|Discovered by||William Lassell|
|Discovery date||24 October 1851|
|Pronunciation||/ / or / /|
|Adjectives||Arielian / /|
Mean orbit radius
Average orbital speed
|Inclination||0.260° (to Uranus's equator)|
|Dimensions||1162.2 × 1155.8 × 1155.4 km|
|578.9±0.6 km (0.0908 Earths)|
Ariel is the fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus and so has an extreme seasonal cycle.
It was discovered in October 1851 by William Lassell and named for a character in two different pieces of literature. As of 2019, much of the detailed knowledge of Ariel derives from a single flyby of Uranus performed by the space probe Voyager 2 in 1986, which managed to image around 35% of the moon's surface. There are no active plans at present to return to study the moon in more detail, although various concepts such as a Uranus orbiter and probe have been proposed.
After Miranda, Ariel is the second-smallest of Uranus' five major rounded satellites and the second-closest to its planet. Among the smallest of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), it is believed to be composed of roughly equal parts ice and rocky material. Its mass is approximately equal in magnitude to Earth's hydrosphere.
Like all of Uranus' moons, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.
Discovered on 24 October 1851 by William Lassell, it is named for a sky spirit in Alexander Pope's The Rape of the Lock and Shakespeare's The Tempest .
Both Ariel and the slightly larger Uranian satellite Umbriel were discovered by William Lassell on 24 October 1851.Although William Herschel, who discovered Uranus's two largest moons Titania and Oberon in 1787, claimed to have observed four additional moons, this was never confirmed and those four objects are now thought to be spurious.
All of Uranus's moons are named after characters from the works of William Shakespeare or Alexander Pope's The Rape of the Lock . The names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell. I.Ariel is named after the leading sylph in The Rape of the Lock . It is also the name of the spirit who serves Prospero in Shakespeare's The Tempest . The moon is also designated Uranus
Among Uranus's five major moons, Ariel is the second closest to the planet, orbiting at the distance of about 190,000 km. Its orbit has a small eccentricity and is inclined very little relative to the equator of Uranus. Its orbital period is around 2.5 Earth days, coincident with its rotational period. This means that one side of the moon always faces the planet; a condition known as tidal lock. Ariel's orbit lies completely inside the Uranian magnetosphere. The trailing hemispheres (those facing away from their directions of orbit) of airless satellites orbiting inside a magnetosphere like Ariel are struck by magnetospheric plasma co-rotating with the planet. This bombardment may lead to the darkening of the trailing hemispheres observed for all Uranian moons except Oberon (see below). Ariel also captures magnetospheric charged particles, producing a pronounced dip in energetic particle count near the moon's orbit observed by Voyager 2 in 1986.
Because Ariel, like Uranus, orbits the Sun almost on its side relative to its rotation, its northern and southern hemispheres face either directly towards or directly away from the Sun at the solstices. This means it is subject to an extreme seasonal cycle; just as Earth's poles see permanent night or daylight around the solstices, so Ariel's poles see permanent night or daylight for half a Uranian year (42 Earth years), with the Sun rising close to the zenith over one of the poles at each solstice.The Voyager 2 flyby coincided with the 1986 southern summer solstice, when nearly the entire northern hemisphere was dark. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. A number of such events occurred in 2007–2008, including an occultation of Ariel by Umbriel on 19 August 2007.
Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with Miranda, which could have been partially responsible for the heating of that moon (although the maximum heating attributable to a former 1:3 resonance of Umbriel with Miranda was likely about three times greater). billion years ago, would have increased Ariel's orbital eccentricity, resulting in tidal friction due to time-varying tidal forces from Uranus. This would have caused warming of the moon's interior by as much as 20 K.Ariel may have once been locked in the 4:1 resonance with Titania, from which it later escaped. Escape from a mean motion resonance is much easier for the moons of Uranus than for those of Jupiter or Saturn, due to Uranus's lesser degree of oblateness. This resonance, which was likely encountered about 3.8
Ariel is the fourth largest of the Uranian moons, and may have the third greatest mass. g/cm3, which indicates that it consists of roughly equal parts water ice and a dense non-ice component. The latter could consist of rock and carbonaceous material including heavy organic compounds known as tholins. The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon, which is porous and thus transmits little solar heat to layers below. Water ice absorption bands are stronger on Ariel's leading hemisphere than on its trailing hemisphere. The cause of this asymmetry is not known, but it may be related to bombardment by charged particles from Uranus's magnetosphere, which is stronger on the trailing hemisphere (due to the plasma's co-rotation). The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.The moon's density is 1.66
Except for water, the only other compound identified on the surface of Ariel by infrared spectroscopy is carbon dioxide (CO2), which is concentrated mainly on its trailing hemisphere. Ariel shows the strongest spectroscopic evidence for CO2 of any Uranian satellite,and was the first Uranian satellite on which this compound was discovered. The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the energetic charged particles coming from Uranus's magnetosphere or solar ultraviolet radiation. This hypothesis would explain the asymmetry in its distribution, as the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the outgassing of primordial CO2 trapped by water ice in Ariel's interior. The escape of CO2 from the interior may be related to past geological activity on this moon.
Given its size, rock/ice composition and the possible presence of salt or ammonia in solution to lower the freezing point of water, Ariel's interior may be differentiated into a rocky core surrounded by an icy mantle. km) is about 64% of the radius of the moon, and its mass is around 56% of the moon's mass—the parameters are dictated by the moon's composition. The pressure in the center of Ariel is about 0.3 GPa (3 kbar). The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely by some, but possible by others.If this is the case, the radius of the core (372
Ariel is the most reflective of Uranus's moons.Its surface shows an opposition surge: the reflectivity decreases from 53% at a phase angle of 0° (geometrical albedo) to 35% at an angle of about 1°. The Bond albedo of Ariel is about 23%—the highest among Uranian satellites. The surface of Ariel is generally neutral in color. There may be an asymmetry between the leading and trailing hemispheres; the latter appears to be redder than the former by 2%. Ariel's surface generally does not demonstrate any correlation between albedo and geology on the one hand and color on the other hand. For instance, canyons have the same color as the cratered terrain. However, bright impact deposits around some fresh craters are slightly bluer in color. There are also some slightly blue spots, which do not correspond to any known surface features.
The observed surface of Ariel can be divided into three terrain types: cratered terrain, ridged terrain, and plains.The main surface features are impact craters, canyons, fault scarps, ridges, and troughs.
The cratered terrain, a rolling surface covered by numerous impact craters and centered on Ariel's south pole, is the moon's oldest and most geographically extensive geological unit. km wide and trend mainly in an east- or northeasterly direction. The floors of many canyons are convex; rising up by 1–2 km. Sometimes the floors are separated from the walls of canyons by grooves (troughs) about 1 km wide. The widest graben have grooves running along the crests of their convex floors, which are called valles . The longest canyon is Kachina Chasma, at over 620 km in length (the feature extends into the hemisphere of Ariel that Voyager 2 did not see illuminated).It is intersected by a network of scarps, canyons (graben), and narrow ridges mainly occurring in Ariel's mid-southern latitudes. The canyons, known as chasmata , probably represent graben formed by extensional faulting, which resulted from global tensional stresses caused by the freezing of water (or aqueous ammonia) in the moon's interior (see below). They are 15–50
The second main terrain type—ridged terrain—comprises bands of ridges and troughs hundreds of kilometers in extent. It bounds the cratered terrain and cuts it into polygons. Within each band, which can be up to 25 to 70 km wide, are individual ridges and troughs up to 200 km long and between 10 and 35 km apart. The bands of ridged terrain often form continuations of canyons, suggesting that they may be a modified form of the graben or the result of a different reaction of the crust to the same extensional stresses, such as brittle failure.
The youngest terrain observed on Ariel are the plains: relatively low-lying smooth areas that must have formed over a long period of time, judging by their varying levels of cratering. km. The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel. A few of these areas appear to be less than 100 million years old, suggesting that Ariel may still be geologically active in spite of its relatively small size and lack of current tidal heating.The plains are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain. In the latter case they are separated from the cratered terrain by sharp boundaries, which in some cases have a lobate pattern. The most likely origin for the plains is through volcanic processes; their linear vent geometry, resembling terrestrial shield volcanoes, and distinct topographic margins suggest that the erupted liquid was very viscous, possibly a supercooled water/ammonia solution, with solid ice volcanism also a possibility. The thickness of these hypothetical cryolava flows is estimated at 1–3
Ariel appears to be fairly evenly cratered compared to other moons of Uranus; km across, and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few of the craters are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the preexisting crustal structure. In the cratered plains there are a few large (about 100 km in diameter) light patches that may be degraded impact craters. If this is the case they would be similar to palimpsests on Jupiter's moon Ganymede. It has been suggested that a circular depression 245 km in diameter located at 10°S 30°E is a large, highly degraded impact structure.the relative paucity of large craters suggests that its surface does not date to the Solar System's formation, which means that Ariel must have been completely resurfaced at some point of its history. Ariel's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently. The largest crater observed on Ariel, Yangoor, is only 78
Ariel is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity.The precise composition of the subnebula is not known; however, the higher density of Uranian moons compared to the moons of Saturn indicates that it may have been relatively water-poor. Significant amounts of carbon and nitrogen may have been present in the form of carbon monoxide (CO) and molecular nitrogen (N2), instead of methane and ammonia. The moons that formed in such a subnebula would contain less water ice (with CO and N2 trapped as clathrate) and more rock, explaining the higher density.
The accretion process probably lasted for several thousand years before the moon was fully formed. K at a depth of about 31 km. After the end of formation, the subsurface layer cooled, while the interior of Ariel heated due to decay of radioactive elements present in its rocks. The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust reaching estimates of 30 MPa, which may have led to cracking. Some present-day scarps and canyons may be a result of this process, which lasted for about 200 million years.Models suggest that impacts accompanying accretion caused heating of Ariel's outer layer, reaching a maximum temperature of around 195
The initial accretional heating together with continued decay of radioactive elements and likely tidal heating may have led to melting of the ice if an antifreeze like ammonia (in the form of ammonia hydrate) or some salt was present. K. The ocean, however, is likely to have frozen long ago. The freezing of the water likely led to the expansion of the interior, which may have been responsible for the formation of the canyons and obliteration of the ancient surface. The liquids from the ocean may have been able to erupt to the surface, flooding floors of canyons in the process known as cryovolcanism.The melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle. A layer of liquid water (ocean) rich in dissolved ammonia may have formed at the core–mantle boundary. The eutectic temperature of this mixture is 176
Thermal modeling of Saturn's moon Dione, which is similar to Ariel in size, density, and surface temperature, suggests that solid state convection could have lasted in Ariel's interior for billions of years, and that temperatures in excess of 173 K (the melting point of aqueous ammonia) may have persisted near its surface for several hundred million years after formation, and near a billion years closer to the core.
The apparent magnitude of Ariel is 14.8; cm aperture, Ariel, due to its proximity to Uranus's glare, is often not visible to telescopes of 40 cm aperture.similar to that of Pluto near perihelion. However, while Pluto can be seen through a telescope of 30
The only close-up images of Ariel were obtained by the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. The closest approach of Voyager 2 to Ariel was 127,000 km (79,000 mi)—significantly less than the distances to all other Uranian moons except Miranda. The best images of Ariel have a spatial resolution of about 2 km. They cover about 40% of the surface, but only 35% was photographed with the quality required for geological mapping and crater counting. At the time of the flyby the southern hemisphere of Ariel (like those of the other moons) was pointed towards the Sun, so the northern (dark) hemisphere could not be studied. No other spacecraft has ever visited the Uranian system. The possibility of sending the Cassini spacecraft to Uranus was evaluated during its mission extension planning phase. It would have taken about twenty years to get to the Uranian system after departing Saturn, and these plans were scrapped in favour of remaining at Saturn and eventually destroying the spacecraft in Saturn's atmosphere.
On 26 July 2006, the Hubble Space Telescope captured a rare transit made by Ariel on Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Such events are rare and only occur around equinoxes, as the moon's orbital plane about Uranus is tilted 98° to Uranus's orbital plane about the Sun.Another transit, in 2008, was recorded by the European Southern Observatory.
Miranda, also designated Uranus V, is the smallest and innermost of Uranus's five round satellites. It was discovered by Gerard Kuiper on 16 February 1948 at McDonald Observatory in Texas, and named after Miranda from William Shakespeare's play The Tempest. Like the other large moons of Uranus, Miranda orbits close to its planet's equatorial plane. Because Uranus orbits the Sun on its side, Miranda's orbit is perpendicular to the ecliptic and shares Uranus' extreme seasonal cycle.
Umbriel is a moon of Uranus discovered on October 24, 1851, by William Lassell. It was discovered at the same time as Ariel and named after a character in Alexander Pope's poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a rocky core and an icy mantle. The surface is the darkest among Uranian moons, and appears to have been shaped primarily by impacts. However, the presence of canyons suggests early endogenic processes, and the moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.
Triton is the largest natural satellite of the planet Neptune, and was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt.
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of Cronus (Saturn). It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as "ice giants" to distinguish them from the other giant planets. Uranus's atmosphere is similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K, and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. The interior of Uranus is mainly composed of ices and rock.
Puck is an inner moon of Uranus. It was discovered in December 1985 by the Voyager 2 spacecraft. The name Puck follows the convention of naming Uranus's moons after characters from Shakespeare. The orbit of Puck lies between the rings of Uranus and the first of Uranus's large moons, Miranda. Puck is approximately spherical in shape and has diameter of about 162 km. It has a dark, heavily cratered surface, which shows spectral signs of water ice.
Oberon, also designated Uranus IV, is the outermost major moon of the planet Uranus. It is the second-largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies who appears as a character in Shakespeare's A Midsummer Night's Dream. Its orbit lies partially outside Uranus's magnetosphere.
Cressida is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 9 January 1986, and was given the temporary designation S/1986 U 3. It was named after Cressida, the Trojan daughter of Calchas, a tragic heroine who appears in William Shakespeare's play Troilus and Cressida. It is also designated Uranus IX.
Desdemona is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 13 January 1986, and was given the temporary designation S/1986 U 6. Desdemona is named after the wife of Othello in William Shakespeare's play Othello. It is also designated Uranus X.
Juliet is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 3 January 1986, and was given the temporary designation S/1986 U 2. It is named after the heroine of William Shakespeare's play Romeo and Juliet. It is also designated Uranus XI.
Belinda is an inner satellite of the planet Uranus. Belinda was discovered from the images taken by Voyager 2 on 13 January 1986 and was given the temporary designation S/1986 U 5. It is named after the heroine of Alexander Pope's The Rape of the Lock. It is also designated Uranus XIV.
Titania, also designated Uranus III, is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1,578 kilometres (981 mi). Discovered by William Herschel in 1787, it is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream. Its orbit lies inside Uranus's magnetosphere.
Portia is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 3 January 1986, and was given the temporary designation S/1986 U 1. The moon is named after Portia, the heroine of William Shakespeare's play The Merchant of Venice. It is also designated Uranus XII.
Wunda is a large crater on the surface of Uranus' moon Umbriel. It is 131 km in diameter and is located near the equator of Umbriel. The crater is named after Wunda, a dark spirit of Australian aboriginal mythology.
Uranus, the seventh planet of the Solar System, has 27 known moons, most of which are named after characters that appear in, or are mentioned in, the works of William Shakespeare and Alexander Pope. Uranus's moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner and major moons all have prograde orbits, while orbits of the irregulars are mostly retrograde. The inner moons are small dark bodies that share common properties and origins with Uranus's rings. The five major moons are ellipsoidal, indicating that they reached hydrostatic equilibrium at some point in their past, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, about one-twentieth the mass of the Earth's Moon. The orbits of the regular moons are nearly coplanar with Uranus's equator, which is tilted 97.77° to its orbit. Uranus's irregular moons have elliptical and strongly inclined orbits at large distances from the planet.
The rings of Uranus are intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789; modern astronomers are divided on whether he could have seen them, as they are very dark and faint.
The exploration of Uranus has, to date, been through telescopes and a lone probe by NASA's Voyager 2 spacecraft, which made its closest approach to Uranus on January 24, 1986. Voyager 2 discovered 10 moons, studied the planet's cold atmosphere, and examined its ring system, discovering two new rings. It also imaged Uranus' five large moons, revealing that their surfaces are covered with impact craters and canyons.
Mommur Chasma is the largest 'canyon' on the known part of the surface of Uranus' moon Oberon. This feature probably formed during crustal extension at the early stages of moon's evolution, when the interior of Oberon expanded and its ice crust cracked as a result. The canyon is an example of graben or scarp produced by normal fault(s). The chasma was first imaged by Voyager 2 spacecraft in January 1986.
The Messina Chasmata are the largest canyon or system of canyons on the surface of the Uranian moon Titania, named after a location in William Shakespeare's comedy Much Ado About Nothing. The 1,492 km (927 mi)- long feature includes two normal faults running NW–SE, which bound a down-dropped crustal block forming a structure called a graben. The graben cuts impact craters, which probably means that it was formed at a relatively late stage of the moon's evolution, when the interior of Titania expanded and its ice crust cracked as a result. The Messina Chasmata have only a few superimposed craters, which also implies being relatively young. The feature was first imaged by Voyager 2 in January 1986.
Rousillon Rupes is a scarp on the surface of the Uranian moon Titania named after "Bertram, count of Rousillon" in William Shakespeare's comedy All's Well That Ends Well. The 402 km long feature is a normal fault situated near the equator and running perpendicular to it. The scarp cuts impact craters, which probably means that it was formed at a relatively late stage of moon's evolution, when the interior of Titania expanded and its ice crust cracked as a result. Rousillon Rupes has only few crater superimposed on it, which also implies its relatively young age. The scarp was first imaged by Voyager 2 spacecraft in January 1986.
The Kachina Chasmata are the longest canyon or system of canyons on the surface of the Uranian moon Ariel. The name comes from a spirit in Hopi mythology. The 622 km long and 50 km wide chasmata arise from a system of normal faults running from the north-west to south-east. The faults bound down-dropped crustal blocks forming structures called graben. The canyons cut the cratered terrain, which means that they were formed at a relatively late stage of the moon's evolution, when the interior of Ariel expanded and its ice crust cracked as a result. The floor of the canyons is not visible on the images obtained by the Voyager 2 spacecraft in January 1986; thus, whether it is covered by smooth plains like the floors of other Arielian graben is currently unknown.
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