Oberon (moon)

Template:Short description {{#invoke:other uses|otheruses}} Template:Featured article {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Oberon | image = {{#invoke:InfoboxImage|InfoboxImage|image=Oberon in true color by Kevin M. Gill.jpg|upright={{#if:||1.1}}|alt=}} | caption = Oberon, as imaged by Voyager 2, January 1986. A number of bright-rayed craters are visible, with the largest, Hamlet, at center. At the lower left limb rises an 11 km high mountain. | headerstyle = {{#if:|background-color:|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

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| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}

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| header60 = Proper orbital elements

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({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsec Template:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsec Template:\yr}}

| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = | label72 = Template:Longitem | data72 = | label73 = Template:Longitem | data73 = Template:Val (Template:Val)<ref name="Thomas 1988" /> | label74 = Template:Longitem | data74 = | label75 = Template:Longitem | data75 = | label76 = Flattening | data76 = | label77 = Circumference | data77 = | label78 = Template:Longitem | data78 = Template:Val Template:Efn | label79 = Volume | data79 = Template:Val Template:Efn | label80 = Mass | data80 = Template:Val<ref>Jacobson (2023), as cited in French et al. (2024)<ref name="French et al. 2024"/></ref> | label81 = Template:Longitem | data81 = Template:Val (calculated) | label82 = Template:Longitem | data82 = Template:Gr m/s² Template:Efn | label83 = Template:Longitem | data83 = | label84 = Template:Longitem | data84 = Template:V2 km/sTemplate:Efn | label85 = Template:Longitem | data85 = presumed synchronous<ref name="Smith Soderblom et al. 1986" /> | label86 = Template:Longitem | data86 = | label87 = Template:Longitem | data87 = | label88 = Template:Longitem | data88 = | label89 = Template:Longitem | data89 = | label90 = Template:Longitem | data90 = | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if: |Template:Longitem |Albedo}} | data93 = Template:Plainlist | label94 = Temperature | data94 = 70–80 K<ref name="Grundy Young et al. 2006" />

| data100 = {{#if:|

Surface temp.	min	mean	max
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| header110 = Atmosphere

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Oberon Template:IPAc-en, also designated Template:Nowrap, is the outermost and second-largest major moon of the planet Uranus. It is the second-most massive of the Uranian moons, and the tenth-largest 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.

Oberon likely 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 namingEdit

Oberon was discovered by William Herschel on January 11, 1787; on the same day, he discovered Uranus's largest moon, Titania.<ref name="Herschel 1787" /><ref name="Herschel 1788" /> He later reported the discoveries of four more satellites,<ref name="Herschel 1798" /> although they were subsequently revealed as spurious.<ref name="Struve 1848" /> For nearly fifty years following their discovery, Titania and Oberon would not be observed by any instrument other than William Herschel's,<ref name="Herschel 1834" /> although the moon can be seen from Earth with a present-day high-end amateur telescope.<ref name="Newton Teece 1995" />

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.<ref name="Kuiper 1949" /> 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,<ref name="Lassell 1852" /> who had discovered the other two moons, Ariel and Umbriel, the year before.<ref name="Lassell 1851" /> It is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission.<ref name=podcast>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The adjectival form of the name is Oberonian, Template:IPAc-en.<ref name="Shakespeare" />

Oberon was initially referred to as "the second satellite of Uranus" and in 1848 was given the designation Template:Nowrap by Lassell,<ref name="Lassell 1848" /> although he sometimes used Herschel's numbering (where Titania and Oberon are II and IV).<ref name="Lassell 1850" /> 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 Template:Nowrap.<ref name="Lassell, letter 1851" />

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed an O (the initial of Oberon) combined with the low globe of Jérôme Lalande's Uranus symbol as the symbol of Oberon (File:Oberon symbol (fixed width).svg). This symbol is not widely used.<ref name=moons>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

OrbitEdit

Oberon orbits Uranus at a distance of about 584,000 km, being the farthest from the planet among its five major moons.Template:Efn Oberon's orbit has a small orbital eccentricity and inclination relative to the equator of Uranus.<ref name="orbit" /> Its orbital period is around 13.5 days, coincident with its rotational period. In other words, Oberon is tidally locked, with one face always pointing toward the planet.<ref name="Smith Soderblom et al. 1986" /> Oberon spends a significant part of its orbit outside the Uranian magnetosphere.<ref name="Ness Acuña et al. 1986" /> As a result, its surface is directly struck by the solar wind.<ref name="Grundy Young et al. 2006" /> 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.<ref name="Ness Acuña et al. 1986" /> This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).<ref name="Grundy Young et al. 2006" />

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.<ref name="Grundy Young et al. 2006" /> 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.<ref name="Hidas Christou et al. 2008" />

Composition and internal structureEdit

File:Oberon Earth Moon Comparison.png

Size comparison of Earth, the Moon, and Oberon.

Oberon is the second-largest and second-most massive of the Uranian moons after Titania, and the ninth-most massive moon in the Solar System.Template:Efn It is the tenth-largest moon by size however, since Rhea, the second-largest moon of Saturn and the ninth-largest moon, is nearly the same size as Oberon although it is about 0.4% larger, despite Oberon having more mass than Rhea.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Oberon's density of 1.68 g/cm³, 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.<ref name="Hussmann Sohl et al. 2006" /> The latter could be made of rock and carbonaceous material including heavy organic compounds.<ref name="Smith Soderblom et al. 1986" /> The presence of water ice is supported by spectroscopic observations, which have revealed crystalline water ice on the surface of the moon.<ref name="Grundy Young et al. 2006" /> 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.<ref name="Grundy Young et al. 2006" /> 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.<ref name="Grundy Young et al. 2006" /> Meteorite impacts tend to sputter (knock out) ice from the surface, leaving dark non-ice material behind.<ref name="Grundy Young et al. 2006" /> The dark material itself may have formed as a result of radiation processing of methane clathrates or radiation darkening of other organic compounds.<ref name="Smith Soderblom et al. 1986" /><ref name="Bell McCord 1991" />

Oberon may be differentiated into a rocky core surrounded by an icy mantle.<ref name="Hussmann Sohl et al. 2006" /> 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).<ref name="Hussmann Sohl et al. 2006" /> 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 (close to the water–ammonia eutectic temperature of 176 K).<ref name="Hussmann Sohl et al. 2006" /> However, the internal structure of Oberon depends heavily on its thermal history, which is poorly known at present. Albeit more recent publications seem to be in favour of active subterranean oceans throughout the larger moons of Uranus.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Surface features and geologyEdit

File:Oberon-NASA names en.png

A photo of Oberon. All named surface features are captioned.

Oberon is the second-darkest large moon of Uranus after Umbriel.<ref name="Karkoschka 2001, Hubble" /> 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%.<ref name="Karkoschka 2001, Hubble" /> Its surface is generally red in color, except for fresh impact deposits, which are neutral or slightly blue.<ref name="Helfenstein Hillier et al. 1990" /> 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.<ref name="Bell McCord 1991" /> 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.<ref name="Bell McCord 1991" /> 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, similar to Saturn's moon Iapetus.<ref name="Buratti Mosher 1991" />

Two primary classes of geological features dominate Oberon's surface: impact craters and chasmata ('canyons'—deep, elongated, steep-sided depressions<ref name="USGS-Nomenclature" /> which would probably be described as rift valleys or escarpments if on Earth).<ref name="Smith Soderblom et al. 1986" /> 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.<ref name="Plescia1987" /> The crater diameters range up to 206 kilometers for the largest known crater,<ref name="Plescia1987" /> Hamlet.<ref name="USGS: Uranus: Oberon: Hamlet" /> Many large craters are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice.<ref name="Smith Soderblom et al. 1986" /> The largest craters, Hamlet, Othello and Macbeth, have floors made of a very dark material deposited after their formation.<ref name="Plescia1987" /> A peak with a height of about 11 km was observed in some Voyager images near the south-eastern limb of Oberon,<ref name="Moore Schenk et al. 2004" /> which may be the central peak of a large impact basin with a diameter of about 375 km.<ref name="Moore Schenk et al. 2004" /> Oberon's surface is intersected by a system of canyons, which, however, are less widespread than those found on Titania.<ref name="Smith Soderblom et al. 1986" /> The canyons' sides are probably scarps produced by normal faults Template:Efn which can be either old or fresh: the latter transect the bright deposits of some large craters, indicating that they formed later.<ref name="Croft 1989" /> The most prominent Oberonian canyon is Mommur Chasma.<ref name="Mommur" />

The geology of Oberon was influenced by two competing forces: impact crater formation and endogenic resurfacing.<ref name="Croft 1989" /> The former acted over the moon's entire history and is primarily responsible for its present-day appearance.<ref name="Plescia1987" /> 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.<ref name="Croft 1989" /> The canyons obliterated parts of the older surface.<ref name="Croft 1989" /> The cracking of the crust was caused by the expansion of Oberon by about 0.5%,<ref name="Croft 1989" /> 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),<ref name="Plescia1987" /> while others think that the impacts excavated dark material buried beneath the pure ice (crust).<ref name="Helfenstein Hillier et al. 1990" /> In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior.<ref name="Helfenstein Hillier et al. 1990" />

Named surface features on Oberon<ref name="usgs" />
Feature	Named after	Type	Length (diameter), km	Coordinates
Mommur Chasma	Mommur, French folklore	Chasma	537	Template:Coord
Antony	Mark Antony	Crater	47	Template:Coord
Caesar	Julius Caesar		76	Template:Coord
Coriolanus	Coriolanus		120	Template:Coord
Falstaff	Falstaff		124	Template:Coord
Hamlet	Hamlet		206	Template:Coord
Lear	King Lear		126	Template:Coord
MacBeth	Macbeth		203	Template:Coord
Othello	Othello		114	Template:Coord
Romeo	Romeo		159	Template:Coord
Surface features on Oberon are named for male characters and places associated with Shakespeare's works.<ref name="Strobell Masursky 1987" />

Origin and evolutionEdit

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.<ref name="Mousis 2004" /> 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.Template:Efn<ref name="Smith Soderblom et al. 1986" /> Significant amounts of carbon and nitrogen may have been present in the form of carbon monoxide and N₂ instead of methane and ammonia.<ref name="Mousis 2004" /> 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.<ref name="Smith Soderblom et al. 1986" />

Oberon's accretion probably lasted for several thousand years.<ref name="Mousis 2004" /> The impacts that accompanied accretion caused heating of the moon's outer layer.<ref name="Squyres Reynolds et al. 1988" /> The maximum temperature of around 230 K was reached at the depth of about 60 km.<ref name="Squyres Reynolds et al. 1988" /> 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.<ref name="Smith Soderblom et al. 1986" /> 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,<ref name="Hillier & Squyres 1991" /> implying that any endogenous activity from this cause ceased billions of years ago.<ref name="Smith Soderblom et al. 1986" />

The initial accretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice<ref name="Hillier & Squyres 1991" /> if some antifreeze like ammonia (in the form of ammonia hydrate) or some salt was present.<ref name="Hussmann Sohl et al. 2006" /> 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.<ref name="Hussmann Sohl et al. 2006" /> The eutectic temperature of this mixture is 176 K.<ref name="Hussmann Sohl et al. 2006" /> 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.<ref name="Plescia1987" /> Still, present knowledge of the evolution of Oberon is very limited. Although recent analysis concluded that its more likely that the larger moons of Uranus having active subsurface oceans.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

ExplorationEdit

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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,<ref name="Stone 1987" /> the best images of this moon have spatial resolution of about 6 km.<ref name="Plescia1987" /> The images cover about 40% of the surface, but only 25% of the surface was imaged with a resolution that allows geological mapping.<ref name="Plescia1987" /> 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.<ref name="Smith Soderblom et al. 1986" /> No other spacecraft has ever visited the Uranian system.