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Tianwei I

Uranus's moons

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Ariel is 27 known Uranus The fourth largest satellite among satellites. Triton orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus, so it has an extreme seasonal period. It is composed of William Lassell William Lassell discovered it on October 24, 1851, and named it after the characters in two different literary works.^[1-2] Almost all the detailed knowledge of Triton comes from the flyby of Uranus system in 1986 Voyager 2 , which photographed about 35% of the satellite's surface. Although some people put forward various concepts such as Uranus orbiter and probe,^[3] However, there are no active plans to study the satellite in more detail.

Triton is the second smallest of the five major spherical satellites of Uranus, only larger than Miranda, and the second closest to Uranus.^[4] Among the 19 smallest known spherical satellites in the solar system, it is the 14th in diameter and is considered to be composed of roughly equal ice and rock materials. Its mass is about equal to that of the earth Hydrosphere Total mass.^[5]

Like all the moons of Uranus, Mimas may have formed around Uranus shortly after its formation Accretion disk 。 Like other large satellites, Triton may also undergo geological differentiation, with its rock core wrapped by ice mantle. The surface of Triton is very complex, and the system composed of cliffs, canyons and ridges cuts across Impact crater The surface of the earth. It is likely that due to tidal heating, its surface shows newer signs of geological activity than other Uranian satellites.

Chinese name: Tianwei I
Foreign name: Ariel
Alias: Ariel
Classification: satellite
Discoverer: William Lassell
Discovery time: October 24, 1851
Quality: 1.251✕10 twenty-one kg^[6] （±0.021）
Average density: 1.592 g/m³
Diameter: About 1157.8 km^[7] （1162.2 ×1155.8 ×1155.4 km）

surface temperature: 60 K
Escape speed: 0.559 km/s
Albedo: zero point five three
Apparent magnitude: 14.4 etc^[8]
Rotation period: synchronization
Semimajor axis: 191020 km^[4]
Eccentricity: zero point zero zero one two^[4]
Revolution period: 2.520 days^[4]
Rail inclination: 0.260 degrees^[4]
Gravitational acceleration: 0.269 m/s2

catalog

one Discovery and naming
two track
three Composition and internal structure

six Observation and exploration

Discovery and naming

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Tianwei Yiyou William Lassell (William Lassell) discovered it on October 24, 1851, and Alexander Pope (Alexander Pope) William Shakespeare Of《 storm 》It is named after the sky spirit in. William Russell discovered Triton and the larger Triton on October 24, 1851.^[1-2] All the moons of Uranus are named after the characters in the works of William Shakespeare or Alexander Pope. John Herschel (John Herschel) proposed the names of all four known Uranus satellites in 1852 at the request of William Russell.^[9] Ariel is named after the leading sky spirit in the Story of Hair Seizure.^[10] This is also in Shakespeare's The Tempest^[11] In Prospero (Prospero) The name of the service wizard. Triton is also known as Uranus I.^[2] John Herschel's father William Herschel William Herschel discovered the two largest moons of Uranus in 1787 Titania (Titania) and Tianwei IV (Oberon), and he also claimed to have observed four other satellites,^[12] However, it was never confirmed, and it was later believed that these four celestial bodies did not exist.^[13-15]

track

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Among the five major satellites of Uranus, Triton is the second closest to Uranus, with an orbital half length of about 190000 kilometers. its Track eccentricity Smaller, relative to the equator of Uranus Rail inclination Very small.^[4] Its orbital period is about 2.5 Earth days, consistent with its rotation period. This one is called Tidal locking The state of means that one side of the satellite is always facing the planet.^[16] Triton's orbit is completely located in the orbit of Uranus Magnetosphere Inside.^[17] There is no atmosphere satellite running inside the magnetosphere like Triton Trailing hemisphere (Opposite to the direction of its orbit), it will be bombarded by the magnetospheric plasma rotating with the planet.^[18] This kind of bombardment may lead to the phenomenon of the following hemisphere darkening observed by all Uranus except Triton (see below).^[17] Enceladus captured charged particles in the magnetosphere and generated them near the orbit of the satellite observed by Voyager 2 in 1986 High-energy particle The number has decreased significantly.^[19]

The larger satellite in the Uranus system, Ariel at the top

Because Iapetus is like Uranus equator Almost lying around the sun, so its northern and southern hemispheres To (summer solstice or winter solstice) both face directly or directly away from the sun. This means that it is affected by extreme seasonal cycles, just like the poles of the Earth see polar night and polar day before and after the day, the poles of Triton appear in every half of the Uranian year (42 Earth years) Polar night and polar day , the sun near one of the poles approaches it every solstice zenith Location.^[17] Voyager 2 happened to fly by at the solstice in 1986, when almost the entire northern hemisphere was dark. Every 42 years, Uranus reaches its Equinox (vernal equinox or autumnal equinox) position, when its equatorial plane intersects with the earth, Uranus satellites interact with each other eclipse It will be possible. A number of such events occurred in 2007-2008, including the cover feeding of Enceladus on Enceladus on August 19, 2007.^[20]

Triton did not participate in any orbital resonance with other Uranus satellites. But in the past it might have Tianwei Five (Miranda) has a 5:3 resonance, which may partially lead to the heating of the satellite (although the maximum heating may be attributed to the 1:3 resonance of Europa and Rhea before about three times).^[21] Triton may have been locked in 4:1 resonance with Triton and escaped later.^[22] Because Uranus has a small oblateness Jupiter or Saturn The average motion resonance of the satellites of Uranus is much easier.^[22] This resonance may have occurred about 3.8 billion years ago, and it will increase the orbital eccentricity of Triton, resulting in the time-varying tidal force generated by Uranus Tidal friction 。 This will warm the interior of the satellite up to 20K.^[22]

Composition and internal structure

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Triton is the fourth largest satellite of Uranus in volume and possibly the third largest in mass. The density of Triton is 1.66 g/cm3,^[23] This indicates that it is composed of approximately equal water ice blocks and dense non ice materials.^[5] Non ice materials may consist of rocks and carbonaceous materials, the latter including Torin (Tholins).^[16] Infrared spectrum observation confirmed the existence of water ice, which revealed that the crystalline water ice on the surface of the satellite was porous, so there was almost no solar heat transfer to the lower layer.^[17]^[24] On the leading hemisphere of Triton, water ice Absorption band Stronger than in the trailing hemisphere.^[17] The reason for this asymmetry is not clear, but it may be related to the bombardment of charged particles from Uranus's magnetosphere, which is stronger in the following hemisphere (due to the common rotation of plasma).^[17] High energy particles tend to sputter water ice, decompose methane in the form of inclusion hydrate in the water ice, and darken other organic matters, leaving dark carbon rich residues.^[17]

The clearest image of Triton taken by Voyager 2

Except for water, other compounds identified on the surface of Triton by infrared spectroscopy are carbon dioxide （CO two ）, which is mainly concentrated in the following hemisphere. Triton presented the strongest CO2 spectral evidence of all Uranus satellites,^[17] It is also the first Uranus satellite with carbon dioxide.^[17] The source of carbon dioxide is not completely clear. Under the action of Uranus magnetosphere or high-energy charged particles generated by solar ultraviolet radiation, it may be produced by local carbonate or organic matter. This hypothesis can explain the asymmetry of its distribution, because the trailing hemisphere is more strongly affected by the magnetosphere than the leading hemisphere. Another possible source is the original CO captured by water ice inside Triton two Be released. The escape of carbon dioxide from the interior may be related to the past geological activities of the satellite.^[17]

Due to its size, the composition of rock/ice and the salt or ammonia that may exist in the solution freezing point The interior of Triton may be divided into ice mantle Surrounding rock core.^[5] In this case, the radius of the core (372km) is about 64% of the satellite's radius and its mass is about 56% of the satellite's mass. These parameters are determined by the satellite's composition. The pressure of Tianwei No.1 Center is about 0.3 GPa (3000 bar).^[5] Although some people believe that underground oceans are unlikely,^[5] But others believe that the current state of the frozen mantle is not clear.^[25]

surface

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Albedo and color

Triton is the largest satellite of Uranus reflectivity The highest.^[26] Its surface shows opposite fluctuations, reflectivity (geometry Albedo ）From 53% when the phase angle is 0 ° to 35% when the phase angle is about 1 °. The spectral albedo of Triton is about 23%, which is the highest among Uranus.^[26] The surface of Triton is usually neutral.^[27] There may be asymmetry between the leading hemisphere and the following hemisphere;^[28] The latter seems to be slightly 2% redder than the leading hemisphere. On the one hand, the surface of Triton usually shows no correlation between albedo and geology, and on the other hand, there is no correlation between albedo and color. For example, the color of a canyon is the same as that of a multi pit terrain. However, the bright impact deposits around some of the newer craters are slightly bluish in color.^[27-28] There are also some bluish spots that do not correspond to any known surface feature.^[28]

Surface characteristics

The observed surface of Triton can be divided into three terrain types: multi pit terrain, ridge Topography and plains.^[29] The main surface features are impact crater, canyon, fault Steep slopes, ridges and valleys.^[30] The multi crater terrain is centered on the south pole of Triton and covers the undulating surface of many impact craters. It is the oldest and most widely distributed geological unit of the satellite.^[29] Crater terrain and steep canyons, Graben It is interwoven with the network of narrow ridges, which is mainly distributed in the south central latitude of Triton.^[29] ^[31] The canyon may represent a graben formed by an extensional fault, which is caused by the freezing of water (or ammonia water) inside the moon (see below) Tensile stress Caused by.^[16]^[29] They are 15 to 50 kilometers wide and mainly develop in the east and northeast.^[29] The ground of many canyons is convex and can rise 1-2 kilometers.^[31] In some cases, the platform is separated from the canyon wall by a groove (valley) about 1km wide.^[31] The widest graben has grooves on its protruding platform peak, which are called valleys.^[16] The longest canyon is Kachina Chasma, with a total length of more than 620 kilometers (this feature extends to the hemisphere of Tianwei that cannot be observed by Traveler 2).^[30]^[32]

The second main terrain type is the ridge terrain, including the belt terrain consisting of ridges and valleys stretching for hundreds of kilometers. This limits the multi pit terrain and cuts it into various polygons. Within each banded terrain, independent ridges and valleys may be 25 to 70 kilometers wide, up to 200 kilometers long, and 10 to 35 kilometers apart. Banded ridge terrain often forms continuous canyons, indicating that they may be Graben Or the earth's crust Tensile stress (such as brittle failure).^[29]

The transverse valley cutting the surface of Triton

The youngest terrain observed on Triton is plain According to the changing velocity of crater formation, the formation of relatively low flat areas must be a long process.^[29] The plain is located at the bottom of the canyon with some irregular depressions in the multi pit terrain.^[16] In the latter case, they are separated from the multi pit terrain by a steep boundary, which in some cases is leaf shaped.^[29] The most likely origin of these plains is volcanic process. Their linear vents are geometrically shaped, similar to those on Earth Shield volcano The unique terrain edge indicates that the liquid ejected is very viscous, which may be a supercooled water/ammonia solution or a solid Ice volcano Role of.^[31] These hypothetical low temperatures Lava flow The thickness of is estimated to be 1-3 km.^[31] Therefore, the canyon must have been formed when Tianwei Yi was still undergoing endogenous surface renovation.^[29] Some of these areas seem to be less than 100 million years old, which indicates that although the scale of Triton is relatively small and the current tidal heating is low, it may still be geologically active.^[33]

Compared with other Uranus moons, the impact craters of Io appear to be fairly uniform.^[16] The relative scarcity of large impact craters indicates that its surface cannot be traced back to the early formation of the solar system, which means that at some point in its history, Triton has completed the whole process of surface renovation.^[29] Astronomers believe that the past geological activities of Triton were driven by tidal heating, and its orbit was higher than its eccentricity.^[22] Yangoor is the largest impact crater observed on Triton, only 78 kilometers wide,^[30] And shows signs of subsequent deformation. All the large impact craters on Triton have flat ground and Central peak And very few impact craters are surrounded by bright jet deposits. Many impact craters are polygonal, indicating that their appearance is affected by pre-existing crustal structure. On the plain of the impact crater, there are some huge bright spots (about 100 km in diameter), which may be degraded impact craters. If this is the case, they will be similar to Ganymede Superficial Variational complementary structure （palimpsests）。^[29] It has been suggested that the 245 km diameter circular depression located at 10 ° S 30 ° E is a large and highly degraded impact structure.^[34]

The huge bright spots on the impact crater plain are similar to the complementary structure on the surface of Ganymede

The deep valley on Tianwei I is mostly named after the mythical goblin
name	coordinate	Length (km)	Name Source
Brownie Valley	16°S 37.6°E	three hundred and forty-three	Brownie, the goblin of Scotland and Northern England
Kachina Valley	33.7°S 246°E	six hundred and twenty-two	Kachina， The goblin of the Hebi nationality
Chubby Valley	28.3°S 326.9°E	four hundred and sixty-seven	Chubby, the English goblin
Korigan Valley	27.6°S 347.5°E	three hundred and sixty-five	Corrigan, the goblin of Brittany, France
Kra Valley	32.1°S 354.2°E	one hundred and forty-two	Kra， Akan goblin
Pixie Valley	20.4°S 5.1°E	two hundred and seventy-eight	Pixie, the English goblin
Silver Valley	48.6°S 353°E	three hundred and forty-nine	Silver, the English goblin

The impact crater on Triton is mostly named after the goblins in mythology and drama
name	coordinate	Diameter (km)	Name Source
Aban impact pit	15.5°S 251.3°E	twenty	Aban， Zoroastrian water god
Agape impact crater	46.9°S 336.5°E	thirty-four	Agape， The goblin of Edmund Spencer
Ataksak impact crater	53.1°S 224.3°E	twenty-two	Ataksak， Inuit Goddess of Sky and Light
Bei Fangna impact crater	17°S 31.9°E	twenty-one	Befana, Italian witch, similar to Santa Claus
Berelena impact crater	22.5°S 327.9°E	twenty-nine	Berelenna, the goblin of the blue bird in Maurice Maitlink's works
Harmony Goddess Impact Pit	22.3°S 23°E	twenty	Harmony Goddess, the seven spinning sisters of Lithuanian mythology, is somewhat similar to Moira in Greek mythology and the three Nunn goddesses in Nordic mythology.
Djadek impact crater	12°S 251.1°E	twenty-two	Djadek， Czech goblin, considered to be the same kind of goblin as Dumoy
Dumoi impact crater	71.5°S 339.7°E	seventy-one	Dumoy, Russian goblin
Fenwara impact crater	15.8°S 19°E	thirty-one	Fanwalla, the king of Irish goblins
Gwen impact crater	77.5°S 22.5°E	thirty-four	Gwen Epp Niz, the goblin king of Glastonbury Tower
Yuweng impact crater	37.8°S 33.7°E	forty	Yu Weng of Bordeaux, Knight of French epic martial arts songs
Laica impact crater	21.3°S 44.4°E	thirty	Laica， Inca mythology
Mabu impact pit	38.8°S 352.2°E	thirty-four	Queen Mab, the goblin queen mentioned by Romeo and Juliet
Melusina impact crater	52.9°S 8.9°E	fifty	Melusina, the spirit of Melusina's chronicles in French literature
Ouna impact crater	21.9°S 244.4°E	thirty-nine	Erna, the Irish goblin
Rima impact crater	18.3°S 260.8°E	forty-one	Rima， Female Taishan in William Henry Hudson's novel Green Mansions
Yangoor impact pit	68.7°S 279.7°E	seventy-eight	Yangoor， A goblin that brings a new day in Australian aboriginal mythology

Origin and evolution

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Triton is formed by an accretion disk or a sub nebula. This gas and dust disk may have existed since the formation of Uranus, or it may have been caused by a huge impact that caused Uranus to tilt.^[35] Although the composition of the sub nebula is not clear; However, compared with Saturn, Uranus has a higher density, indicating that it may be relatively water poor.^[16] A large amount of carbon and nitrogen may carbon monoxide (CO) and molecular nitrogen (N two ）Exists in the form of methane and ammonia 。^[35] Satellites formed in such sub nebulae will contain less water ice (CO and N two It is captured by water ice to form inclusion complex) and more rocks, which can explain its higher density.^[16]

Before the formation of Triton, accretion The process may have lasted for thousands of years.^[35] The model shows that the impact with accretion causes the outer layer of Triton to heat up, reaching a maximum temperature of 195 K at a depth of about 31 km.^[36] After the formation, the underground layer cools, while the interior of Triton is heated due to the decay of radioactive elements in its rocks.^[16] The cooled near surface layer shrinks while the interior expands. This caused a huge tensile stress in the crust of the satellite that may lead to fracture, estimated to be 30 MPa.^[29]^[37] This process lasted about 200 million years, and many cliffs and canyons today are the result of this process. [42]^[37]

If water ice is mixed with ammonia (in the form of ammonia hydrate) or some salt Antifreeze The early accretion heating, the continuous decay of radioactive elements and possible tidal heating may all lead to ice melting.^[36] Melting may separate ice from rock and form the internal structure of rock core surrounded by ice mantle.^[5] A layer of liquid water (ocean) rich in dissolved ammonia may be formed at the core mantle boundary. The eutectic temperature of the mixture of water and antifreeze is 176K.^[5] But this layer of ocean is probably already frozen. The freezing of water is likely to cause internal expansion, which may be the reason for the formation of the canyon and the disappearance of the ancient surface.^[29] Liquid from the ocean can be ejected to the surface through the activity process of ice volcanoes, flooding the bottom of the canyon to form a plain.^[36]

Enceladus Its thermal model is similar to that of Triton in size, density and surface temperature, indicating that solid convection may have lasted for billions of years in Triton. During the hundreds of millions of years after its formation, the temperature near the surface of Triton may have remained above 173 K（ ammonia (melting point), which has remained close to the rock core for nearly one billion years.^[29]

Observation and exploration

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Tianweiyi Apparent magnitude 14.8,^[8] Brightness vs Perihelion The nearby Pluto is similar. Although Pluto can be seen through a telescope with an aperture of 30 cm glare A telescope with an aperture of 40 cm usually cannot see Triton.^[38] The Voyager 2 spacecraft took the only detailed pictures of the moon, which flew over Uranus Sunday in January 1986. The closest distance to the moon was 127000 kilometers (79000 miles), which is far less than all other Uranus satellites except the moon.^[39] Space of the best image of Triton resolving power About 2 kilometers,^[29] They cover about 40% of the earth's surface, but only 35% of the photos have Geological mapping And crater counting.^[29] During the flyby, the southern hemisphere of Triton (like other satellites) points to the sun, so it is impossible to study the northern (dark) hemisphere.^[16] The Uranus system has not been visited by other spacecraft except Voyager 2.^[40] stay Cassini In the mission expansion plan phase, NASA once evaluated the possibility of sending Cassini spacecraft to Uranus,^[3] After leaving Saturn, it will take about 20 years for the spacecraft to enter the Uranus system. But eventually the plan was canceled, so Cassini stayed in Saturn and was eventually destroyed in the atmosphere of Saturn.^[3]

Enhanced color image of the surface of Triton

On July 26, 2006, the Hubble Space Telescope captured a rare transit of Triton on Uranus Cloud top You can see the shadow of Triton. Such events are very rare and only occur around the equinox of Uranus (vernal equinox or autumnal equinox), because the orbital plane of the satellite around Uranus inclines 98 ° relative to the orbital plane of Uranus around the sun.^[41] The European Southern Observatory recorded another transit event in 2008.^[42]

The transit star of Triton on Uranus, leaving a shadow on the cloud top of Uranus

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