# Oberon (moon)

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Discovery The best Voyager 2 image of Oberon [caption 1] William Herschel January 11, 1787 [1] Uranus IV or [2] Oberonian [3] 583520 km [4] 0.0014 [4] 13.463234  d [4] 3.15 km/s (calculated) 0.058° (to Uranus's equator) [4] Uranus 761.4±2.6 km (0.1194 Earths) [5] 7285000 km2 [lower-alpha 1] 1849000000 km3 [lower-alpha 2] (3.076±0.087)×1021 kg [6] 1.63±0.05 g/cm³ [7] 0.354 m/s² [lower-alpha 3] 0.734 km/s [lower-alpha 4] presumed synchronous [8] 0.31 (geometrical)0.14 (Bond) [9] 70–80  K [10] 14.1 [11] .mw-parser-output .reflist{font-size:90%;margin-bottom:0.5em;list-style-type:decimal}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}

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.

## Contents

It is likely that Oberon formed from the accretion disk that surrounded Uranus just after the planet's formation. The moon consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the boundary between the mantle and the core. The surface of Oberon, which is dark and slightly red in color, appears to have been primarily shaped by asteroid and comet impacts. It is covered by numerous impact craters reaching 210 km in diameter. Oberon possesses a system of chasmata (graben or scarps) formed during crustal extension as a result of the expansion of its interior during its early evolution.

The Uranian system has been studied up close only once: the spacecraft Voyager 2 took several images of Oberon in January 1986, allowing 40% of the moon's surface to be mapped.

## Discovery and naming

Oberon was discovered by William Herschel on January 11, 1787; on the same day he discovered Uranus's largest moon, Titania. [1] [12] He later reported the discoveries of four more satellites, [13] although they were subsequently revealed as spurious. [14] For nearly fifty years following their discovery, Titania and Oberon would not be observed by any instrument other than William Herschel's, [15] although the moon can be seen from Earth with a present-day high-end amateur telescope. [11]

All of the moons of Uranus are named after characters created by William Shakespeare or Alexander Pope. The name Oberon was derived from Oberon, the King of the Fairies in A Midsummer Night's Dream . [16] The names of all four satellites of Uranus then known were suggested by Herschel's son John in 1852, at the request of William Lassell, [17] who had discovered the other two moons, Ariel and Umbriel, the year before. [18] The adjectival form of the name is Oberonian,. [19]

Oberon was initially referred to as "the second satellite of Uranus", and in 1848 was given the designation Uranus II by William Lassell, [20] although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV). [21] In 1851 Lassell eventually numbered all four known satellites in order of their distance from the planet by Roman numerals, and since then Oberon has been designated Uranus IV. [22]

## Orbit

Oberon orbits Uranus at a distance of about 584,000 km, being the farthest from the planet among its five major moons. [lower-alpha 5] Oberon's orbit has a small orbital eccentricity and inclination relative to the equator of Uranus. [4] Its orbital period is around 13.5 days, coincident with its rotational period. In other words, Oberon is a synchronous satellite, tidally locked, with one face always pointing toward the planet. [8] Oberon spends a significant part of its orbit outside the Uranian magnetosphere. [23] As a result, its surface is directly struck by the solar wind. [10] This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by the magnetospheric plasma, which co-rotates with the planet. [23] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below). [10]

Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Oberon) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the zenith over one of the poles at each solstice. [10] The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was in darkness. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. One such event, which lasted for about six minutes, was observed on May 4, 2007, when Oberon occulted Umbriel. [24]

## Composition and internal structure

Oberon is the second largest and most massive of the Uranian moons after Titania, and the ninth most massive moon in the Solar System. [lower-alpha 6] Oberon's density of 1.63 g/cm³, [7] which is higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice and a dense non-ice component. [26] The latter could be made of rock and carbonaceous material including heavy organic compounds. [8] The presence of water ice is supported by spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. [10] Water ice absorption bands are stronger on Oberon's trailing hemisphere than on the leading hemisphere. This is the opposite of what is observed on other Uranian moons, where the leading hemisphere exhibits stronger water ice signatures. [10] The cause of this asymmetry is not known, but it may be related to impact gardening (the creation of soil via impacts) of the surface, which is stronger on the leading hemisphere. [10] Meteorite impacts tend to sputter (knock out) ice from the surface, leaving dark non-ice material behind. [10] The dark material itself may have formed as a result of radiation processing of methane clathrates or radiation darkening of other organic compounds. [8] [27]

Oberon may be differentiated into a rocky core surrounded by an icy mantle. [26] If this is the case, the radius of the core (480 km) is about 63% of the radius of the moon, and its mass is around 54% of the moon's mass—the proportions are dictated by the moon's composition. The pressure in the center of Oberon is about 0.5  GPa (5  kbar). [26] The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze, Oberon may possess a liquid ocean layer at the core–mantle boundary. The thickness of this ocean, if it exists, is up to 40 km and its temperature is around 180 K. [26] However, the internal structure of Oberon depends heavily on its thermal history, which is poorly known at present.

## Surface features and geology

Oberon is the second-darkest large moon of Uranus after Umbriel. [9] Its surface shows a strong opposition surge: its reflectivity decreases from 31% at a phase angle of 0° (geometrical albedo) to 22% at an angle of about 1°. Oberon has a low Bond albedo of about 14%. [9] Its surface is generally red in color, except for fresh impact deposits, which are neutral or slightly blue. [28] Oberon is, in fact, the reddest among the major Uranian moons. Its trailing and leading hemispheres are asymmetrical: the latter is much redder than the former, because it contains more dark red material. [27] The reddening of the surfaces is often a result of space weathering caused by bombardment of the surface by charged particles and micrometeorites over the age of the Solar System. [27] However, the color asymmetry of Oberon is more likely caused by accretion of a reddish material spiraling in from outer parts of the Uranian system, possibly from irregular satellites, which would occur predominately on the leading hemisphere. [29]

Scientists have recognized two classes of geological feature on Oberon: craters and chasmata ('canyons'—deep, elongated, steep-sided depressions [30] which would probably be described as rift valleys or escarpments if on Earth). [8] Oberon's surface is the most heavily cratered of all the Uranian moons, with a crater density approaching saturation—when the formation of new craters is balanced by destruction of old ones. This high number of craters indicates that Oberon has the most ancient surface among Uranus's moons. [31] The crater diameters range up to 206 kilometers for the largest known crater, [31] Hamlet. [32] Many large craters are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice. [8] The largest craters, Hamlet, Othello and Macbeth, have floors made of a very dark material deposited after their formation. [31] A peak with a height of about 11 km was observed in some Voyager images near the south-eastern limb of Oberon, [33] which may be the central peak of a large impact basin with a diameter of about 375 km. [33] Oberon's surface is intersected by a system of canyons, which, however, are less widespread than those found on Titania. [8] The canyons' sides are probably scarps produced by normal faults [lower-alpha 7] which can be either old or fresh: the latter transect the bright deposits of some large craters, indicating that they formed later. [34] The most prominent Oberonian canyon is Mommur Chasma. [35]

The geology of Oberon was influenced by two competing forces: impact crater formation and endogenic resurfacing. [34] The former acted over the moon's entire history and is primarily responsible for its present-day appearance. [31] The latter processes were active for a period following the moon's formation. The endogenic processes were mainly tectonic in nature and led to the formation of the canyons, which are actually giant cracks in the ice crust. [34] The canyons obliterated parts of the older surface. [34] The cracking of the crust was caused by the expansion of Oberon by about 0.5%, [34] which occurred in two phases corresponding to the old and young canyons.

The nature of the dark patches, which mainly occur on the leading hemisphere and inside craters, is not known. Some scientists hypothesized that they are of cryovolcanic origin (analogs of lunar maria), [31] while others think that the impacts excavated dark material buried beneath the pure ice (crust). [28] In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior. [28]

Named surface features on Oberon [36]
FeatureNamed afterTypeLength (diameter), kmCoordinates
Mommur Chasma Mommur, French folklore Chasma 537
Antony Mark Antony Crater 47
Caesar Julius Caesar 76
Coriolanus Coriolanus 120
Falstaff Falstaff 124
Hamlet Hamlet 206
Lear King Lear 126
MacBeth Macbeth 203
Othello Othello 114
Romeo Romeo 159
Surface features on Oberon are named for male characters and places associated with Shakespeare's works. [37]

## Origin and evolution

Oberon 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. [38] The precise composition of the subnebula is not known; however, the relatively high density of Oberon and other Uranian moons compared to the moons of Saturn indicates that it may have been relatively water-poor. [lower-alpha 8] [8] Significant amounts of carbon and nitrogen may have been present in the form of carbon monoxide and N2 instead of methane and ammonia. [38] 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. [8]

Oberon's accretion probably lasted for several thousand years. [38] The impacts that accompanied accretion caused heating of the moon's outer layer. [39] The maximum temperature of around 230 K was reached at the depth of about 60 km. [39] After the end of formation, the subsurface layer cooled, while the interior of Oberon heated due to decay of radioactive elements present in its rocks. [8] The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust leading to cracking. The present-day system of canyons may be a result of this process, which lasted for about 200 million years, [40] implying that any endogenous activity from this cause ceased billions of years ago. [8]

The initial accretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice [40] if some antifreeze like ammonia (in the form of ammonia hydrate) or some salt was present. [26] Further 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. [26] The eutectic temperature of this mixture is 176 K. [26] If the temperature dropped below this value the ocean would have frozen by now. Freezing of the water would have led to expansion of the interior, which may have also contributed to the formation of canyon-like graben. [31] Still, present knowledge of the evolution of Oberon is very limited.

## Exploration

So far the only close-up images of Oberon have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest approach of Voyager 2 to Oberon was 470,600 km, [41] the best images of this moon have spatial resolution of about 6 km. [31] The images cover about 40% of the surface, but only 25% of the surface was imaged with a resolution that allows geological mapping. [31] At the time of the flyby the southern hemisphere of Oberon was pointed towards the Sun, so the dark northern hemisphere could not be studied. [8] No other spacecraft has ever visited the Uranian system.

## Notes

1. A number of bright-rayed craters are visible. Hamlet, just below center, has dark material on its floor; to its upper left is smaller Othello. Above the limb at lower left rises an 11 km high mountain, probably the central peak of another crater. Mommur Chasma runs along the terminator at upper right.
1. Surface area derived from the radius r: ${\displaystyle 4\pi r^{2}}$.
2. Volume v derived from the radius r: ${\displaystyle 4\pi r^{3}/3}$.
3. Surface gravity derived from the mass m, the gravitational constant G and the radius r: ${\displaystyle Gm/r^{2}}$.
4. Escape velocity derived from the mass m, the gravitational constant G and the radius r: 2Gm/r.
5. The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.
6. The eight moons more massive than Oberon are Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, Triton, and Titania. [25]
7. Some canyons on Oberon are graben. [31]
8. For instance, Tethys, a Saturnian moon, has a density of 0.97 g/cm³, which means that it contains more than 90% water. [10]

## Related Research Articles

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.

Nereid, or Neptune II, is the third-largest moon of Neptune. Of all known moons in the Solar System, it has the most eccentric orbit. It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949.

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.

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. It was discovered in 1672 by Giovanni Domenico Cassini.

Tethys, or Saturn III, is a mid-sized moon of Saturn about 1,060 km (660 mi) across. It was discovered by G. D. Cassini in 1684 and is named after the titan Tethys of Greek mythology.

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.

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.

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 planet Neptune has 14 known moons, which are named for minor water deities in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, 17 days after the discovery of Neptune itself; over a century passed before the discovery of the second natural satellite, Nereid. Neptune's outermost moon Neso, which has an orbital period of about 26 Julian years, orbits farther from its planet than any other moon in the Solar System.

The naming of moons has been the responsibility of the International Astronomical Union's committee for Planetary System Nomenclature since 1973. That committee is known today as the Working Group for Planetary System Nomenclature (WGPSN).

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.

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Two moons in the Solar System are larger than the planet Mercury : Ganymede and Titan, and seven are larger and more massive than the dwarf planet Pluto.

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