Oberon

Click image for description
Discovery
Discovered by	William Herschel
Discovery date	January 11, 1787
Designations
Alternate name	Uranus IV
Adjective	Oberonian
Orbital characteristics
Semi-major axis	583 520 km
Mean orbit radius	583 519 km
Eccentricity	0.0014
Orbital period	13.463 234 d
Inclination	0.058° (to Uranus' equator)
Satellite of	Uranus
Physical characteristics
Mean radius	761.4 km (0.1194 Earths)[1]
Surface area	7 285 000 km²
Volume	1 849 000 000 km³
Mass	3.014 × 1021 kg (5.046 × 10−4 Earths)[1]
Mean density	1.63 g/cm³[1]
Equatorial surface gravity	0.346 m/s2
Escape velocity	0.73 km/s
Rotation period	presumed synchronous[2]
Albedo	0.31 (geometrical), 0.14 (bond)[3]
Temperature	~61 K
Apparent magnitude	13.94 [4]

Oberon (pronounced /ˈoʊbərɒn/ OH-bər-on), also designated as Uranus IV, is the outermost of the major moons of the planet Uranus. It is the second largest and second most massive of Uranian moons. It is the ninth most massive moon in the solar system.^[5] Oberon was discovered by William Herschel in 1787 and named after a character in Shakespeare's A Midsummer Night's Dream. Oberon's orbit lies 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. In addition to craters, Oberon possesses a system of canyons (grabens), which were formed as a result of the expansion of the interior during the early phases of the moon’s evolution. Oberon probably formed in the accretion disk that surrounded Uranus just after its 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

Oberon was discovered on January 11, 1787 by William Herschel; the same year he discovered Uranus's largest moon, Titania.^[6][7] He would later report four more satellites, which would turn out to be spurious.^[8] Remarkably, for nearly fifty years following their discovery, Titania and Oberon would not be observed by any other instrument than William Herschel's.^[9] However, the moon can be seen from Earth with a present-day amateur telescope.^[10]

Name

All of the moons of Uranus are named for characters from William Shakespeare or Alexander Pope. Oberon was named after Oberon, the King of the Fairies in A Midsummer Night's Dream.

The name "Oberon", and the names of all four satellites of Uranus then known, were suggested by Herschel's son John Herschel in 1852 at the request of William Lassell, who had discovered Ariel and Umbriel the year before.^[11] Lassell had earlier endorsed Herschel's 1847 naming scheme for the seven then-known satellites of Saturn and had named his newly-discovered eighth satellite Hyperion in accordance with Herschel's naming scheme in 1848. The adjectival form of the name is Oberonian, pronounced /ˌɒbəˈroʊniən/.

Oberon was initially called simply "the second satellite of Uranus"; and in 1848 it was given the designation Uranus II by William Lassell, although he sometimes also used William Herschel's numbering (where Titania and Oberon are II and IV).^[12][13][14] Oberon is now designated Uranus IV.

Composition and internal structure

Oberon's density of 1.63 g/cm^3[1] indicates that it consists of water ice and a dense non-ice component in roughly equal proportions.^[15] The latter can include rock and heavy organic compounds. The presence of water ice is supported by spectroscopic observations, which have found crystalline water ice on the surface.^[16] Water ice absorption bands are stronger on Oberon's trailing hemisphere, opposite to what is observed in other Uranian moons, where the leading hemisphere exhibits stronger water ice bands.^[16] The cause of this asymmetry is not known, but it may be related to "impact gardening" of the surface, which is stronger on the leading hemisphere.^[16] Meteorite impacts tend to sputter ice from the surface leaving dark material behind. While no other compounds has been identified on the surface, plausible chemicals include ammonia, carbon dioxide, various salts and organic compounds.^[16][2]

Oberon's internal structure is dictated by its composition. It may be differentiated into a rocky core surrounded by an icy mantle.^[15] 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 pressure in the center of Oberon is about 0.5 GPa (5 kar).^[15] 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.^[15] However the internal structure of Oberon depends heavily on its thermal history, which is poorly known at present.

Surface features and geology

A map of Oberon. The large crater with the dark floor (right of center) is Hamlet.

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. At the time of the flyby the southern hemisphere of the moon was pointed towards the Sun, so the northern hemisphere could not be studied.^[2]

Oberon is the second darkest large moon of Uranus after Umbriel. Its surface shows a strong opposition surge: its reflectivity decreases from 31% at a phase angle of 0 (geometrical albedo) to 20% at a phase angle of about 1°. Oberon's bond albedo is very low at about 14%.^[3] Its surface is generally slightly red in color, except for fresh impact deposits, which are blue.

So far, scientists have recognized three classes of geological feature on Oberon: craters, chasmata (canyons), and mountains. Oberon has the most heavily cratered surface of all the Uranian moons, with a crater density that may be close to saturation. The craters' diameters range from a few kilometers at the low end to 206 kilometers for the largest known crater, Hamlet.^[17] Many large craters are surrounded by bright impact ejecta consisting of relatively fresh ice.^[2] The largest craters, Hamlet, Othello and Macbeth, have dark floors. A peak with height of about 11 km was observed in some Voyager images near the south-eastern limb of Oberon^[18] which may be the central peak of a large impact basin with diameter of about 375 km.^[18] Oberon's surface is intersected by a system of canyons, which, however, are less widespread than those found on Titania.^[2] 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.^[19]

The geology of Oberon was influenced by two competing forces: impact crater formation and endogenic resurfacing.^[19] The former acted over the moon's entire history and is primarily responsible for its present-day appearance.^[2] The latter processes were active for some time 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.^[19] The canyons obliterated parts of the older surface. The cracking of the crust was caused by the expansion of Oberon by about 0.5%,^[19] 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, while others think that the impacts simply excavated dark material from depth. In the latter case Oberon should be at least partially differentiated.

Origin and evolution

Oberon is thought to have formed in the accretion disc that existed around Uranus for some time after its formation.^[20] The accretion of Oberon probably lasted for several thousand years.^[20] The impacts that accompanied accretion caused heating of the moon's outer layer.^[21] The maximum temperature of around 230 K was reached at the depth of about 60 km.^[21] 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 result of this process, which lasted for about 200 million years, is the present-day system of canyons.^[21]

The initial accretional heating was probably strong enough to melt the ice if some antifreeze like ammonia was present.^[21] Further radiogenic heating may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle.^[15] However the present knowledge of the thermal evolution of Oberon is very limited.

See also

• List of geological features on Oberon

References

External links

• Oberon profile by NASA's Solar System Exploration
• Oberon profile on The Nine Planets

v • d • e Natural satellites of the Solar System Planetary satellites Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian Other satellite systems Plutonian · Eridian · Haumean · Asteroid satellites Largest satellites Ganymede · Titan · Callisto · Io · Moon · Europa · Triton Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys  · Enceladus · Miranda · Proteus · Mimas Inner satellites · Trojans · Irregulars · List · List by diameter · Timeline of discovery · Naming

Privacy: I log your IP address, the request you submitted, and when you dunnit. I also spy on you through cameras in your neighbour's eyes. What, you think he's a person? No, he's my own private Terminator.