Oberon (moon)

Oberon
	The best Voyager 2 image of Oberon obtained on January 24, 1986
Discovery
Discovered by	William Herschel
Discovery date	January 11, 1787
Designations
Alternative names	Uranus IV
Adjectives	Oberonian
Orbital characteristics
Semi-major axis	583 520 km
Eccentricity	0.0014
Orbital period (sidereal)	13.463 234 d
Inclination	0.058° (to Uranus's equator)
Satellite of	Uranus
Physical characteristics
Mean radius	762 ± 4 km (0.1194 Earths)
Surface area	7 285 000 km²
Volume	1 849 000 000 km³
Mass	3.014 ± 0.075 kg (5.046 Earths)
Mean density	1.63 ± 0.05 g/cm³
Surface gravity	0.346 m/s2
Escape velocity	0.727 km/s
Synodic rotation period	presumed synchronous
Albedo	0.31 (geometrical),; 0.14 (bond)
Temperature	~61 K
Apparent magnitude	14.1

Oberon (Template:PronEng),^{[note 5]} also designated Uranus IV, is the outermost major moon of the planet Uranus. It is the second largest and second most massive of Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after a character in Shakespeare's A Midsummer Night's Dream. Its orbit lies partially outside Uranus's magnetosphere.

Oberon consists of approximately equal amounts of ice and rock, and is likely differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the core/mantle boundary. The surface of Oberon, which is dark 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 canyons (grabens) formed as a result of the expansion of its interior during its early evolution. This moon probably formed from the accretion disk that surrounded Uranus just after the planet's formation.

As of 2008, the Uranian system has been studied up close only once: by the spacecraft Voyager 2 in January 1986. It took several images of Oberon, which allowed mapping of about 40% of the moon’s surface.

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]^[7] He later reported the discoveries of four more satellites,^[8] although they were subsequently revealed as spurious.^[9] For nearly fifty years following their discovery, Titania and Oberon would not be observed by any instrument other than William Herschel's,^[10] although the moon can be seen from Earth with a present-day high end amateur telescope.^[6]

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.^[11] 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,^[12] who had discovered the other two moons, Ariel and Umbriel, the year before.^[13] The adjectival form of the name is Oberonian, Template:Pron-en.

Oberon was initially referred to as "the second satellite of Uranus", and in 1848 was given the designation Uranus II by William Lassell,^[14] although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV).^[15] 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.^[16]

Orbit

Oberon orbits Uranus at the distance of about 584,000 km, being the furthest from the planet among its five major moons.^{[note 6]} Oberon's orbit has a small orbital eccentricity and inclination (relative to the equator of Uranus).^[2] 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.^[4] Oberon spends a significant part of its orbit outside the Uranian magnetosphere. As a result, its surface is directly struck by the solar wind.^[17] 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.^[18] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).^[17] 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 hemispheres spend 42 years in a complete darkness, and another 42 years in continuous sunlight.^[17] 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 last for about six minutes, was observed on May 4, 2007, when Oberon occulted Umbriel.^[19]

Composition and internal structure

Oberon is the largest and most massive of Uranian moons after Titania, and the ninth most massive moon in the Solar System.^{[note 7]} Oberon's density of 1.63 g/cm³,^[3] 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.^[21] The latter could include rocks and heavy organic compounds. The presence of water ice is supported by spectroscopic observations, which have revealed crystalline water ice on the surface of the moon.^[17] 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.^[17] 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.^[17] Meteorite impacts tend to sputter (knock out) ice from the surface, leaving dark non-ice material behind. While no other compounds have been identified on the surface, plausible candidates for this dark material include rocks, carbon dioxide, various salts and organic compounds.^[4]^[17]

Oberon may be differentiated into a rocky core surrounded by an icy mantle.^[21] 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 parameters are dictated by moon's composition. The pressure in the center of Oberon is about 0.5 GPa (5 kbar).^[21] 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.^[21] 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.^[5] Its surface shows a strong opposition surge: its reflectivity decreases from 31% at a phase angle of 0° (geometrical albedo) to 20% at an angle of about 1°. Oberon has a very low bond albedo of about 14%.^[5] Its surface is generally slightly red in color, except for fresh impact deposits, which are slightly blue.^[22] The trailing and leading hemispheres are asymmetrical: the former is less red than the latter, because it contains less dark material.^[23] The reddening of the surfaces may be a result of space weathering caused by bombardment of the surface by charged particles and micrometeorites over the age of the Solar System.^[23]

Scientists have recognized two classes of geological feature on Oberon: craters and chasmata (canyons).^[4] The ancient surface of Oberon 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.^{[note 8]}^[24] The crater diameters range from a few kilometers at the low end to 206 kilometers for the largest known crater,^[24] Hamlet.^[25] Many large craters are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice.^[4] The largest craters, Hamlet, Othello and Macbeth, have floors made of a very dark material deposited after their formation.^[24] A peak with a height of about 11 km was observed in some Voyager images near the south-eastern limb of Oberon,^[26] which may be the central peak of a large impact basin with a diameter of about 375 km.^[26] Oberon's surface is intersected by a system of canyons, which, however, are less widespread than those found on Titania.^[4] The canyons are probably normal faults or grabens, which can be either old or fresh: the latter transect the bright deposits of some large craters, indicating that they formed later.^[27] The most prominent among Oberonian canyons is Mommur Chasma.^[28]

The geology of Oberon was influenced by two competing forces: impact crater formation and endogenic resurfacing.^[27] The former acted over the moon's entire history and is primarily responsible for its present-day appearance.^[24] 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.^[27] The canyons obliterated parts of the older surface.^[27] The cracking of the crust was caused by the expansion of Oberon by about 0.5%,^[27] 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),^[24] while others think that the impacts excavated dark material buried beneath the pure ice (crust).^[22] In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop of the non-differentiated interior.^[22]

Named surface features on Oberon^[29]
Feature	Named after	Type	Length (diameter), km	Latitude,°	Longitude,°
Mommur Chasma	Mommur, English folklore	Chasma	537	−16.3	323.5
Antony	Mark Antony	Crater	47	−27.5	65.4
Caesar	Julius Caesar		76	−26.6	61.1
Coriolanus	Coriolanus		120	−11.4	345.2
Falstaff	Falstaff		124	−22.1	19.0
Hamlet	Hamlet		206	−46.1	44.4
Lear	King Lear		126	−5.4	31.5
MacBeth	Macbeth		203	−58.4	112.5
Othello	Othello		114	−66.0	42.9
Romeo	Romeo		159	−28.7	89.4

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.^[30] 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.^{[note 9]}^[4] Significant amounts of nitrogen and carbon may have been present in the form of carbon monoxide and N₂ instead of ammonia and methane.^[30] 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.^[4]

Oberon's accretion probably lasted for several thousand years.^[30] The impacts that accompanied accretion caused heating of the moon's outer layer.^[31] The maximum temperature of around 230 K was reached at the depth of about 60 km.^[31] 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.^[4] 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,^[31] implying that any endogenous activity ceased billions years ago.^[4]

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) was present.^[31] 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.^[21] The eutectic temperature of this mixture is 176 K.^[21] If the temperature dropped below this value the ocean would have frozen by now. The freezing of the water led to the expansion of the interior, which may have also been responsible for the formation of canyons.^[24] Still, the 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 distance between Voyager 2 and Oberon was only 470,600 km,^[32] the best images of this moon have spatial resolution of about 6 km.^[24] At the time of the flyby the southern hemisphere of Oberon was pointed towards the Sun, so the northern (dark) hemisphere could not be studied.^[4] No other spacecraft ever visited Uranus (and Oberon), and no mission to this planet is planned in the foreseeable future.

Notes

^ Surface area derived from the radius r: $4\pi r^{2}$ .
^ Volume v derived from the radius r: $4\pi r^{3}/3$ .
^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: $Gm/r^{2}$ .
^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: $\sqrt 2Gm/r$ .
^ In US dictionary transcription, Template:USdict.
^ The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.
^ The eight moons more massive than Oberon are Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, Triton, and Titania.^[20]
^ The high number of craters on Oberon means that it has the most ancient surface among Uranus's moons.^[24]
^ For instance, Tethys, a Saturnian moon, has the density of 0.97 g/cm³, which means that it contains more than 90% of water.^[17]

References

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^ ^a ^b Newton, Bill (1995). The guide to amateur astronomy. Cambridge University Press. p. 109. ISBN 9780521444927. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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^ ^a ^b ^c ^d ^e Cite error: The named reference Croft1989 was invoked but never defined (see the help page).
^ "Oberon: Mommur". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 2009-03-06.
^ Cite error: The named reference usgs was invoked but never defined (see the help page).
^ ^a ^b ^c Cite error: The named reference Mousis2004 was invoked but never defined (see the help page).
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External links

Hamilton, Calvin J. (1999). "Oberon profile". NASA's Solar System Exploration. Retrieved May 21, 2009.
Arnett, Bill (December 22, 2004). "Oberon profile". The Nine Planets Solar System Tour. Retrieved March 6, 2009.
Arnett, Bill (November 17, 2004). "Seeing the Solar System". The Nine Planets Solar System Tour. Retrieved March 6, 2009.

[4] Surface area derived from the radius r: $4\pi r^{2}$ .

[5] Volume v derived from the radius r: $4\pi r^{3}/3$ .

[6] Surface gravity derived from the mass m, the gravitational constant G and the radius r: $Gm/r^{2}$ .

[7] Escape velocity derived from the mass m, the gravitational constant G and the radius r: $\sqrt 2Gm/r$ .

[11] In US dictionary transcription, Template:USdict.

[22] The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.

[27] The eight moons more massive than Oberon are Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, Triton, and Titania.^[20]

[32] The high number of craters on Oberon means that it has the most ancient surface among Uranus's moons.^[24]

[39] For instance, Tethys, a Saturnian moon, has the density of 0.97 g/cm³, which means that it contains more than 90% of water.^[17]

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[Newton-10] Newton, Bill (1995). The guide to amateur astronomy. Cambridge University Press. p. 109. ISBN 9780521444927. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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[Mommur-36] "Oberon: Mommur". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 2009-03-06.

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[1]

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