Oberon , also designated , is the outermost and second-largest major moon of the planet

Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...

. It is the second-most massive of the Uranian moons, and the tenth-largest

moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. Discovered by

William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

in 1787, Oberon is named after the mythical king of the fairies who appears as a character in

Shakespeare William Shakespeare ( 23 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's natio ...

's ''

A Midsummer Night's Dream ''A Midsummer Night's Dream'' is a Comedy (drama), comedy play written by William Shakespeare in about 1595 or 1596. The play is set in Athens, and consists of several subplots that revolve around the marriage of Theseus and Hippolyta. One s ...

''. Its orbit lies partially outside Uranus's

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...

. 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 Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

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 crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

s reaching 210 km in diameter. Oberon possesses a system of ''

chasma In planetary nomenclature, a chasma (''plural'': chasmata ) is a deep, elongated, steep-sided Depression (geology), depression. As of 2020, the International Astronomical Union, IAU has named 122 such features in the Solar System, on Venus (63), ...

ta'' (

graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

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 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...

'' 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

on January 11, 1787; on the same day, he discovered Uranus's largest moon, Titania. He later reported the discoveries of four more satellites, although they were subsequently revealed as spurious. For nearly fifty years following their discovery, Titania and Oberon would not be observed by any instrument other than William Herschel's, although the moon can be seen from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

with a present-day high-end amateur telescope. All of the moons of Uranus are named after characters created by

William Shakespeare William Shakespeare ( 23 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...

Alexander Pope Alexander Pope (21 May 1688 Old Style and New Style dates, O.S. – 30 May 1744) was an English poet, translator, and satirist of the Age of Enlightenment, Enlightenment era who is considered one of the most prominent English poets of the early ...

. The name Oberon was derived from

Oberon Oberon () is a king of the fairy, fairies in Middle Ages, medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania ...

, the King of the Fairies in ''

''. 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 William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer. who had discovered the other two moons, Ariel and

Umbriel Umbriel () is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell at the same time as neighboring moon Ariel. It was named after a character in Alexander Pope's 1712 poem '' The Rape of the Lock''. Umb ...

, the year before. It is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission. The adjectival form of the name is ''Oberonian,'' . Oberon was initially referred to as "the second satellite of Uranus" and in 1848 was given the designation ' by Lassell, although he sometimes used Herschel's numbering (where Titania and Oberon are II and IV). In 1851, Lassell eventually numbered all four known satellites in order of their distance from the planet by

Roman numerals Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, eac ...

, and since then Oberon has been designated '. 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 A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...

symbols, proposed an O (the initial of Oberon) combined with the low globe of

Jérôme Lalande Joseph Jérôme Lefrançois de Lalande (; 11 July 1732 – 4April 1807) was a French astronomer, freemason and writer. He is known for having estimated a precise value of the astronomical unit (the distance from the Earth to the Sun) using measu ...

's Uranus symbol as the symbol of Oberon (). This symbol is not widely used.

Orbit

Oberon orbits Uranus at a distance of about 584,000 km, being the farthest from the planet among its five major moons. Oberon's orbit has a small

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...

and

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

relative to the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

of Uranus. 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. Oberon spends a significant part of its orbit outside the Uranian

. As a result, its surface is directly struck by the

solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...

. 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. This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below). 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 Pole or poles may refer to: People *Poles (people), another term for Polish people, from the country of Poland * Pole (surname), including a list of people with the name * Pole (musician) (Stefan Betke, born 1967), German electronic music artist ...

spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the

zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...

over one of the poles at each

solstice A solstice is the time when the Sun reaches its most northerly or southerly sun path, excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries ...

. 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 A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...

and its equatorial plane intersects the Earth, mutual

occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks f ...

s 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.

Composition and internal structure

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. It is the tenth-largest moon by size however, since Rhea, the second-largest moon of

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

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. 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. The latter could be made of rock and

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

aceous material including heavy

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s. The presence of water ice is supported by spectroscopic observations, which have revealed

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

water ice on the surface of the moon. Water ice

absorption band In spectroscopy, an absorption band is a range of wavelengths, frequency, frequencies or energies in the electromagnetic spectrum that are characteristic of a particular transition from initial to final state in a substance. According to quantum ...

s 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. 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.

Meteorite impact An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal eff ...

s tend to sputter (knock out) ice from the surface, leaving dark non-ice material behind. The dark material itself may have formed as a result of radiation processing of

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

clathrates or radiation darkening of other organic compounds. Oberon may be differentiated into a rocky core surrounded by an icy mantle. 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). The current state of the icy mantle is unclear. If the ice contains enough

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

or other

antifreeze An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allow ...

, 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). 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.

Surface features and geology

Oberon is the second-darkest large moon of Uranus after

. 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 The Bond albedo (also called spheric albedo, planetary albedo, and bolometric albedo), named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic radi ...

of about 14%. Its surface is generally red in color, except for fresh impact deposits, which are neutral or slightly blue. 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. 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. 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 In Greek mythology, Iapetus (; ; ), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other sources. Iapetus was linked ...

. Two primary classes of geological features dominate Oberon's surface:

s and

ta ('canyons'—deep, elongated, steep-sided depressions which would probably be described as

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges produced by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear ...

s or

escarpment An escarpment is a steep slope or long cliff that forms as a result of faulting or erosion and separates two relatively level areas having different elevations. Due to the similarity, the term '' scarp'' may mistakenly be incorrectly used inte ...

s if on Earth). 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. The crater diameters range up to 206 kilometers for the largest known crater,

Hamlet ''The Tragedy of Hamlet, Prince of Denmark'', often shortened to ''Hamlet'' (), is a Shakespearean tragedy, tragedy written by William Shakespeare sometime between 1599 and 1601. It is Shakespeare's longest play. Set in Denmark, the play (the ...

. Many large craters are surrounded by bright impact ejecta ( rays) consisting of relatively fresh ice. The largest craters, Hamlet, Othello and Macbeth, have floors made of a very dark material deposited after their formation. A peak with a height of about 11 km was observed in some ''Voyager'' images near the south-eastern limb of Oberon, which may be the central peak of a large impact basin with a diameter of about 375 km. Oberon's surface is intersected by a system of canyons, which, however, are less widespread than those found on Titania. The canyons' sides are probably scarps produced by

normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

s which can be either old or fresh: the latter transect the bright deposits of some large craters, indicating that they formed later. The most prominent Oberonian canyon is Mommur Chasma. The geology of Oberon was influenced by two competing forces:

formation and endogenic resurfacing. The former acted over the moon's entire history and is primarily responsible for its present-day appearance. The latter processes were active for a period following the moon's formation. The endogenic processes were mainly

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

in nature and led to the formation of the canyons, which are actually giant cracks in the ice crust. The canyons obliterated parts of the older surface. The cracking of the crust was caused by the expansion of Oberon by about 0.5%, 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), while others think that the impacts excavated dark material buried beneath the pure ice ( crust). In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior.

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. 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. Significant amounts of

and

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

may have been present in the form of

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

and N₂ instead of methane and

. 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. Oberon's accretion probably lasted for several thousand years. The impacts that accompanied accretion caused heating of the moon's outer layer. The maximum temperature of around 230 K was reached at the depth of about 60 km. 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. 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, implying that any endogenous activity from this cause ceased billions of years ago. The initial accretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice if some antifreeze like ammonia (in the form of ammonia hydrate) or some

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

was present. 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. The eutectic temperature of this mixture is 176 K. 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

. 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.

Exploration

So far the only close-up images of Oberon have been from the ''

'' 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, the best images of this moon have spatial resolution of about 6 km. The images cover about 40% of the surface, but only 25% of the surface was imaged with a resolution that allows geological mapping. At the time of the flyby the southern hemisphere of Oberon was pointed towards the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, so the dark northern hemisphere could not be studied. No other spacecraft has ever visited the Uranian system.

Notes

References

External links

* * * * * {{Authority control 17870111 Discoveries by William Herschel Moons with a prograde orbit