Oberon (moon)

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Oberon (moon)
Oberon, recorded in 1986 by Voyager 2
Central body Uranus
Properties of the orbit
Major semi-axis 583,519 km
Periapsis 582,702 km
Apoapsis 584,336 km
eccentricity 0.0014
Orbit inclination 0.068 °
Orbital time 13.463234 d
Mean orbital velocity 3.15 km / s
Physical Properties
Albedo 0.24
Apparent brightness 13.2 mag
Medium diameter 1522.8 km
Dimensions (3.014 ± 0.075) 10 21 kg
surface 7,285,000 km 2
Medium density (1.63 ± 0.05) g / cm 3
Sidereal rotation 13.463234 days
Axis inclination 0.0 °
Acceleration of gravity on the surface 0.346 m / s 2
Escape speed 726 m / s
Surface temperature −203 ° C to −193 ° C; 70 K to 80 K

Wilhelm Herschel

Date of discovery January 11, 1787
Remarks Most heavily cratered large Uranus moon
Size comparison
Size comparison between Uranus (left) and its largest moons, from left to right Puck, Miranda, Ariel, Umbriel, Titania and Oberon (photo montage to scale)

Oberon (also Uranus IV ) is the eighteenth innermost of the 27 known and the second largest and the outermost of the five large moons of the planet Uranus . It is the tenth largest moon in the solar system .


Oberon was discovered on January 11, 1787 together with Titania as the first and second Uranus moons by the Electoral Hanoverian astronomer Wilhelm Herschel with his self-made reflecting telescope in Slough ( Great Britain ). Herschel had discovered Uranus around six years earlier. He announced the discovery of the two moons after the orbit parameters had been ascertained on February 9, 1787 and continued to observe the system from 1790 to 1796. Herschel later stated the discovery of another four Uranus moons , which later turned out to be nonexistent. For almost 50 years after this discovery, Oberon and Titania were not observed by any telescope other than Herschel's.


The moon was named after the Elven King Oberon from William Shakespeare's Midsummer Night 's Dream . Oberon was Titania's husband .

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

Originally Oberon was referred to as "the second satellite of Uranus," and in 1848 Lassell's moon was named Uranus II , although he sometimes used Herschel's numbering Uranus IV . In 1851, Lassell re-numbered the previously known moons according to their distances from the mother planet, and Oberon has since been definitely referred to as Uranus IV .

Unlike the Earth's moon, Oberon, like all other satellites in the solar system, has no official or widely used astronomical symbol . Oberonsymbols circulating on the internet (e.g. Gender symbol Other dark transparent Background.png) are designs by private individuals. Official recognition or use is not foreseeable.

Track properties


Oberon orbits Uranus on a prograde , almost perfectly circular orbit at an average distance of around 583,519 km (approx. 22,830 Uranus radii) from its center, i.e. around 558,000 km above its cloud ceiling. The orbital eccentricity is 0.0014, the web is 0.068 ° relative to the equator of Uranus inclined .

The orbit of the closest moon Titania is on average 147,000 km from Oberon's orbit. Oberon is the outermost regular moon of Uranus; outside of its orbit there is a large gap to the irregular Uranus moons discovered between 1997 and 2003 , of which the innermost, Francisco , has an average distance of around 3,700,000 km.

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

Oberon's orbit is partially outside the magnetosphere of Uranus. As a result, its surface is hit directly by the solar wind . This is very important because the following hemispheres of atmospheric moons are under constant bombardment of magnetospheric plasma , which rotates with the planet. This can lead to a darkening of the subsequent hemisphere, which has so far been observed in all Uranus moons except for Oberon.

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

At present Oberon has no orbital resonance with other moons, and according to the current state of knowledge also not in its past, which is a great contrast to the four inner large neighbors.


The rotation time is the same as the orbital time and Oberon shows, like the earth's moon , a synchronous rotation , which also takes place within 13 days, 11 hours, 7 minutes and 3 seconds. Its axis of rotation is exactly perpendicular to its plane .

Physical Properties


Oberon has a diameter of around 1520 km. It is almost the second largest moon of Uranus, as it is only about 55 km smaller than the largest moon Titania . In addition, the two moons are also very similar in other physical properties, whereby Oberon with an average density of 1.63 g / cm 3 is slightly lighter than its "sister". They also show different sized traces of geological activity.

In terms of size, Oberon can best be compared with Titania or the Saturn moons Rhea and Iapetus . Voyager 2 has so far only been able to investigate about 40% of the entire moon , mainly the southern hemisphere - as with all Uranus moons.

The total area of ​​Oberon is approximately 7,285,000 km 2 , which is slightly less than the area of Australia excluding the state of Victoria .

internal structure

Oberon has an average density of 1.63 g / cm 3 and its albedo is 0.24, which means that 24% of the incident sunlight is reflected from the surface. The low density and the albedo suggest that Oberon is composed of around 50% water ice , 30% silicate rock and 20% carbon and nitrogen compounds as well as other heavy organic compounds. The ratio of water ice to rock is in line with the other larger moons of Uranus.

The presence of water ice is supported by infrared spectroscopic studies from 2001 to 2005 that revealed crystalline water ice on Titania's surface. This seems to be more prominent on Oberon's subsequent hemisphere; this is in great contrast to the other major moons of Uranus, which have the greater proportion on the leading hemisphere. The reason for this is unknown, but it may have to do with the reshaping of the surface by collecting small particles, as is common in leading hemispheres ( “impact gardening” ). Micrometeorite impacts tend to cathodically atomize water ice, leaving dark residues on the surface. The dark material itself was created by the outgassing processes of methane trapped in ice as gas hydrate or by darkening through the outflow of other organic material.


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


Side view of Oberon

In contrast to the bumps on Titania, which originate from tidal forces, the cratered ice surface of Oberon suggests little activity. It is very similar to the moon Umbriel , but its diameter is around a quarter smaller. In the southern hemisphere, there are large faults that cross the surface. This could also indicate geological activity early after its formation.

Images from the Voyager 2 spacecraft showed a crater -strewn, icy surface that is apparently very old and with little geological activity. Both the number and dimensions of the craters are much larger on Oberon than on Titania or Ariel. In many craters there are deposits of a still unknown, dark substance on the floor. Some of the craters show bright shimmers of ejected material, similar to those on Jupiter's moon Callisto .

Of the great moons of Uranus, Oberon is the second darkest after Umbriel. The surface shows great brightness effects depending on the lighting angle; the reflectivity of 0.31 at a phase angle of 0 ° drops rapidly to 0.22 at around 1 °. The spherical albedo is a relatively low 0.14.

The surface of Oberon appears generally in a red hue, with the exception of the relatively fresh light-colored deposits which appear in a neutral gray to bluish tinge. Oberon has the redest surface of all major Uranus moons. There appears to be a small dichotomy between the leading and trailing hemispheres; the leading one appears a bit more reddish as it contains more dark red material. The red coloring of surfaces is often caused by space erosion caused by the bombardment of charged particles and micrometeorites throughout the history of the solar system. The color asymmetry on Oberon, however, probably arose from the accretion of reddish material from the outer uranium system, possibly from irregular moons, that would predominantly be reflected on the leading hemisphere.

The mean surface temperature is between −193 ° C and −203 ° C (70–80 K ). The acceleration due to gravity on the surface is 0.346 m / s 2 ; this corresponds to around 3.5 percent of the earthly. No evidence of an atmosphere or magnetic field was found on Oberon.

So far, scientists have identified two types of surface structures on the known surface: canyons and impact craters . All surface features on Oberon have been named after male characters and locations from works by William Shakespeare.


The surface of Oberon is cut by a system of canyons called the Chasma (plural Chasmata ). They are less diverse than the Chasmata on Titania. The walls of these canyons may represent steps in terrain created by faults . They can be older or more recent depending on whether they cut or are cut by the craters that are already there.

The most famous and so far only named canyon is Mommur Chasma , which has a length of 537 km. It may have been formed by an expansion of Oberon's interior in its early history when the ice crust broke open as a result of that expansion. This canyon is an example of a trench or terrain step created by faults. The Gazetteer of Planetary Nomenclature of the IAU indicates that Mommur Chasma to the forest home of Oberon in Shakespeare's Midsummer Night's Dream was named, but this is not mentioned in the piece; instead it appears in the French epic Huon von Bordeaux .

Crater area

Oberon has the most cratered surface of all major Uranus moons. The crater density is close to saturation; this means that newer craters can only be created by destroying 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 major Uranus moons. The craters reach a diameter of up to 206 km for the largest crater, Hamlet . Many large craters are surrounded by bright impact deposits and radiation systems composed of relatively fresh ice. The largest craters Hamlet , Othello and MacBeth have very dark crater floors, which were enriched with dark material after their formation.

Several images show a mountain on the horizon that rises 6 kilometers (according to other information 11 km) above its surroundings. It is possibly a central mountain of a large impact basin 345 km in diameter. The mountain can be seen in most of the pictures on the lower left edge of Oberon.

List of named craters on Oberon
Surname Diameter (km) Coordinates Origin of name
Hamlet 206.0 46 ° 06′S 44 ° 24′E / 46.1 ° S 44.4 ° E Hamlet , Prince of Denmark in Hamlet
MacBeth 203.0 58 ° 24′S 112 ° 30′E / 58.4 ° S 112.5 ° E Macbeth , General of Scotland in Macbeth
Romeo 159.0 28 ° 42′S 89 ° 24′E / 28.7 ° S 89.4 ° E Romeo , son of Countess Montague in Romeo and Juliet
Lear 126.0 5 ° 24′S 31 ° 30′E / 5.4 ° S 31.5 ° E Lear , King of Britain in King Lear
Falstaff 124.0 22 ° 06′S 19 ° 00′E / 22.1 ° S 19.0 ° E Falstaff , from Henry IV and The Merry Wives of Windsor
Coriolanus 120.0 11 ° 24′S 345 ° 12′E / 11.4 ° S 345.2 ° E Cajus Marcius Coriolanus , noble Roman from Coriolanus
Othello 114.0 66 ° 00′S 42 ° 54′E / 66.0 ° S 42.9 ° E Othello , the Moor from Othello, the Moor from Venice
Caesar 76.0 26 ° 36′S 61 ° 06′E / 26.6 ° S 61.1 ° E Gaius Iulius Caesar , Roman statesman from Julius Caesar
Antony 47.0 27 ° 30′S 65 ° 24′E / 27.5 ° S 65.4 ° E Mark Antony , from Antony and Cleopatra and Julius Caesar

The geology of the surface has been influenced by two competing forces: shaping by impact and endogenous surface renewal. The former took place over the entire period of the moon's history and is mainly responsible for the appearance of the surface today, while the latter processes also had global influence, but only occurred at a certain point in time after the formation of Oberon. These endogenous processes are mainly of a tectonic nature and caused the formation of the Chasmata, which are huge fractures in the moon's ice crust created by a global expansion of approximately 0.5% and renewing parts of the older surface.

The origin and composition of the dark spots, which are predominantly found on the leading hemisphere and on crater floors, is not known. The hypotheses range from cryovolcanism to the release of dark material through impacts that was deposited under an even thinner ice crust. In this case, Oberon would have to be at least partially differentiated, with a thin ice crust on a dark, non-ice component.


Oberon was probably formed by an accretion disk or by a sub-nebula that may have been around Uranus during its formation or that formed after the (still theoretical) impact that caused the planet to tip over. The exact composition of this sub-nebula is not known, but the higher densities of the Uranus system compared to Saturn's moons closer to the Sun indicate a relative scarcity of water. There may have been significant amounts of nitrogen (N 2 ) and carbon (C) in the form of carbon monoxide (CO) and molecular nitrogen instead of ammonia (NH 3 ) and methane (CH 4 ). Satellites that emerged from such a sub-nebula should contain less water ice and CO and N 2 than gas hydrate trapped in ice and more rock, which would explain the higher densities.

The accretion process may take several thousand years to complete the formation of Oberon. Models show that the impacts accompanying the accretion are likely to cause the outer shell of the moon to heat up to a temperature of around 230 K at a depth of up to 60 km. Once formed, this outer layer cooled while Oberon's interior heated up due to the decomposition of radioactive elements in the rock. The cooling outer shell contracted while the interior expanded . This created severe tension in the moon's crust, which resulted in fractures on the crust and the formation of the canyons. This process, which lasted about 200 million years, indicates that the endogenous formation of the surface must have been completed billions of years ago.

The initial heat of accretion and the subsequent decomposition of radioactive elements may have melted water ice if a substance that depresses the freezing point, such as salt or ammonia, was present in the form of ammonium hydroxide . This should have led to a separation of ice and rock (differentiation) of the core. In this case, a layer of liquid water rich in dissolved ammonia would have formed at the boundary between the mantle and core. The eutectic temperature of this mixture is 176 K. If the temperature has fallen below this value, the existing ocean today should probably have frozen over long ago. This freezing over probably led to the expansion of the interior, which was probably responsible for the formation of the Chasmata. The knowledge of the development of Oberon is currently still very limited.


For 200 years since the discovery by William Herschel in 1787, little was known about Oberon other than the orbit parameters. The moon was too small and too far away to be resolved with terrestrial telescopes.

On January 20, 1986 Oberon has a distance of minimally 470,600 km from the Voyager 2 - spacecraft happened and are photographed and measured. As a result of the high inclination of the planetary system of 98 °, the axes of rotation of Uranus and Oberon pointed towards Earth at this point in time, so that the moons of Uranus could not be approached individually on the equatorial plane, as was previously the case with Jupiter and Saturn, but instead Arranged orbits like a target around the planet and the planet had to be hit. This meant that of Uranus and all of its moons, only the southern hemisphere could be photographed at intervals of about two days - the worst possible position for a flyby. In addition, you had to choose a moon, as a close pass by inevitably required large distances to all others.

Since Voyager 2 was to be steered further to Neptune , the prerequisite for this was a close Uranus flyby. As a result, only the moon near Miranda could be passed. As a result, the best resolution of the photos was about 6 km; they show about 40% of the surface, whereby only about 25% could be used with the necessary quality for geological maps and crater counting.

Web links

Commons : Oberon (moon)  - Collection of images, videos and audio files

Individual evidence

  1. Information table and brief description on solarviews.com (English)
  2. ^ William Herschel: An Account of the Discovery of Two Satellites Revolving Round the Georgian Planet. By William Herschel, LLD. FRS Phil. Trans. R. Soc. Lond. January 1, 1787 77: 125-129; doi : 10.1098 / rstl.1787.0016 ( full text )
  3. Example of a private website with a collection of symbol designs : Denis Moskowitz: Astronomical / Astrological symbols for other planets' moons. April 13, 2014, accessed May 19, 2015 .
  4. Uranus and its moons at JPL
  5. Uranus' Moons. ORACLE ThinkQuest, September 5, 2001, archived from the original on May 22, 2009 ; accessed on February 8, 2010 (English).
  6. ^ Moons of Saturn, Uranus, & Neptune. Laboratory for Atmospheric and Space Physics, accessed February 9, 2010 .
  7. Oberon. seasky, accessed February 12, 2010 .
  8. ^ Oberon (moon) in the Gazetteer of Planetary Nomenclature of the IAU (WGPSN) / USGS
  9. Calvin J. Hamilton: Oberon. solarviews, accessed on February 12, 2010 .