Umbriel
| Umbriel | |
|---|---|
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| Umbriel captured by Voyager 2 | |
| Central body | Uranus |
| Properties of the orbit | |
| Major semi-axis | 266,300 km |
| Periapsis | 265,100 km |
| Apoapsis | 267,500 km |
| eccentricity | 0.0039 |
| Orbit inclination | 0.128 ° |
| Orbital time | 4.144177 d |
| Mean orbital velocity | 4.67 km / s |
| Physical Properties | |
| Albedo | 0.16 |
| Apparent brightness | 14.0 mag |
| Medium diameter | 1169.4 km |
| Dimensions | 1.172 ± 0.135 x 10 21 kg |
| surface | 4,296,000 km 2 |
| Medium density | 1.39 ± 0.16 g / cm 3 |
| Sidereal rotation | 4.144177 days |
| Axis inclination | 0.0 ° |
| Acceleration of gravity on the surface | 0.23 m / s 2 |
| Escape speed | 520 m / s |
| Surface temperature | −188 ± 1 ° C to −198 ° C / 85–75 K |
| discovery | |
| Explorer | |
| Date of discovery | October 24, 1851 |
| Remarks | darkest large Uranus moon |
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| Size comparison between Uranus (left) and its largest moons, from left to right Puck, Miranda, Ariel, Umbriel, Titania and Oberon (photo montage to scale) | |
The moon Umbriel (also Uranus II ) is the sixteenth innermost of the 27 known and the third largest of the five large moons of the planet Uranus .
Discovery and naming
Umbriel was discovered on October 24, 1851 together with Ariel as the third and fourth moons of Uranus by the British astronomer William Lassell with a 60 cm reflector telescope at a self-built observatory in Liverpool .
The moon was named after the malevolent dark spirit in Alexander Pope's verse epic The Robbery . The name is based on the Latin word umber , which means shadow .
The names of the first four Uranus moons discovered ( Oberon , Titania , Umbriel and Ariel) were suggested by John Herschel , the son of Wilhelm Herschel , at the request of Lassell. Wilhelm Herschel was the discoverer of Oberon, Titania and Uranus himself.
According to the naming conventions of the IAU, names of dark, rather malicious characters from legend and literature are used for surface structures on Umbriel.
Track properties
Orbit
Umbriel orbits Uranus on a prograde , almost perfectly circular orbit at an average distance of around 266,300 km (approx. 10,419 Uranus radii) from its center, i.e. around 240,700 km above its cloud ceiling. The orbit eccentricity is 0.0039, the orbit is 0.128 ° inclined to the equator of Uranus .
The orbit of the next inner moon Ariel is on average 75,000 km away from Umbriel's orbit, that of the next outer moon Titania about 170,000 km.
Umbriel orbits Uranus in 4 days, 3 hours, 27 minutes and 37 seconds.
Umbriel's orbit is entirely in the magnetosphere of Uranus. The subsequent hemispheres of atmospheric moons like Umbriel are therefore under constant bombardment by 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. Umbriel also traps magnetospherically charged particles, which leads to an increased number of these particles in Umbriel's orbit and was observed by the Voyager 2 spacecraft .
Since Umbriel, 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 Umbriel 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 equinoxes , when the equatorial plane crosses with the direction of 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; Titania was covered by Umbriel on August 15, 2007 and December 8, 2007, and so was Ariel, which was covered by Umbriel on August 19, 2007. Umbriel, in turn, was covered by Oberon for six minutes on May 4, 2007.
At present, Umbriel has no orbital resonance with other moons. However, in his story he may have been in a 3: 1 resonance with Miranda , possibly responsible for the internal heating of this moon.
rotation
The rotation time is the same as the orbital time and Umbriel shows, like the earth's moon , a synchronous rotation , which thus also takes place within 4 days, 3 hours, 27 minutes and 37 seconds. Its axis of rotation is almost exactly perpendicular to its plane .
Physical Properties
size
Umbriel has a diameter of 1169.4 km. It is thus the third largest Uranus moon and slightly larger than its inner orbit neighbor Ariel, but it seems to have less mass than this. In terms of size, Umbriel is also comparable to Saturn's moon Dione and Pluto's large moon Charon .
The size of Umbriel's surface is about 4,296,000 km 2 , which corresponds almost exactly to the area of the European Union . With Voyager 2 of the surface could be explored in more detail during their flyby about 40%; - as with all Uranus moons mainly the southern hemisphere.
internal structure
Umbriel has an average density of 1.39 g / cm³. Based on the low density, it is assumed that Umbriel is composed of around 60% water ice . It also has shares of silicate rock and carbon compounds such as methane and the organic heavy tholine . The presence of water ice is supported by infrared spectroscopic studies that revealed crystalline water ice on Umbriel's surface. This seems to be stronger on Umbriel's leading hemisphere. The reason for this is unknown, but it appears to have come from the bombardment of charged particles from Uranus' magnetosphere, which is more represented in the following hemisphere due to the co-rotation of the plasma. These energetic particles tend to sputter water ice, decompose methane trapped in ice as gas hydrate , and darken other organic matter, resulting in carbon-rich deposits on the surface.
Apart from the water ice, only carbon dioxide (CO 2 ) could be detected spectroscopically unequivocally and this compound is mainly concentrated on the following hemisphere. Its origin has not yet been adequately clarified. It could be produced locally from carbonates or organic matter by the influence of the charged particles of Uranus' magnetosphere, or by solar ultraviolet radiation . The former hypothesis would explain the asymmetry in the distribution, since the following hemisphere is under greater influence from the magnetosphere. Another possible source is the outgassing of CO 2 trapped in water ice in Umbriel's interior. The release of CO 2 from the interior may be related to past geological activity of the moon.
Umbriel is possibly a differentiated body with a rock core and a coat of water ice. If so, the diameter of the core would be 634 km, which is 54% of the total diameter, and a core mass of 40% of the total mass - these parameters are dictated by the composition of the moon. The pressure in the center of Umbriel is about 2.4 kbar . According to previous studies, it is unlikely that an underground ocean like that on Jupiter's moon Europa could exist in the ice mantle of Umbriel .
surface
The surface of Umbriel is old and encrusted with few traces of geological activity. Apart from the ubiquitous impact craters from impacts , scientists have not yet been able to determine any other surface-changing features.
Umbriel has the darkest surface of all major Uranus moons with a geometric albedo of 0.16, i.e. That is, 16% of the incident sunlight is reflected. The surface shows great brightness effects depending on the lighting angle; the reflectivity of 0.26 at a phase angle of 0 ° drops rapidly to 0.19 at around 1 °. The spherical albedo is 0.10; For comparison, Ariel has a spherical albedo of 0.23. In general, the brightness of the surface appears relatively homogeneous; it mostly shows no strong variations in color or albedo.
In contrast to Oberon, another dark Uranus moon, the surface of Umbriel appears in a slightly bluish hue, while fresh impact deposits appear even a little bluer. There appears to be a minimal dichotomy between the leading and trailing hemispheres; a slight reddish coloration of the leading hemisphere - in contrast to its inner neighbor Ariel - possibly originates from the weathering by charged particles and impacts of micrometeorites since the formation of the solar system . The color asymmetry 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 maximum surface temperature of Umbriel is −188 ° C (85 K ); on average, however, it is only an estimated −198 ° C (75 K).
Crater area
There are far more and larger craters on Umbriel's surface than on Titania or Ariel - only Oberon has more of them. The diameters of the observed craters range from a few kilometers to 210 km for the largest, called Wokolo . All recognizable craters have central mountains , but no signs of radiation systems .
Near the equator is the most noticeable feature on its surface, presumably a crater with a 131 km ring of bright material. Its origin has not yet been fully clarified, but it can obviously also be linked to an impact event . When an asteroid hit , lighter ice could have been ejected from deeper crustal layers. The presumed impact crater is called Wunda and the bright ring filling it, which is at least 10 km wide, is called Fluorescent Cheerio .
Also near the equator are the two interesting craters Vuver with a particularly deep valley floor and Skynd , which is noticeable due to its unusually bright central mountain . Studies of models by Umbriel also show a possible large impact structure 400 km in diameter and approximately 5 km deep.
| Surname | Diameter (km) | Coordinates | Origin of name |
|---|---|---|---|
| Wokolo | 208.0 | 30 ° 00′S 1 ° 48′E / 30 ° S 1.8 ° E | Wokolo ( Bambara people in West Africa ) |
| Malingee | 164.0 | 22 ° 54′S 13 ° 54′E / 22.9 ° S 13.9 ° E | Malingee ( Australian Aboriginal mythology) |
| Wunda | 131.0 | 7 ° 54′S 273 ° 36′E / 7.9 ° S 273.6 ° E | Wunda ( Australian Aboriginal mythology) |
| Vuver | 98.0 | 4 ° 42′S 311 ° 36′E / 4.7 ° S 311.6 ° E | Vuver ( Finnish mythology ) |
| Gob | 88.0 | 12 ° 42′S 27 ° 48′E / 12.7 ° S 27.8 ° E | Gob ( paganism ) |
| Kanaloa | 86.0 | 10 ° 48′S 345 ° 42′E / 10.8 ° S 345.7 ° E | Kanaloa ( Hawaiian religion ) |
| Skynd | 72.0 | 1 ° 48′S 331 ° 42′E / 1.8 ° S 331.7 ° E | Skynd ( Danish folklore ) |
| Peri | 61.0 | 9 ° 12′S 4 ° 18′E / 9.2 ° S 4.3 ° E | Peri ( Persian mythology ) |
| Minepa | 58.0 | 42 ° 42′S 8 ° 12′E / 42.7 ° S 8.2 ° E | Minepa ( Makua people in Mozambique ) |
| Alberich | 52.0 | 33 ° 36′S 42 ° 12′E / 33.6 ° S 42.2 ° E | Alberich ( Germanic mythology ) |
| Setibos | 50.0 | 30 ° 48′S 346 ° 18′E / 30.8 ° S 346.3 ° E | Setibos (from Patagonia ) |
| Zlyden | 44.0 | 23 ° 18′S 326 ° 12′E / 23.3 ° S 326.2 ° E | Zlyden ( Slavic Mythology ) |
| Fin | 43.0 | 37 ° 24′S 44 ° 18′E / 37.4 ° S 44.3 ° E | Fin ( Danish folklore ) |
Other types of terrain
As with other Uranus moons, the surface is cut up by a system of canyons that generally run from northeast to southwest. However, they were not officially recognized by the IAU because 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 heavily cratered surface may have remained stable since the solar system was formed. The only signs of internal geological activity are the canyons and dark polygonal structures tens to hundreds of kilometers in diameter. These features were identified by the precise photometry of the Voyager images and are more or less uniformly distributed; like the canyons, they run in a northeast-southwest direction. Some of these polygons correspond to depressions several kilometers deep and may have been created by an early episode of tectonic activity.
At this point in 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 ejected by an impact or an eruption . On the other hand, Umbriel's crust could also consist entirely of this material, which would have prevented the formation of bright surface features such as radiation systems. Nevertheless, the existence of the Wunda crater and another bright spot near the South Pole seem to contradict the latter hypothesis .
Emergence
Umbriel was probably formed by an accretion disk or 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 contain CO and N 2 than gas hydrate enclosed in ice and more rock, which would explain the higher densities.
The accretion process may take several thousand years to complete the formation of Umbriel. Models show that the impacts accompanying accretion would cause the outer shell of the moon to heat up to a temperature of around 180 K to a depth of up to 3 km. Once formed, this outer layer cooled while Umbriel's interior heated up due to the decomposition of radioactive elements in the rock. The cooling outer shell contracted while the interior expanded . This caused strong stresses in the moon's crust with a pressure of up to an estimated 3 kbar , which led to fractures on the crust. 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 there must have been a layer of liquid water rich in dissolved ammonia at the boundary between the mantle and core. The eutectic temperature of this mixture is 176 K. However, this ocean has probably long since frozen over. Of all the Uranus moons, Umbriel is considered to be the least likely for endogenous surface renewal, although this was possible in its very early history.
exploration
For about 135 years since William Lassell's discovery in 1851, little was known about Umbriel other than orbit parameters. The moon was too small and too far away to be resolved with terrestrial telescopes. Due to its dark surface and its proximity to Uranus, it is the most difficult of all major Uranus moons to observe; a telescope opening of at least 40 cm is necessary.
On January 20, 1986 Umbriel was a relatively close distance of a minimum of 325,000 km from the Voyager 2 - spacecraft happens, photographed and measured. As a result of the high axis inclination of the planetary system of 98 °, the rotation axes of Uranus and Umbriel 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 rather each other Arranged orbits like a target around the planet and the planet had to be hit. This meant that of all of Uranus' moons, only the southern hemisphere could be photographed every 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 5.2 km; they show about 40% of the surface, whereby only about 20% could be used with the necessary quality for geological maps and crater counting.
Web links
- Polish moon page: Umbriel - description and further links (English)
- Umbriel ... A Shadowy World Gaurds Its Secrets - Description and various images
- Satellite Viewer - orbit simulation of Uranus' moons
- USGS - List of Named Structures on Umbriel
Individual evidence
- ↑ Categories for Naming Features on Planets and Satellites - IAU Working Group for Planetary System Nomenclature (WGPSN)