Umbriel (moon)

William Lassell

The moon Umbriel (also Uranus II ) is the second of the 27 well-known as well as the third largest of the five large moons of the planet Uranus.

• 3.1 size
• 3.2 Internal structure
• 3.3 Surface 3.3.1 crater terrain
• 3.3.2 Other types of terrain

Discovery and designation

Umbriel was discovered on October 24, 1851 along with Ariel as the third and fourth moons of Uranus by the British astronomer William Lassell with a 60 cm reflecting telescope on a home-built observatory in Liverpool.

The moon was named after the evil dark spirit in Alexander Pope's epic poem The Rape of the Lock. The name is inspired by the Latin word umbra, the shadow means.

The names of the first four moons discovered Uranus ( Oberon, Titania, Umbriel and Ariel ) were purchased from John Herschel, the son of William Herschel, proposed on demand by Lassell. William Herschel was the discoverer of Oberon, Titania and Uranus itself

After the naming conventions of the IAU names are used by dark, rather malignant figures from legend and literature for surface structures on Umbriel.

Web properties

Orbit

Umbriel Uranus orbits on a prograde, nearly perfectly circular orbit at an average distance of about 266,300 kilometers (about 10.419 Uranus radii ) from the center, or around 240,700 km on whose cloud tops. The orbital eccentricity is 0.0039, the orbit is inclined 0.128 degrees from the equator of Uranus.

The orbit of the next inner moon Ariel is on average 75,000 kilometers away from Umbriel's orbit, that of the next outer moon Titania about 170,000 km.

Umbriel Uranus rotates in 4 days, 3 hours, 27 minutes and 37 seconds.

Umbriel's orbit lies entirely in the magnetosphere of Uranus. The following hemispheres of airless moons Umbriel are as a result. Magnetosphärischem under constant attack from 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. Umbriel also catches a magneto- spherical charged particles, which leads to an increased number of these particles in Umbriel's orbit and could be observed by the Voyager 2 spacecraft.

Since Umbriel 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 Umbriel be 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 equinoxes, in which the equatorial plane intersects with the direction to the earth and which 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; Titania Umbriel was covered by 15 August 2007 and 8 December 2007, as well as Ariel, which was covered on August 19, 2007 by Umbriel. Umbriel was in turn covered on 4 May 2007 for six minutes from Oberon.

Currently has no orbital resonance with Umbriel other moons. In his history he was but possibly in a 3:1 resonance with Miranda, who was possibly responsible for the internal heating of this moon.

Rotation

The rotation period is equal to the orbital period and Umbriel has with how the Earth's moon, a synchronous rotation, which therefore also takes place within 4 days, 3 hours, 27 minutes and 37 seconds. Its axis of rotation is almost exactly perpendicular to its orbital plane.

Physical Properties

Size

Umbriel has a diameter of 1169.4 km. He is the third largest Uranus moon and slightly larger than the moon Ariel, but he seems to have less mass than this.

From the size Umbriel is best compared with Ariel, Saturn's moon Dione or the Pluto Charon. 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 ​​Umbriel is about 4.296 million km2, which corresponds almost exactly to the area of the European Union.

Internal construction

Umbriel has an average density of 1.39 g / cm ³. On the basis of low density, it is assumed that Umbriel is composed of about 60% of water ice. He also owns part of silicate rock and carbon compounds such as methane and the organic heavy Tholin. The presence of water ice is supported by infrared spekroskopische investigations that brought crystalline water ice on Umbriel's surface to light. This seems to be more present on Umbriel's leading hemisphere. The reason for this is unknown, but it seems from the bombardment of charged particles of Uranus ' magnetosphere to stem that has a stronger presence on the trailing hemisphere by the co- rotation of the plasma. These energetic particles tend to sputtering of water ice, the decomposition of methane trapped in ice as clathrate hydrate and the darkening of other organic material, resulting in carbon-rich deposits on the surface.

Apart from the water ice previously could only carbon dioxide ( CO2) can be spectroscopically proved beyond doubt and this compound is mainly focused on the following hemisphere. Its origin is not yet sufficiently clarified. It could be produced locally from carbonates or organic material through the influence of the charged particles of Uranus ' magnetosphere, or by solar ultraviolet radiation. The former hypothesis would explain the asymmetry in the distribution because the following hemisphere is under stronger influence of the magnetosphere. Another possible source is the outgassing of CO2 that is trapped in water ice in Umbriel's heart. The release of CO2 from the inside is possibly connected with the past geological activity of the moon.

Umbriel 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 634 km, which corresponds to 54 % of the total diameter, core, and a mass of 40 % of total mass -, these parameters are dictated by the composition of the moon. The pressure in the center of Umbriel is about 2.4 kbar. That could exist in the ice mantle of Umbriel an underground ocean as on the Jovian moon Europa, is considered by previous studies as unlikely.

Surface

The surface of Umbriel is old and crusty and has few traces of geological activity. Besides the ubiquitous impact craters by Impacts have so far not notice any other surface-altering characteristics scientists.

Umbriel has the darkest area of ​​all large moons of Uranus with a geometric albedo of 0.16, ie, 16 % of the incident sunlight is reflected. The surface shows depending on the lighting angle high brightness effects; the reflectivity of 0.26 at a phase angle of 0 ° to 0.19 decreases rapidly from about 1 °. The Bond albedo is 0.10; Ariel has, for comparison, a spherical albedo of 0.23. In general, the brightness of the surface appears to be relatively homogeneous; it shows mostly no strong variations in the color or albedo.

Unlike Oberon, another dark Uranus moon, the surface of Umbriel appears in a slightly bluish tint, while fresh Impaktablagerungen appear even slightly bluer. There seems to exist a minimal dichotomy of the leading and the trailing hemisphere; a slight reddish tint which - in contrast to the inner neighbors Ariel - leading hemisphere may be derived from the weathering by charged particles and micrometeorites impacts since the formation of the solar system. The color asymmetry was probably built by the accretion of a reddish material from the outer Uranus system, possibly irregular moons that would be reflected mainly on the leading hemisphere.

The maximum surface temperature of Umbriel is -188 ° C (85 K); on average there are only about an estimated -198 ° C (75 K).

Crater terrain

On Umbriel's surface there are far more and larger craters than on titania or Ariel - only Oberon has more of it. The diameters of the observed craters range from a few kilometers to 210 km for the largest, Wokolo called. All recognizable craters have central mountains, but no signs of radiation systems.

Near the equator, the most striking feature is on its surface, presumably a crater with a 131 km large ring of bright material. Its origin is not yet fully understood, but obviously it can be also associated with an impact event. When an asteroid impacts brighter ice could have been ejected from deeper crustal layers. The probable impact crater named Wunda and it fills bright ring, which is at least 10 km wide, Fluorescent cheerio is called.

Also close to the equator, the two interesting craters Vuver can be seen with a very deep valley and Skynd that really stand out for its unusually bright central peak. Studies of models of Umbriel also show a possible large impact structure of 400 km in diameter and a depth of about 5 km.

Other landforms

As with other Uranian moons, the surface is cut by a system of canyons that run generally from northeast to southwest. However, they were not officially recognized by the IAU, as the resolution of the images was too low. This, and the generally weak appearance of this moon prevented geological mapping of these structures.

Umbriel's hard cratered surface may remained stable since the formation of the solar system. The only signs of internal geological activity are the canyons and dark polygonal structures of several tens to hundreds of kilometers in diameter. These features were identified by the precise photometry of the Voyager images and are distributed more or less uniformly; they run like the canyons nordöslich -southwest direction. Some of these polygons correspond to lowering of a few kilometers depth, and may be caused by an early episode of tectonic activity.

At the present time there is no explanation for the dark and uniform appearance of Umbriel. Its surface may be covered by a relatively thin layer of dark material that was ejected by an impact or a rash. On the other hand, could Umbriel's crust and overall made ​​from this material, which would have prevented the formation of bright surface features such as radiation systems. Nevertheless, the existence of the crater Wunda and another bright spot near the South Pole seems to contradict the latter hypothesis.

Formation

Umbriel was probably shaped by an accretion disk or 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 Umbriel 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 180 K to a depth of up to 3 km. After the formation of this outer layer cooled, while Umbriel's affairs by the decomposition aufheizte radioactive elements in the rock. The cooling outer shell contracted while expanding the interior. This has caused severe stress in the crust of the moon at a pressure of up to 3 kbar estimated, leading to breakage of the crust. 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. This ocean is however probably already frozen over. Of all the Uranian moons Umbriel is considered to be the least likely for endogenous surface renewal, though this in its very early history was possible.

Research

Since the discovery in 1851 by William Lassell was about 135 years out of the orbital parameters over Umbriel not much is known. The moon was too small and too far away to resolve it closer with ground-based telescopes. He is observed by all the major Uranian moons hardest by its dark surface and its proximity to Uranus; it is necessary for a telescopic opening of at least 40 cm.

On 20 January 1986 Umbriel a relatively close distance of at least 325,000 km was passed by the Voyager 2 spacecraft and photographed and measured. The rotation axis of Uranus and Umbriel 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 in each case 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 5.2 km; they show about 40 % of the surface, with only about 20 % could be used with the necessary quality for geological mapping and crater counting.