Oberon (moon)

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For other uses, see Oberon (disambiguation).

Oberon
synchronous[8]
Albedo
  • 0.31 (geometrical)
  • 0.14 (Bond)[9]
Temperature70–80 K[10]
14.1[11]

Oberon

Shakespeare's A Midsummer Night's Dream. Its orbit lies partially outside Uranus's magnetosphere
.

Oberon likely formed from the

core and an icy mantle. A layer of liquid water may be present at the boundary between the mantle and the core. The surface of Oberon, which is dark and slightly red in color, appears to have been primarily shaped by asteroid and comet impacts. It is covered by numerous impact craters reaching 210 km in diameter. Oberon possesses a system of chasmata (graben or scarps
) formed during crustal extension as a result of the expansion of its interior during its early evolution.

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

Discovery and naming

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

All of the moons of Uranus are named after characters created by

Oberon, the King of the Fairies in A Midsummer Night's Dream.[16] The names of all four satellites of Uranus then known were suggested by Herschel's son John in 1852, at the request of William Lassell,[17] who had discovered the other two moons, Ariel and Umbriel, the year before.[18] It is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission.[19] The adjectival form of the name is Oberonian, /ˌɒbəˈroʊniən/.[20]

Oberon was initially referred to as "the second satellite of Uranus", and in 1848 was given the designation Uranus II by William Lassell,[21] although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV).[22] 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.[23]

Orbit

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

tidally locked, with one face always pointing toward the planet.[8] Oberon spends a significant part of its orbit outside the Uranian magnetosphere.[24] As a result, its surface is directly struck by the solar wind.[10] This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by the magnetospheric plasma, which co-rotates with the planet.[24] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).[10]

Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Oberon) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the zenith over one of the poles at each solstice.[10] The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was in darkness. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. One such event, which lasted for about six minutes, was observed on May 4, 2007, when Oberon occulted Umbriel.[25]

Composition and internal structure

Size comparison of Earth, the Moon, and Oberon.

Oberon is the second-largest and second-most massive of the Uranian moons after

Meteorite impacts tend to sputter (knock out) ice from the surface, leaving dark non-ice material behind.[10] The dark material itself may have formed as a result of radiation processing of methane clathrates or radiation darkening of other organic compounds.[8][29]

Oberon may be differentiated into a rocky

eutectic temperature of 176 K).[28] 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.[30]

Surface features and geology

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

Oberon is the second-darkest large moon of Uranus after

irregular satellites, which would occur predominately on the leading hemisphere.[32]

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

normal faults[g] which can be either old or fresh: the latter transect the bright deposits of some large craters, indicating that they formed later.[37] The most prominent Oberonian canyon is Mommur Chasma.[38]

The geology of Oberon was influenced by two competing forces:

tectonic in nature and led to the formation of the canyons, which are actually giant cracks in the ice crust.[37] The canyons obliterated parts of the older surface.[37] The cracking of the crust was caused by the expansion of Oberon by about 0.5%,[37]
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

lunar maria),[34] while others think that the impacts excavated dark material buried beneath the pure ice (crust).[31] In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior.[31]

Named surface features on Oberon[39]
Feature Named after Type Length (diameter), km Coordinates
Mommur Chasma Mommur, French folklore Chasma 537 16°18′S 323°30′E / 16.3°S 323.5°E / -16.3; 323.5
Antony Mark Antony Crater 47 27°30′S 65°24′E / 27.5°S 65.4°E / -27.5; 65.4
Caesar Julius Caesar 76 26°36′S 61°06′E / 26.6°S 61.1°E / -26.6; 61.1
Coriolanus
Coriolanus
 120 11°24′S 345°12′E / 11.4°S 345.2°E / -11.4; 345.2
Falstaff
Falstaff
 124 22°06′S 19°00′E / 22.1°S 19.0°E / -22.1; 19.0
Hamlet Hamlet 206 46°06′S 44°24′E / 46.1°S 44.4°E / -46.1; 44.4
Lear King Lear 126 5°24′S 31°30′E / 5.4°S 31.5°E / -5.4; 31.5
MacBeth Macbeth 203 58°24′S 112°30′E / 58.4°S 112.5°E / -58.4; 112.5
Othello Othello 114 66°00′S 42°54′E / 66.0°S 42.9°E / -66.0; 42.9
Romeo Romeo 159 28°42′S 89°24′E / 28.7°S 89.4°E / -28.7; 89.4
Surface features on Oberon are named for male characters and places associated with Shakespeare's works.[40]

Origin and evolution

Oberon is thought to have formed from an

obliquity.[41] 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.[h][8] Significant amounts of carbon and nitrogen may have been present in the form of carbon monoxide and N2 instead of methane and ammonia.[41] The moons that formed in such a subnebula would contain less water ice (with CO and N2 trapped as clathrate) and more rock, explaining the higher density.[8]

Oberon's accretion probably lasted for several thousand years.

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,[43] implying that any endogenous activity from this cause ceased billions of years ago.[8]

The initial

eutectic temperature of this mixture is 176 K.[28] 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.[34] 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.[44]

Exploration

Main article: Exploration of Uranus

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

geological mapping.[34] At the time of the flyby the southern hemisphere of Oberon was pointed towards the Sun, so the dark northern hemisphere could not be studied.[8]
No other spacecraft has ever visited the Uranian system.

See also

Notes

  1. ^ Surface area derived from the radius r: .
  2. ^ Volume v derived from the radius r: .
  3. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: .
  4. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: √2Gm/r.
  5. ^ The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.
  6. ^ The eight moons more massive than Oberon are Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, Triton, and Titania.[26]
  7. ^ Some canyons on Oberon are graben.[34]
  8. ^ For instance, Tethys, a Saturnian moon, has a density of 0.97 g/cm3, which means that it contains more than 90% water.[10]

References

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  5. ^ Thomas, P. C. (1988). "Radii, shapes, and topography of the satellites of Uranus from limb coordinates". Icarus. 73 (3): 427–441.
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