William Herschel. © National Portrait Gallery, London
Oberon is the outermost and one among the 27 moons of Uranus. William Herschel discovered it in the year 1787 and it is the second largest as well as the second most massive moon of Uranus. He discovered Oberon and Titania on the same day.
In the entire solar system it is the ninth most massive moon.
Much like all the other moons of Uranus, Oberon was also named in 1852 after a character in Shakespeare’s play, A Midsummer Night’s Dream. Though initially named as Uranus II according to the order in which it was discovered, it was later renamed as Uranus IV according to its distance from the planet.
Physical structure of Oberon
Oberon is assumed to have formed from the diffuse material around Uranus after the planet was formed. The structure of Oberon is that of a rocky core covered by a mantle made of ice. However, the moon consists of equal amounts of rock and ice. It is assumed that a layer of liquid water may be present between the core and the mantle.
The surface of Oberon is slightly dark and red in color and is covered by a number of craters that may have been created by the impact of asteroids and comets. Some of the craters are almost 210 km in diameter. The surface of Oberon also has many deep elongated depressions called chasmata.
Oberon and Uranus. © NASA
These are assumed to have been formed when the interior of the moon was still expanding during early years of its evolution. It is believed that it is remained stable from the days its formation.
The radius of Oberon is about 763 km and has a surface area of 7.2 million km. The density of Oberon is about 1.63 g/cm3 and has a surface gravity of about 0.348 m/s2. The temperature in the moon varies between 70 and 80 degrees Kelvin.
The Oberon’s orbit is at a distance of 584,000 km from Uranus. The orbital period of the moon is 13.5 days. This coincides with its rotational period. As a result, one face of the moon is tidally locked with Uranus, that is, it always faces the planet. Oberon spends a major part of its orbit outside the magnetosphere of Uranus. This is a reason why its surface is struck by solar wind directly. For those satellites that orbit inside the magnetosphere, the trailing hemispheres of these moons are continuously struck by magnetospheric plasma which leads to darkening of these hemispheres unlike that of Oberon.
Uranus orbits the sun almost on its side. The moons of Uranus orbit the planet on its equatorial plane. This causes the satellites to experience extreme seasonal cycles. The north and south poles of Oberon have 42 years of continuous darkness and the next 42 years of continuous sunlight. During each solstice, the sun rises close to the zenith of one of the poles. During the summer solstice, the northern hemisphere is left totally darkened.
Oberon’s internal structure
Oberon’s density is indicative of the fact that it contains equal proportions of water ice as well as a non-ice dense component. The non-ice material is believed to be made up of carbonaceous material, organic compounds, and rock. Spectroscopic findings have corroborated the presence of water ice on the surface of Oberon.Internal structure of Oberon. © NASA images
Whereas other Uranian moons exhibit more water ice presence in their leading hemispheres, Oberon exhibits stronger signatures in its trailing hemisphere. This discrepancy is seen as the result of formation of soil due to impacts, which is more predominant on its leading hemisphere. Impacts from meteorites generally knock off ice from the surface and leave behind material that is non ice and dark. This darkening of this material is assumed to have been caused by radiation processing of compounds of an organic nature. The radius of the core of Oberon is about 480 km. The pressure at the center of the core is about 5 kbar. The exact composition of the icy mantle is unclear. In case the mantle contains a sufficient quantity of antifreeze, there is every chance that there is an ocean that exists between the icy mantle and the rocky core. The thickness of the ocean can be up to 40 km and temperature about 180 degrees Kelvin.
Oberon’s geology and surface characteristics
Oberon is a dark moon of Uranus. The surface of Oberon is red in color and shows tinges of blue where there have been fresh impacts and deposits thereof.
Oberon’s geology and surface characteristics. © NASA
The leading hemisphere of Oberon is redder than its trailing hemisphere. Meteorites of a very small size and charged particles that bombard the surface are seen to have caused the reddening of the moon’s surface. Another theory has it that the reddening may have been caused by reddish material accretion from the outer sections including irregular satellites that are seen to impact the leading hemisphere more. Oberon’s surface is seen to be made up of primarily craters and chasmata. Oberon has the most number of craters when compared to the other moons of Uranus. These high numbers are indicative of the fact that the surface of Oberon is the most ancient one when compared with that of the other moons. The largest crater is Hamlet which is about 206 km in diameter. The craters are surrounded by rays of fresh ice. All known craters (Othello, Macbeth, and Hamlet) have floors that are dark due to deposition of material. Some of the basins formed by impacts have large peaks at the center. Voyager imager shows one such peak that is about 11 km in height.
Oberon’s surface also has a number of chasmata or canyons that intersect the landscape. The more recent ones are brighter. Mommur Chasma is the most prominent canyon in Oberon. These canyons are cracks on the crust of ice. Tectonic activity for a period after the formation of the moon has been responsible for the formation of ice. The cracking of the crust is believed to have taken place in two phases giving rise to old and comparatively new canyons.
As regards the nature of the dark patches seen at the bottom of the craters, some scientists believe that they are of volcanic origin, whereas others feel that high impacts may have brought out dark material from below the ice.
Voyager 2. © NASA
Formed form the accretion of dust and debris, the comparatively high density of Oberon indicates that it may have been poor in water content and containing more rocky material. The accretion is assumed to have taken place over many thousand years. The impacts caused the outer layer of the moon to heat up. After the moon was formed, the layer below the surface cooled but the core remained heated due to radioactive decay. The outer layer contracted while the inner core expanded for about 200 million years. These stresses are assumed to have created the canyons. Any such activity caused by the above mentioned happening are believed to have ceased billions of years ago.
Pictures of Oberon have been gathered by Voyager 2 probe during its flight past Uranus in January 1986. The pictures were shot from a distance of 470,600 km. There have been no pictures of the darkened northern hemisphere of the moon during Voyager’s flight.