, whose pressure is lower than 70000 times the atmospheric pressure at the earth's sea level.[6]Voyager 2 only captured about 40% of Triton's surface, and Triton will still be the focus of future exploration missions returning to Neptune.
On October 10, 1846, just 17 days after the discovery of Neptune, British astronomersWilliam Lassell William Lassell discovered Triton.[14]WhenJohn Herschel (John Herschel) When he received the news of the discovery of Neptune, he wrote to Russell and suggested that he look for possible satellites.Russell discovered Triton eight days after receiving the letter.[14-15]Russell also claimed to have discovered Neptune rings for some time.[16]Although Neptune was later confirmed to have ringsNeptune ringIt was so dark that Russell could not observe it at all.Russell is a brewer. He uses the metal mirror with an aperture of about 61 cm (24 inches) built by himselfReflecting telescope(also known as the "two foot" mirror) discovered Triton.[17]The telescope was later donated toRoyal Observatory Greenwich , but was finally removed.[17]Triton is named after the Greek sea god Triton (son of Poseidon).Greek sea godPoseidonPoseidon and Roman PoseidonNipton(Neptune, the source of Neptune's name).This name was first proposed by Camille Flammarion in his Astronomie Populaire published in 1880,[18]It was officially adopted decades later.[19]Until the discovery of Neptune's second satellite, Nereid, in 1949, this satellite was usually called "Neptune satellite".Russell never named his many satellites directly.Then he found outTitanThe name Hyperion previously proposed by John Herschel was selected.[20]findTianwei I、EnceladusIs also selectedWilliam ShakespeareAriel, Umbriel, etc.
revolution
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Triton is unique among all the large satellites in the solar system, because it is retrograde around Neptune (that is, it rotates in the direction of rotation of the planet).Most of the outer irregular moons of Jupiter and Saturn, including some of Uranus, also haveRetrograde orbit。However, these satellites are far away from their planets and relatively small, the largest of whichIapetusPhoebe is only 8% of the diameter of Triton (0.03% of the mass).
Triton has two orbitsdip angleThe rotation angle of Neptune is 30 ° relative to the orbit of Neptune, and the orbit of Triton is 157 ° relative to the rotation angle of Neptune (the inclination angle exceeding 90 ° indicates retrograde motion).Triton's orbit moves forward relative to Neptune's rotation, with a period of about 678 Earth years (4.1 Neptune years),[11-12]This makes Triton's inclination to Neptune's orbit vary between 127 ° and 173 °.The recently measured inclination is 130 °, and Triton's orbit is close to the maximum deviation from Neptune's orbital plane.
Triton's orbit (red) is opposite to other satellites (green) and Neptune's rotation
Triton's orbit around Neptune is nearly perfectly circular, and its eccentricity is almost zero.Since the formation of the Neptune system, the viscoelastic damping generated by tide alone cannot make Triton's orbit round, and the gas resistance generated by the moving debris disk may play an important role.The tidal deceleration effect also causes Triton's orbit to gradually fall, and the orbital height from Triton to Neptune has been less than the distance between the Earth and the moon.It is predicted that Triton will enter the Roche limit of Neptune in 3.6 billion years.This will lead to the collision with Neptune's atmosphere, or the rupture of Triton, forming a new Neptune ring similar to Saturn's ring.
rotation
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Triton's spin quiltTidal locking , located around NeptuneSynchronous orbitAlways keep one face toward Neptune.Its equator is almost perfectly aligned with its orbital plane.[21]The rotation axis of Triton forms a 40 degree angle with the orbital plane of Neptune, so at some time of the year, every pole of Neptune is very close to facing the sun, just like the polar axis of Uranus.When Neptune revolves around the sun, the polar regions of Triton face the sun alternately, causing the polar regions to illuminate the sun one after another, resulting in seasonal changes.Scientists observed this change in 2010.[22-23]
origin
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A satellite in a retrograde orbit cannot be with a planetSolar nebulaThe same area of Triton was formed, so Triton was captured from other places.Triton may have originated fromKuiper belt[4]The Kuiper Belt is a circular region composed of a group of small ice bodies, extending from the inside of the orbit of Neptune to about 50 astronomical units away from the sun.This zone is considered to be the mostShort period cometIt is also the home of some large objects similar to planets, includingPluto。Pluto is considered to be the largest object in the Kuiper Belt, locked in resonance orbit with Neptune.Triton is only slightly larger than Pluto and has almost the same composition. Scientists believe that both have the same origin.[24]
The proposal of Triton capture theory can explain several characteristics of Neptune system, includingnereid (Nereid) Extremely eccentric orbit, and a small number of satellites compared with other giant stars.The initial eccentric orbit of Triton will intersect the orbits of irregular satellites, interfere with the orbits of smaller regular satellites, and disperse them through gravitational interaction.[11-12]
After Triton is captured, the eccentric orbit will also lead to the occurrence oftidal heating This may keep Triton in a fluid state for one billion years. The evidence of internal differentiation of Triton supports this inference.This source of internal heat disappeared after tide locking and orbit rounding.[25]
Scientists propose two mechanisms for Triton capture.If it is attracted by the planetary gravity, the passing celestial body must lose enough energy to decelerate to a speed lower than that required for escape.The early theory of how Triton slowed down was that it collided with another celestial body, or that it happened to encounter an celestial body when passing Neptune (this is unlikely), or that it was a satellite or protosatellite orbiting Neptune (this is very likely).[6]A recent hypothesis suggests that Triton was part of a binary system before it was captured.When the binary system encounters Neptune, it is disintegrated by gravitational interaction, one of which is thrown out of the system, and the other (Triton) is bound by Neptune.The greater the mass of the companion star, the more likely this event is to occur.[4]A similar mechanism can also be used to explain the process of Mars capturing its satellite.[26]This hypothesis is supported by several evidences, including the fact that binary systems are very common in large Kuiper Belt objects.[27-28]This process is brief and gentle, so that Triton will not be damaged by collision.Events like this may have been common during the formation of Neptune, and then occurred outwardPlanetary migrationThe same is true when.[4]However, the simulation in 2017 showed that Triton, after being captured by Neptune, may at least collide with another satellite before its orbital eccentricity decreases, leading to collisions between other satellites.[29-30]
physical property
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Triton occupies an absolute dominant position in Neptune's satellite
Triton is the seventh largest satellite in the solar system, and also the sixteenth largest celestial bodyEris Slightly larger.It accounts for more than 99.5% of the mass of all the satellites orbiting Neptune, including Neptune's ring and 13 other known satellites. It is even greater than the total mass of all the known satellites smaller than Neptune in the solar system.Triton has a diameter of 5.5% of Neptune's, and is the largest satellite among the gas giant planets relative to the planet, althoughtitanIt is larger in mass than Saturn.Its density (2.061 g/cmthree）, radius, surface temperature and chemical composition are similar to Pluto.[31]
Triton's surface is covered with a transparent layer of nitrogen ice.Voyager 2 has only observed and studied 40% of Triton's surface, but Triton's surface may be completely covered by such thin nitrogen ice.Like Pluto, Triton's crust is composed of 55% nitrogen ice mixed with other ice.Water ice accounts for 15-35%, frozencarbon dioxide(Dry ice) accounts for 10-20%.Ice contains trace 0.1%methaneAnd 0.05%carbon monoxide。[6]Ammonia ice may also exist on the surface, because there are signs of ammonia dihydrate in the lithosphere.The average density of Triton means that it may be composed of about 30 – 45% water ice (containing relatively small amounts of volatile ice material), and the rest is rock material.[6]Triton's surface area is 23 million square kilometers, which is equivalent to 4.5% of the Earth's surface area or 15.5% of the Earth's continental area.TritonAlbedoIt is abnormally high, reflecting 60 – 95% sunlight. Since the first observation, the albedo has changed very little.In contrast, the moon reflects only 11%.[32]Methane ice will be converted intoTorin, resulting in the red color on Triton's surface.[6]
Since the surface of Triton shows a long melting history, it is assumed that the internal model of Triton, like the Earth, is divided into solid core, mantle and shell.Water is the most abundant volatile substance in the solar system, which forms Triton's mantle and encloses the core composed of rocks and metals.There is enough production inside TritonRadioactive decayIt is similar to Europa and many other icy objects in the outer layer of the solar system.[6][33-35]Scientists believe that this is not enough to drive convection in Triton's icy crust. The strong tilt tide has generated enough additional heat to complete this process. At the same time, it has also observed signs of recent surface geological activities.[35]The black matter ejected from Triton's surface is suspected to contain organic compounds,[34]If there is liquid water in Triton, it is speculated that there may be an environment suitable for certain life forms.[34][36-37]
atmosphere
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Clouds on Triton's surface photographed by Voyager 2
TritonnitrogenThe atmosphere is very fragile, with traces of carbon monoxide and methane near the surface.[13][38-39]Like Pluto's atmosphere, Triton's atmosphere is thought to be the result of the evaporation of nitrogen ice on its surface.[24]Its surface temperature is at least 35.6 K (− 237.6 ° C), because Triton's nitrogen ice is in the hexagonal crystal state with higher temperature, and the hexagonal nitrogen ice and cubic nitrogen ice will undergo phase transition at this temperature.[40]The upper limit of surface temperature 40K can be calculated from the equilibrium vapor pressure of nitrogen in Triton's atmosphere.[41]This is more than Pluto's averageEquilibrium temperature44 K (- 229.2 ° C) is lower.The surface atmospheric pressure of Triton is only 1.4-1.9 Pa (0.014-0.019 mbar).[6]Triton surfaceturbulenceAtroposphere("weather zone"), which can rise to an altitude of 8 kilometers.GeyserPlumeThe streaks left on Triton's surface indicate that the troposphere is affected bymonsoonIt can move objects more than one micron in size.[42]Unlike other atmospheres, Triton's atmosphere does notstratosphere, but there is aThermosphere, there is aExosphere。[6]Because it can radiate from the sun andNeptuneMagnetosphereAbsorbing heat, the temperature of Triton's upper atmosphere is 95 ± 5 K, higher than its surface temperature.[13][43]Mist shrouds most of Triton's troposphere, which is mainly produced by the interaction of sunlight and methanehydrocarbonAnd nitriles.There are also clouds of condensed nitrogen in Triton's atmosphere, between 1 and 3 kilometers from its surface.[6]
In 1997, when Triton's edge passed in front of the background star, scientists made observations on Earth.These observations indicate that Triton has a denser atmosphere than that inferred from Voyager 2 data.[44]Other observations show that the surface temperature of Triton increased by 5% from 1989 to 1998.[45]These observations indicate that Triton is in an unusually warm summer in the southern hemisphere, which happens every few hundred years.Theories about this warming, including changes in the frost pattern on Triton's surface and changes in the albedo of ice, will lead to more heat absorption.[46]Another theory holds that temperature change is the result of dark red material deposition in geological process.Because Triton's spectral albedo is the highest in the solar system, it is sensitive to small changes in spectral albedo.[47]
Surface characteristics
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Physical characteristics
All details about Triton's surface are derived fromVoyager 2Data obtained from a distance of 40000 km during the flyby in 1989.[48]Voyager 2 photographed 40% of Triton's surface, showing massive exposed rock formations, ridges, valleys, gullies, depressions, plateaus, ice fields and a few impact craters.Triton is relatively flat, and the observed topographic changes will not exceed one kilometer.[6]Almost all the observed impact craters are concentrated in the leading hemisphere of Triton.[49]The analysis of crater density and distribution shows that the surface of Triton is very young geologically, and the ages of different regions are estimated from 50 million to 6 million years.[50]Triton has 55% of its surface covered by nitrogen ice, 15% to 35% by water ice and 10% to 20% by carbon dioxide ice.[51]Surface showsTorinSediment, which is a kind of leading organic compound possibly related to the origin of life.[52]
Explanatory map of Triton landform
Ice volcano
Triton is geologically active, with a young surface and relatively few impact craters.Although Triton's crust is composed of various ice materials, its underground process is similar to the process of generating volcanoes and rifts on the earth, but the contrast with water and ammonia is liquid rock.[6]Triton's entire surface is cut by complex valleys and ridges, which may begeological structure andIce volcanoResult of action.Most surface features on Triton are endogenous, that is, the result of internal geological processes rather than external processes (such as impact).And most of them are the result of volcanic eruption or natural eruption, rather than the result of geological structure.[6]
The area near Leviathan volcano, with caldera in its center
One of the largest ice volcano features found on Triton isLeviathanLeviathan Patera,[53]A feature similar to a caldera, it is near the equator of Triton, with a diameter of about 100 kilometers.The caldera is surrounded by aVolcanic domeExtending about 2000 kilometers along its longest axis, Leviathan volcano is the second largestMount Arbor(Albs Mons), the second largest volcano in the solar system.This geological feature is also connected with two huge ice lava lakes seen in the northwest of the crater.The smaller ice lava lake is at the top left of the right picture.It is believed that the ice lava on Triton is mainly water ice containing a small amount of ammonia, so these lakes will become stable surface liquid when meltingWater body。This is the first place where such water bodies have been found except for the Earth. Among the known ice bodies, only Triton has an ice lava lake, althoughTianwei I（Ariel），Ganymede（Ganymede），Charon (Charon) andtitanSimilar ice lava phenomenon can also be seen on Titan.[54]
In 1989, Voyager 2 observed a very small amount of geyser like nitrogen eruption, and carried dust under Triton's surface, which was up to 8 kilometers high.[31][55]So in the solar system, Triton andearth、Io、EuropaandEnceladusIs one of the few celestial bodies that have observed some form of active eruption.[56]Easiest to observeGeyserThe eruptions are Hili and Mahilani (respectivelyZuluWater spirit andTongaName of sea spirit).[57]
Imagination of Triton - Surface Geyser
All the intermittent springs observed are located between 50 ° and 57 ° south latitude, that is, the area where Triton's surface is directly exposed to sunlight.This shows that although solar heating is very weak in Triton, which is far away from the sun, it plays a crucial role.It is believed that Triton's surface may be composed of a translucent nitrogen ice layer covered on a darker substrate, resulting in a "solid"greenhouse effect”。Solar radiation passes through the thin ice sheet, slowly heating and evaporating the nitrogen ice underground until enough air pressure is accumulated to allow it to penetrate the ice shell and erupt.[6][42]As long as the temperature is only 4K higher than the surrounding surface temperature of 37K, an eruption will be generated that reaches the above observed height.[55]Although usually called "ice volcano", this nitrogen plume activity is different from the large-scale low-temperature volcanic eruption on Triton and the volcanic process driven by internal heat of other celestial bodies.Scientists believe that in the spring of every Martian year, the carbon dioxide geyser on Mars will eject from the Antarctic ice cap in the same way as Triton geyser.[58]
Black stripes on the surface of Triton's Antarctic ice cap photographed by Voyager 2
Each geyser eruption of Triton can last for up to a year. During this period, about 100 million cubic meters of nitrogen ice will be erupted due to sublimation.The fugitive dust may be deposited in the visible streaks 150 km downwind, and the more dispersed sediment may even float farther.[55]Voyager 2's image of Triton's southern hemisphere shows many similar dark matter stripes.[59]During the Voyager 2 flyby from 1977 to 1989, Triton's surface changed from a reddish Pluto like color to a lighter color, indicating that the lighter nitrogen frost covered the older reddish material.[6]The volatiles of Triton equator erupt and deposit at the poles, which may redistribute enough mass in 10000 years, thus causingPolar shift。[60]The right picture shows the black stripes on the surface of Triton's Antarctic ice cap taken by Voyager 2, which are believed to be the dust left by nitrogen geyserssediment。
Ice cap, plain, ridge
One or two large glacial lava lakes of Triton
The Antarctic region of Triton is covered by an ice cap composed of frozen nitrogen and methane with high reflectivity scattered from volcanic vents and geysers.Little is known about Triton's North Pole because it was on the dark side during Voyager 2's flyby, but scientists believe that Triton's North Pole also has an ice cap.[40]TritonplateauFor example, Cipango Planum covers and erases the older landforms, so it is almost certainly the result of ice lava scouring the original landforms.There are some pits distributed on the plain, such as Leviathan Patera, which may be lava vents.The composition of the lava is unknown, and it is suspected to be a mixture of ammonia and water.[7][6]The right picture shows one or two large ice bodies of TritonLava lake, west of Leviathan Volcano.They add up to almostKrakenThe size of the sea (Kraken Mare) is similar.These features are abnormally free of crescent depressions, indicating that they are very young and have been in a melting state recently.
Triton's roughly circular "ring wall plain"
On Triton, four roughly circular“Circular plain”。They are the flattest areas found so far, with a height change of less than 200 meters.Scientists believe they were formed by the eruption of ice lava.[6]In the right figure, Tuonela Planitia (left) and Ruach Planitia (middle) are two of the ice lava "ring wall plains" on Triton's surface.They generally lack impact craters, which is relatively new evidence of geological activity.The plains near Triton's east are dotted with maculae.Some black spots are simple patches with diffusion boundaries, while others include a dark central patch surrounded by a white halo with clear boundaries.The diameter of the black spot is usually about 100 kilometers, and the width of the halo is between 20 and 30 kilometers.[6]
Triton's surface is crisscrossed with a complex pattern of many broad ridges and valleys, which may beFreeze-thaw cycleResults.[61]Many appear to be derived from natural geological structures, which may be extended by strata orStrike slip faultCaused by.[62]The long double ice ridges with central grooves are very similar to the stripes on Europa (although Europa has a larger scale[5]）, may have a similar origin,[6]This may be along thefaultThe daily tide experienced by Triton before its orbit is completely roundedstressCause.[5]These faults with parallel ridges erupt from the inside and cross the complex terrain, with valleys in the equatorial region.Ridges, gullies, cracks (Sulci, such as Yasu Sulci, Ho Sulci and Lo Sulci),[63]Triton is in the middle age in its geological history, which is formed at the same time in many cases and tends to gather into groups.[62]
Hami melon topography
The terrain of Hamigua photographed by Voyager 2 from 130000 kilometers away
Triton's western hemisphere consists of a series of strange cracks and depressions, similar toHami melonThe skin is called "Cantaloupe terrain".Although there are few meteorite craters here, it is considered to be the oldest landform of Triton.[64]This terrain may cover most of Triton's western hemisphere.[6]Hamigua terrain only exists on Triton, which is mainly composed of dirty water ice and contains depressions with a diameter of 30 – 40 kilometers.[64]These caves (cavi) cannot be meteorite craters because they are similar in size and have smooth curves.The main assumptions that form them areDiapirismThat is to say, the material with smaller density is "lumped" by the stratum with larger density to rise.[6][65]Other formation assumptions include collapse, or flood caused by ice volcanism.[64]The right picture shows the Hami melon terrain photographed by Voyager 2 from 130000 km away, which crosses the double ridge iceberg similar to Europa. The Slidr Sulci (vertical) and Tano Sulci (horizontal) form a prominent "X".
Due to the continuous geological activities, there are relatively few impact craters on Triton's surface.A quantitative survey of Triton crater conducted by Voyager 2 found that only 179 craters were indisputably from the impact source,Tianwei FiveMiranda observed 835, while the surface area of Triton is only 3% of that of Triton.[66]The largest crater observed on Triton is believed to have been caused by impact, with a diameter of 27 kilometers (17 miles) and is called Mazomba.[66-67]Although larger craters have been observed, they are generally believed to be caused by volcanism.[66]
The bright Antarctic cap above Triton Hami melon terrain
The few impact craters of Triton are almost all concentrated in the leading hemisphere in the same direction as the orbital movement, most of which are concentrated near the equator with longitude between 30 ° and 70 °,[66]This is caused by Triton sweeping the material around Neptune.[50]Triton is tide locked, and one side is always facing the planet.Astronomers predict that this situation will affect the leading hemisphere more frequently and violentlyTrailing hemisphereThe impact should be small.[66]Voyager 2 only photographed 40% of Triton's surface, so the cause is still unknown.However, the observed asymmetry of crater distribution goes beyond the interpretation based on the number of impacts, which also implies that the surface age of the area without crater (≤ 6 million years) is younger than that of the area with crater (≤ 50 million years).[49]
Observation and detection
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Photos of Triton taken by Voyager 2 spacecraft
The orbital parameters of Triton have been measured with high precision since the 19th century, and it has a retrograde orbit with a great inclination to the orbit plane of Neptune.But it was not until 1930 that Triton was observed in detail for the first time.Before Voyager 2 flew by in 1989, scientists knew little about the satellite.[6]Before Voyager 2, astronomers suspected that Triton might have a liquid nitrogen ocean and a nitrogen/methane atmosphere with density up to 30% of the Earth's.Like the famous overestimation of the density of the Martian atmosphere, it turned out to be incorrect later.Like Mars, Triton was thought to have a denser atmosphere early in its history.[68]
Gerard Kuiper Gerard Kuiper first tried to measure the diameter of Triton in 1954.At the beginning, he obtained a measurement value of 3800km, and the subsequent measurement attempts yielded values ranging from 2500 to 6000 km, from slightly smaller than the diameter of the moon (3474.2 km) to half the diameter of the earth.[69]The data obtained when Voyager 2 arrived at Neptune on August 25, 1989 made the calculation of Triton's diameter (2706 kilometers) more accurate.[70]In the 1990s, scientists used Triton'sOccultationVarious observations were made to determine the presence of atmosphere and strange surfaces.The observation results at the end of 1997 showed that Triton was warming up and the atmosphere became more dense than that of Voyager 2 in 1989.[44]
In the past decades, NASA scientists have repeatedly proposed a new concept of carrying out exploration missions for the Neptune system in the 2010s.All these missions have Triton as the main target, and these plans often include an independent TritonLander, andHuygensThe lander detected Titan similarly.However, any efforts on Neptune and Triton are only at the proposal stage. The funds used by NASA for exploration missions in the outer solar system are mainly concentrated on Jupiter and Saturn systems.[71]One of the Triton Hopper landers will extract nitrogen ice from Triton's surface and process nitrogen into small rocketpropellantSo that the lander can fly or "jump" on the surface of Triton.[72-73]In 2019, a flyover detection mission was formally proposed as a part of NASA's "Discovery Program", called "Trident".[74]
NASA's illustration details the Trident mission
A study in 2012 calculated how the thickness of Triton's surface ice shell affects tidal dissipation and the crystallization process of the underground ocean. The results show that if Triton's ice shell is thin, then the tidal force effect will be obvious and the heating effect will be stronger. Otherwise, if the ice shell is thick, Triton will be stronger,The thermal effect produced by tidal force is weak, but even the liquid ocean will be a nitrogen rich ocean. In addition, the specific size of Triton's rocky core is still unknown, which will depend on the heat released by the radioactive isotope decay of the core.
Although there is still much debate, scientists believe that Triton's underground ocean can serve as a habitat for extraterrestrial life.Europa is one of the candidates for the alien life habitat. Even though the probability of Triton's life is far less than that of Europa, it cannot be excluded.Researchers speculate that Triton may have silicon-based life in the underground ocean, which is not based on carbon, but there is not enough research to show thatsilaneBehavior in special planetary environments.[75]
