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 spacecraft 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 by||William Lassell|
|Discovery date||24 October 1851|
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)|
|Mass||(1.353±0.120)×1021 kg (2.26×10−4 Earths)|
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. 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 I.
Among Uranus's five major moons, Ariel is the second closest to the planet, orbiting at the distance of about 190,000 km.[f] 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). 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 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 is the fourth largest of the Uranian moons, and may have the third greatest mass.[g] The moon's density is 1.66 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.
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. If this is the case, the radius of the core (372 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.
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%.[h] 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. 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 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).
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. 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 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.
Ariel appears to be fairly evenly cratered compared to other moons of Uranus; the relative paucity of large craters[i] 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 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.
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.[j] 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. Models suggest that impacts accompanying accretion caused heating of Ariel's outer layer, reaching a maximum temperature of around 195 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.
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. 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 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.
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; similar to that of Pluto near perihelion. However, while Pluto can be seen through a telescope of 30 cm aperture, Ariel, due to its proximity to Uranus's glare, is often not visible to telescopes of 40 cm aperture.
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.
Ariel may refer to:
Ariel (name), a given name (including a list of people and characters with the name)Bio of a Space Tyrant
Bio of a Space Tyrant series is a six-book science-fiction series by Piers Anthony based within the Solar System. The series revolves around the character Hope Hubris and his family, and charts Hope's ascent from poor Hispanic refugee to Tyrant of Jupiter, a single person heading the Executive, Judicial and Legislative branches of the government. It is considerably more adult-themed than many of Anthony's earlier works.
The novels are set in a future several hundred years distant at a point where the nations of Earth have expanded into and filled the Solar System. Various planets, moons and asteroids within the Solar System have political, religious and geographical affiliations similar to those on 1980s Earth. Many events in the series parallel modern-day situations; for instance, Mars is controlled by the countries formerly comprising the Middle East, and has the largest supply of iron, the primary fuel in Anthony's universe. Ganymede parallels Cuba, and has a Communist government allied with North Saturn's, which corresponds to the Soviet Union. A "Missile Crisis" is an event in the series; many other such correspondences abound.
The series is presented as Hope's first-person autobiography, and includes his image as a tyrant and womanizer. It is clear from the introductions and epilogues that Hope died before his memoir's publication and that his career as military man, politician, executive and statesman was greatly misunderstood by the public, especially his various affairs with women, a major focus of the series.Brownie Chasma
The Brownie Chasma is a Chasma located on Ariel. They are named after Brownies, which are spirits believed to help with household tasks.Discovery and exploration of the Solar System
Discovery and exploration of the Solar System is observation, visitation, and increase in knowledge and understanding of Earth's "cosmic neighborhood". This includes the Sun, Earth and the Moon, the major planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, their satellites, as well as smaller bodies including comets, asteroids, and dust.
The Solar System — our Sun’s system of planets, moons, and smaller debris — is humankind’s cosmic backyard. Small by factors of millions compared to interstellar distances,
the spaces between the planets are daunting, but technologically surmountableFirst Contact?
First Contact? is a juvenile science fiction novel, the thirteenth in Hugh Walters' Chris Godfrey of U.N.E.X.A. series.It was published in the UK by Faber in 1971, in the US by T.Nelson Books in 1973.Kachina Chasmata
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.During the Voyager 2 flyby in 1986 the northern hemisphere of Ariel was not illuminated by the Sun because the spacecraft arrived during a southern solstice on Uranus. Nevertheless, because it was still illuminated by light reflected from Uranus, scientists using advanced processing methods were able to detect some details in the dark hemisphere. These analyses revealed a continuation of the Kachina Chasmata into the dark hemisphere, possibly as far as to the opposite limb. As the total length of the feature appears to be 1800–2200 km, it may be comparable to Ithaca Chasma on Tethys.Kewpie Chasma
The Kewpie Chasma is a Chasma on the surface of Ariel..List of geological features on Ariel
This list of geological features on Ariel itemizes the named geological features on the moon of Uranus called Ariel. Nearly all of the features are named for bright spirits of world mythologies. All information in the tables below comes from the United States Geological Survey.Natural satellite
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another small Solar System body).
In the Solar System there are six planetary satellite systems containing 185 known natural satellites. Four IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris. As of September 2018, there are 334 other minor planets known to have moons.The Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System (although there are minor-planet systems with even greater ratios, notably the Pluto–Charon system). At 3,474 km (2,158 miles) across, the Moon is 0.27 times the diameter of Earth.Solar System
The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, at least four of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.Yangoor (crater)
Yangoor is the largest known crater on the surface of the Uranian moon Ariel. The name comes from a spirit that brings day in Australian Aboriginal mythology. It is about 80 km in diameter and is located approximately 450 km from Ariel's south pole. The northwestern edge of the crater was erased by formation of ridged terrain. The crater lacks bright ejecta deposits and was imaged for the first time by the Voyager 2 spacecraft in January 1986.
|Ranked by size|