Oberon (moon)

Oberon (moon)

Overview
Oberon also designated , is the outermost major moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of the planet Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. Discovered by William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

 in 1787, Oberon is named after the mythical king of the fairies
Oberon
Oberon is a legendary king of the fairies.Oberon may also refer to:-People:* Merle Oberon , British actress* Oberon Zell-Ravenheart , Neopagan activist-Media and entertainment:* Oberon...

 who appears as a character in Shakespeare's A Midsummer Night's Dream
A Midsummer Night's Dream
A Midsummer Night's Dream is a play that was written by William Shakespeare. It is believed to have been written between 1590 and 1596. It portrays the events surrounding the marriage of the Duke of Athens, Theseus, and the Queen of the Amazons, Hippolyta...

. Its orbit lies partially outside Uranus's magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

.

It is likely that Oberon formed from the accretion disk that surrounded Uranus just after the planet's formation.
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Encyclopedia
Oberon also designated , is the outermost major moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of the planet Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. Discovered by William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

 in 1787, Oberon is named after the mythical king of the fairies
Oberon
Oberon is a legendary king of the fairies.Oberon may also refer to:-People:* Merle Oberon , British actress* Oberon Zell-Ravenheart , Neopagan activist-Media and entertainment:* Oberon...

 who appears as a character in Shakespeare's A Midsummer Night's Dream
A Midsummer Night's Dream
A Midsummer Night's Dream is a play that was written by William Shakespeare. It is believed to have been written between 1590 and 1596. It portrays the events surrounding the marriage of the Duke of Athens, Theseus, and the Queen of the Amazons, Hippolyta...

. Its orbit lies partially outside Uranus's magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

.

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
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...

 and rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

, and is probably differentiated into a rocky core and an icy mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

. 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
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s reaching 210 km in diameter. Oberon possesses a system of chasma
Chasma
Chasma is a term used in astrogeology to refer to "a deep, elongated, steep-sided depression". The plural is chasmata. An example is Eos Chasma on Mars. Below are images of some of the major chasmata of Mars...

ta
(graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....

 or scarps
Fault scarp
A fault scarp is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults. They are exhibited either by differential movement and subsequent erosion along an old inactive geologic fault , or by a movement on a recent active fault...

) 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
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

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
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

 on January 11, 1787; on the same day he discovered Uranus's largest moon, Titania
Titania (moon)
Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1578 km. Discovered by William Herschel in 1787, Titania is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream...

. 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
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 with a present-day high-end amateur telescope.

All of the moons of Uranus are named after characters created by William Shakespeare
William Shakespeare
William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

 or Alexander Pope
Alexander Pope
Alexander Pope was an 18th-century English poet, best known for his satirical verse and for his translation of Homer. He is the third-most frequently quoted writer in The Oxford Dictionary of Quotations, after Shakespeare and Tennyson...

. The name Oberon was derived from Oberon, the King of the Fairies in A Midsummer Night's Dream
A Midsummer Night's Dream
A Midsummer Night's Dream is a play that was written by William Shakespeare. It is believed to have been written between 1590 and 1596. It portrays the events surrounding the marriage of the Duke of Athens, Theseus, and the Queen of the Amazons, Hippolyta...

. The names of all four satellites of Uranus then known were suggested by Herschel's son John
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

 in 1852, at the request of William Lassell
William Lassell
William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...

, who had discovered the other two moons, Ariel
Ariel (moon)
Ariel is the brightest and 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....

 and Umbriel
Umbriel (moon)
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...

, the year before. The adjectival form of the name is Oberonian, ˌ.

Oberon was initially referred to as "the second satellite of Uranus", and in 1848 was given the designation by William Lassell, although he sometimes used William 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
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...

, and since then Oberon has been designated .

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
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 and inclination relative to the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

 of Uranus. Its orbital period is around 13.5 days, coincident with its rotational period. In other words, Oberon is a synchronous satellite
Synchronous orbit
A synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...

, tidally locked, with one face always pointing toward the planet. Oberon spends a significant part of its orbit outside the Uranian magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

. As a result, its surface is directly struck by the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

. 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 spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

 over one of the poles at each solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

. The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was unilluminated. Once every 42 years, when Uranus has an equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

 and its equatorial plane intersects the Earth, mutual occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

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 most massive of the Uranian moons after Titania
Titania (moon)
Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1578 km. Discovered by William Herschel in 1787, Titania is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream...

, and the ninth most massive moon in the Solar System. Oberon's density of 1.63 g/cm³, which is higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 and a dense non-ice component. The latter could be made of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 and carbonaceous
Carbonaceous
Carbonaceous is the defining attribute of a substance rich in carbon. Particularly, carbonaceous hydrocarbons are very unsaturated, high-molecular-weight hydrocarbons, having an elevated carbon:hydrogen ratio....

 material including heavy organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s. The presence of water ice is supported by spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption band
Absorption band
An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite a particular transition in a substance...

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
Impact gardening
Impact gardening is the process by which impact events stir the outermost crusts of moons and other celestial objects with no atmospheres. In the particular case of the Moon, this is more often known as lunar gardening...

 (the creation of soil via impacts) of the surface, which is stronger on the leading hemisphere. Meteorite impacts 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
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 clathrates
Clathrate hydrate
Clathrate hydrates are crystalline water-based solids physically resembling ice, in which small non-polar molecules or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded water molecules...

 or radiation darkening of other organic compounds.

Oberon may be differentiated into a rocky core surrounded by an icy mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

. 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
KBAR
KBAR may refer to:* KBAR-FM, a radio station licensed to Victoria, Texas, United States* KBAR , a radio station licensed to Burley, Idaho, United States...

). The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze
Antifreeze
Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....

, 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. 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
Umbriel (moon)
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...

. Its surface shows a strong opposition surge
Opposition surge
The opposition surge is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer...

: its reflectivity decreases from 31% at a phase angle of 0° (geometrical albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

) to 22% at an angle of about 1°. Oberon has a low Bond albedo
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...

 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
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...

 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 satellite
Irregular satellite
In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....

s, which would occur predominately on the leading hemisphere.

Scientists have recognized two classes of geological feature on Oberon: craters
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 and chasma
Chasma
Chasma is a term used in astrogeology to refer to "a deep, elongated, steep-sided depression". The plural is chasmata. An example is Eos Chasma on Mars. Below are images of some of the major chasmata of Mars...

ta ('canyons'—deep, elongated, steep-sided depressions which would probably be described as rift valley
Rift valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion...

s or escarpment
Escarpment
An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...

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
Hamlet (crater)
Hamlet is the largest crater on the known part of the surface of Uranus' moon Oberon. It has diameter of about 206 km and is named after the title character of the play Hamlet, by William Shakespeare. The crater has a dark floor and is surrounded by a system of bright rays, which are ice...

. Many large craters are surrounded by bright impact ejecta (rays
Ray system
A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking a bit like many thin spokes coming from the hub of a wheel. The rays can extend for lengths up to several times the diameter of their originating crater, and are often accompanied by...

) 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
Fault scarp
A fault scarp is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults. They are exhibited either by differential movement and subsequent erosion along an old inactive geologic fault , or by a movement on a recent active fault...

 produced by normal faults which can be either old or fresh: the latter transect
Transect
A transect is a path along which one records and counts occurrences of the phenomena of study .It requires an observer to move along a fixed path and to count occurrences along the path and, at the same time, obtain the distance of the object from the path...

 the bright deposits of some large craters, indicating that they formed later. The most prominent Oberonian canyon is Mommur Chasma
Mommur Chasma
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...

.

The geology of Oberon was influenced by two competing forces: impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 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 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
Cryovolcano
A cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...

 origin (analogs of lunar maria), while others think that the impacts excavated dark material buried beneath the pure ice (crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

). In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior.
Named surface features on Oberon
Feature Named after Type Length (diameter), km Coordinates
Mommur Chasma
Mommur Chasma
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...

Mommur
Mommur
In the story of Huon of Bordeaux, Mommur is an enchanted forest that is ruled by Oberon, king of the fairies. Variously, the name is also applied to the capital city of Oberon, and to the fairy land in which the city is located. The wood in Shakespeare's A Midsummer Night's Dream may be based on...

, French folklore
French folklore
French folklore encompasses the fables, folklore and fairy tales and legends of the Gauls, Franks, Normans, Bretons, Occitans, and other peoples living in France.-Folklore from the Middle Ages:...

Chasma
Chasma
Chasma is a term used in astrogeology to refer to "a deep, elongated, steep-sided depression". The plural is chasmata. An example is Eos Chasma on Mars. Below are images of some of the major chasmata of Mars...

537 16.3°N 323.5°W
Antony Mark Antony
Antony and Cleopatra
Antony and Cleopatra is a tragedy by William Shakespeare, believed to have been written sometime between 1603 and 1607. It was first printed in the First Folio of 1623. The plot is based on Thomas North's translation of Plutarch's Lives and follows the relationship between Cleopatra and Mark Antony...

Crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

47 27.5°N 65.4°W
Caesar Julius Caesar
Julius Caesar (play)
The Tragedy of Julius Caesar, also known simply as Julius Caesar, is a tragedy by William Shakespeare, believed to have been written in 1599. It portrays the 44 BC conspiracy against...

76 26.6°N 61.1°W
Coriolanus Coriolanus
Coriolanus (play)
Coriolanus is a tragedy by William Shakespeare, believed to have been written between 1605 and 1608. The play is based on the life of the legendary Roman leader, Gaius Marcius Coriolanus.-Characters:*Caius Martius, later surnamed Coriolanus...

120 11.4°N 345.2°W
Falstaff Falstaff
Falstaff
Sir John Falstaff is a fictional character who appears in three plays by William Shakespeare. In the two Henry IV plays, he is a companion to Prince Hal, the future King Henry V. A fat, vain, boastful, and cowardly knight, Falstaff leads the apparently wayward Prince Hal into trouble, and is...

124 22.1°N 19.0°W
Hamlet
Hamlet (crater)
Hamlet is the largest crater on the known part of the surface of Uranus' moon Oberon. It has diameter of about 206 km and is named after the title character of the play Hamlet, by William Shakespeare. The crater has a dark floor and is surrounded by a system of bright rays, which are ice...

Hamlet
Hamlet
The Tragical History of Hamlet, Prince of Denmark, or more simply Hamlet, is a tragedy by William Shakespeare, believed to have been written between 1599 and 1601...

206 46.1°N 44.4°W
Lear King Lear
King Lear
King Lear is a tragedy by William Shakespeare. The title character descends into madness after foolishly disposing of his estate between two of his three daughters based on their flattery, bringing tragic consequences for all. The play is based on the legend of Leir of Britain, a mythological...

126 5.4°N 31.5°W
MacBeth Macbeth
Macbeth
The Tragedy of Macbeth is a play by William Shakespeare about a regicide and its aftermath. It is Shakespeare's shortest tragedy and is believed to have been written sometime between 1603 and 1607...

203 58.4°N 112.5°W
Othello Othello
Othello
The Tragedy of Othello, the Moor of Venice is a tragedy by William Shakespeare, believed to have been written in approximately 1603, and based on the Italian short story "Un Capitano Moro" by Cinthio, a disciple of Boccaccio, first published in 1565...

114 66.0°N 42.9°W
Romeo Romeo
Romeo and Juliet
Romeo and Juliet is a tragedy written early in the career of playwright William Shakespeare about two young star-crossed lovers whose deaths ultimately unite their feuding families. It was among Shakespeare's most popular archetypal stories of young, teenage lovers.Romeo and Juliet belongs to a...

159 28.7°N 89.4°W
Surface features on Oberon are named for characters and places associated with Shakespeare's works.

Origin and evolution


Oberon is thought to have formed from an accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...

 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 carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 may have been present in the form of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 and N2 instead of methane and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

. 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
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...

 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
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....

) or some salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 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 graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....

. 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
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, so the dark northern hemisphere could not be studied. No other spacecraft has ever visited the Uranian system, and no mission to this planet is planned in the foreseeable future.

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