Oberon (moon)

William Herschel

Oberon ( Uranus IV) is the eighteenth of the 27 known and with a diameter of about 1520 km is the second largest and outermost of the five large moons of the planet Uranus. He is after Saturn's second largest moon Rhea 's tenth- largest moon in the solar system.

  • 3.1 size
  • 3.2 Internal structure
  • 3.3 Surface 3.3.1 Chasmata
  • 3.3.2 crater terrain

Discovery and designation

Oberon was discovered on January 11th, 1787 along with Titania as first and second moons of Uranus from Kurhannoverianischen astronomers William Herschel with his homemade telescope in Slough ( UK). Herschel had discovered Uranus around six years earlier. He announced the discovery of the two moons after ensuring the orbital parameters on February 9, 1787 known and observed the system from 1790 to 1796 on. Herschel gave later to the discovery of four more moons of Uranus, which later, however, proved to be non-existent. Almost 50 years after this discovery, Oberon and Titania were no other telescope except observed by Herschel more.

The moon was named after the fairy king Oberon from William Shakespeare's A Midsummer Night's Dream. Oberon was the husband of Titania.

All the moons of Uranus are named after characters from Shakespeare or Alexander Pope. The names of the first four moons discovered Uranus ( Oberon, Titania, Ariel and Umbriel ) were in 1852 by John Herschel, son of the discoverer, on request of William Lassell, who previously Ariel and Umbriel discovered a year proposed.

Originally Oberon has been called " the second satellite of Uranus ", and in 1848 received the moon by Lassell the name Uranus II, although he sometimes used Herschel's numbering Uranus IV. 1851 numbered Lassell, the previously known moons on the distances to the mother planet new, and since then Oberon was definitely called Uranus IV.

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Orbit

Oberon orbiting Uranus on a prograde, nearly perfectly circular orbit at an average distance of about 583,519 kilometers (about 22.830 Uranus radii ) from the center, or around 558,000 km on whose cloud tops. The orbital eccentricity is 0.0014, the orbit is inclined 0.068 degrees from the equator of Uranus.

The orbit of the next inner moon Titania is situated in the middle of 147,000 km Oberon's orbit. Oberon is the outermost regular Uranus moon; outside its orbit is a big gap to the 1997-2003 discovered irregular Uranian moons, of which the innermost, Francisco, a mean distance of approximately 3.7 million kilometers has.

Oberon orbits Uranus in 13 days, 11 hours, 7 minutes and 3 seconds.

Oberon's orbit lies partially outside the magnetosphere of Uranus. Thus its surface is taken directly from the solar wind. This is very important because the subsequent hemispheres of airless moons thus are under constant attack from magnetosphärischem plasma which co-rotates with the planet. This can lead to a darkening of the following hemisphere, which so far could be observed for all Uranian moons except Oberon.

Since Oberon as Uranus orbits the sun practically relative to the rotation on the side, its northern or southern hemisphere away at the time of the solstice, either directly to the sun or away from it, leading to extreme seasonal effects. This means that the poles of Oberon are illuminated by the sun are in permanent darkness for half a Uranus year of 42 years or so. During the solstice, the sun is therefore close to the zenith over the poles. During the Voyager 2 flyby in 1986, which occurred almost at the solstice, which showed Südhemisphären of Uranus and its moons towards the sun, while the northern hemispheres were in complete darkness. During the equinox, in which the equatorial plane intersects with the direction to the earth and that also happens every 42 years mutual occultations and eclipses of the moons of Uranus Uranus are possible. A number of these rare events took place last from 2007 to 2008; Umbriel was covered by Oberon on 4 May 2007 for about six minutes.

Currently owns Oberon no orbital resonance with other moons, and according to current knowledge, not in his past, which is a great contrast to the four inner big neighbor.

Rotation

The rotation period is equal to the orbital period and Oberon has with how the Earth's moon, a synchronous rotation, which therefore also takes place within 13 days, 11 hours, 7 minutes and 3 seconds. Its rotation axis is exactly perpendicular to its orbital plane.

Physical Properties

Size

Oberon has a diameter of about 1520 km. He is just short of the second largest Uranus moon, because it is smaller by only about 55 km, the largest moon Titania. In addition, the two moons are very similar in other physical properties, Oberon with an average density of 1.63 g/cm3 again slightly lighter than its "sister". They also show different sized traces of geological activity.

From the size Oberon is best compared with titania or Saturn 's moons Rhea and Iapetus. Of the entire moon has been able by Voyager 2 only about 40%, mainly the southern hemisphere - as with all Uranian moons - are explored in more detail.

The total area of ​​7.285 million km2 Oberon is about, this is slightly less than the area of Australia without the State of Victoria.

Internal construction

Oberon has a mean density of 1.63 g/cm3, and its albedo is 0.24 i.e. 24 % of the incident solar light is reflected from the surface. The low density and let the albedo suggests that Oberon about 50 % water ice, 30 % silicate rock and 20 % carbon compounds and nitrogen compounds and other heavy organic compounds is composed of. The ratio of water ice to rock is in line with the other major moons of Uranus.

The presence of water ice is supported by infrared spectroscopic investigations from 2001 to 2005 that brought crystalline water ice on Titania's surface to light. This seems to be more present on Oberon's subsequent hemisphere; this is a great contrast to the other large Uranian moons that have the greater share on the leading hemisphere. The reason for this is unknown, but it may have to do with the forming of the surface by collecting small particles, as it usually occurs on the leading hemispheres ( "impact gardening "). Impacts of micrometeorites tend to sputtering of water ice that leave dark residue on the surface. The dark material itself was created by Ausgasungsprozesse of trapped gas hydrate than in ice methane or darkening by efflux of other organic material.

Oberon is possibly a differentiated body with a rocky core and a shell of water ice. If this is the case, the diameter of the core would be 960 km, which corresponds to 63 % of the total diameter, core, and a mass of 54% of total mass -, these parameters are dictated by the composition of the moon. The pressure in the center of Oberon is about 5 kbar. The current status of the ice shell is still unclear. If the ice contains enough the freezing point of water -reducing substances such as salt or ammonia could exist like on Jupiter 's moon Europa between core and ice mantle of Oberon possibly a subterranean ocean. The depth of this ocean would be in this case, about 40 km, the temperature would be about -93 ° around the C (180 K). The current internal structure is highly dependent on the thermal history of Oberon, which is not well known.

Surface

In contrast to the bumps on Titania derived from tidal forces, can the cratered surface of the ice by Oberon suspect very little activity. It so closely resembles the moon Umbriel, however in diameter around a quarter less. In the southern hemisphere, there are big faults that intersect the surface. This could also indicate a geologic activity early after its formation.

Recordings of the Voyager 2 spacecraft revealed a landscape dotted with craters, icy surface, which is obviously very old and very little has geologic activity. Both the number and the dimensions of the craters on Oberon much higher than on titania or Ariel. In many craters are found on the bottom sediments of an unknown, dark substance. Some of the craters show bright glimmer of ejected material, similar to Jupiter's moon Callisto.

Of the major Uranian moons Oberon is by Umbriel the zweitdunkelste. The surface shows depending on the lighting angle high brightness effects; the reflectivity of 0.31 at a phase angle of 0 ° to 0.22 decreases rapidly from about 1 °. The Bond albedo is relatively low 0.14.

The surface of Oberon generally appears in a red hue, with the exception of the relatively fresh bright deposits that appear in a neutral gray to bluish. Oberon has the reddest of all surface of Uranus' large moons. There seems to exist a little dichotomy of the leading and the trailing hemisphere; the leading appears more reddish because it contains more dark red material. The red color of surfaces is often caused by space erosion, caused by the bombardment of charged particles and micrometeorites in the period of the whole history of the solar system. The color asymmetry on Oberon was, however, probably due to the accretion of a reddish material from the outer Uranus system, possibly irregular moons that would be reflected mainly on the leading hemisphere.

The average surface temperature is between -193 ° C and -203 ° C ( 70-80 K). The gravitational acceleration at the surface is 0.346 m/s2; this corresponds to approximately 3.5 percent of the earthly. In Oberon no evidence of an atmosphere or a magnetic field have been found.

On the surface, previously known scientists have so far identified two types of surface structures: canyons and craters. All surface features on Oberon are named after male characters and places from the works of William Shakespeare.

Chasmata

The surface of Oberon is cut by a system of canyons, which are called Chasma (plural Chasmata ). They appear less far -ranging as the Chasmata on Titania. The walls of these canyons probably represent terrain levels caused by faults. They may be older or more recent, depending on whether they intersect the existing craters or be cut from them.

The most well-known and so far only named Canyon is Mommur Chasma, which has a length of 537 km. He might have been formed by an extension of Oberon's heart in its early history, as the icy crust set off by this expansion. This canyon is an example of a trench or a ground level, which is caused by dislocations. The Gazetteer of Planetary Nomenclature of the IAU indicates that Mommur Chasma is named after the forest home of Oberon in Shakespeare's A Midsummer Night's Dream, but this was not mentioned in the piece; instead, they appear in the French epic Huon of Bordeaux.

Crater terrain

Oberon has the most cratered surface of all of Uranus' large moons. The crater density is close to saturation, which means that newer craters can only be created by the destruction of older craters and the number of craters thus remains almost constant. The high number of craters suggests that Oberon has the oldest surface of all of Uranus' large moons. The crater reach a diameter of up to 206 km for the largest crater Hamlet. Many large craters are surrounded by bright rays Impaktablagerungen and systems that are composed of relatively fresh ice. The largest crater Hamlet, Othello and Macbeth have very dark crater floors, which have been enriched by its origin with dark material.

Several pictures show on the horizon a mountain, 6 km ( according to other sources 11 km) rises above its surroundings. It is probably around a central mountain of a large impact basin of 345 km in diameter. The mountain can be discerned on most pictures at the bottom left of Oberon.

The geology of the surface was by two competing forces affect: Impacts and endogenous formation by surface renewal. The former took place over the whole period of the history of the moon and is mainly responsible for the present appearance of the surface, while the latter processes also had global influence, however, took place only at a certain time after the formation of Oberon. These endogenous processes are mainly tectonic nature and caused the formation of the Chasmata, the huge cracks in the icy crust of the moon are caused by the global expansion of about 0.5% and renewed parts of the older surface.

The origin and the composition of dark spots that appear mostly on the leading hemisphere and crater soil is not known. The hypotheses range from Kryovulkanismus up to the release of dark material by Impacts, which was annealed under an even thinner crust of ice. In this case, Oberon would have to be at least partially differentiated, with a thin crust of ice on a dark non- ice component.

Formation

Oberon was probably formed by an accretion disk or by a sub- fog, which was possibly to Uranus during its time of origin or formed by the (still theoretical ) impact that could tip the planet on its side. The exact composition of this sub- nebula is not known, but, the higher densities of Uranus system compared to the lying closer to the Sun Saturn's moons to a relative lack of water out. Potentially significant amounts of nitrogen (N2 ) and carbon ( C ) in the form of carbon monoxide ( CO) and molecular nitrogen were present instead of ammonia (NH3 ) and methane ( CH4). Satellites that emerged from such a sub- fog would contain less water ice and CO and N2 as gas hydrate trapped in ice and more rock include, which would explain the higher densities.

The accretion process probably lasted several thousand years until the formation of Oberon was completed. Models show that the accretion accompanying impacts would cause a heating of the outer shell of the moon with a temperature of about 230 K to a depth of up to 60 km. After the formation of this outer layer cooled, while Oberon's affairs by the decomposition aufheizte radioactive elements in the rock. Cooling the outer sheath contracted, while the inside expanded. This caused great tension in the crust of the moon that led to breakthroughs in the crust and the formation of the canyon. This process, which lasted about 200 million years indicates that the endogenous formation of the surface must have been completed already billions of years ago.

Akkretionshitze the initial and subsequent decomposition radioactive elements may lead to a melting water ice when a freezing point -lowering substance, such as a salt of ammonia or in the form of ammonium hydroxide was present. This would lead to a separation of ice and rock (differentiation) of the core. In this case, a layer of liquid water would have led rich of dissolved ammonia, the boundary of mantle and core. The eutectic temperature of this mixture is 176 K. If the temperature falls below this value, the existing ocean today would probably already frozen over. This freezing presumably led to the extension of the interior, which was probably responsible for the formation of Chasmata. Knowledge of the development of Oberon is still very limited at present.

Research

Since the discovery in 1787 by William Herschel was 200 years except for the orbital parameters over Oberon Not much is known. The moon was too small and too far away to resolve it closer with ground-based telescopes.

On January 20, 1986 Oberon was a distance of at least 470,600 kilometers from the Voyager 2 spacecraft passed and photographed and measured. The rotation axis of Uranus and Oberon reported, due to the high axial inclination of the planetary system of 98 °, at which point toward Earth, so that the moons of Uranus could not be served individually as before at Jupiter and Saturn on the equatorial plane, but to their orbits like a target around the planet ruled and the planet had to be made virtually. This meant that all the moons of Uranus and only the southern hemisphere was photographed at intervals of about two days - the worst possible position for a flyby. In addition, you had to opt for a moon, as a close flyby at a necessarily large distances to all other conditional.

Since we wanted to draw on Voyager 2 to Neptune, the condition was a close Uranus flyby. This revealed that only the moon Miranda could be happening close. This was the best resolution of photos about 6 km; they show about 40 % of the surface, with only about 25 % could be used with the necessary quality for geological mapping and crater counting.

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