Umbriel (moon)

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Umbriel
A round spherical body with its left half illuminated. The surface is dark and has a low contrast. There are only a few bright patches. The terminator is slightly to the right from the center and runs from the top to bottom. A large crater with a bright ring on its floor can be seen at the top of the image near the terminator. A pair of large craters with bright central peaks can be seen along the terminator in the upper part of the body. The illuminated surface is covered by a large number of craters.
Umbriel as seen by Voyager 2 in 1986
Discovery
Discovered byWilliam Lassell
Discovery dateOctober 24, 1851
Designations
Pronunciation/ˈʌmbriəl/[note 1]
Uranus II
AdjectivesUmbrielian
Orbital characteristics
266 000 km[2]
Eccentricity0.0039[2]
4.144 d[2]
Inclination0.128° (to Uranus's equator)[2]
Satellite ofUranus
Physical characteristics
Mean radius
584.7 ± 2.8 km (0.092 Earths)[3]
4 296 000 km² (0.008 Earths)[note 2]
Volume837 300 000 km³ (0.0008 Earths)[note 3]
Mass1.172 ± 0.135 kg (2 Earths)[4]
Mean density
1.39 ± 0.16 g/cm³[4]
0.23 m/s² (~0.023 g)[note 4]
0.52 km/s[note 5]
presumed synchronous[5]
0[5]
Albedo0.26 (geometrical),
0.10 (bond)[6]
Surface temp. min mean max
solstice[7] 65 K ~75 K 85 K
15.3 (V-band, opposition)[8]
Atmosphere
Surface pressure
zero

Umbriel is a moon of Uranus discovered on October 24, 1851, by William Lassell. It was discovered at the same time as Ariel and named after a character in Alexander Pope's poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a rocky core and an icy mantle. The surface is the darkest among Uranian moons, and appears to have been shaped primarily by impacts. However, the presence of canyons suggests early endogenic processes, and the moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.

Covered by numerous impact craters reaching 210 km (130 mi) in diameter, Umbriel is the second most heavily cratered satellite of Uranus after Oberon. The most prominent surface feature is a ring of bright material on the floor of Wunda crater. This moon, like all moons of Uranus, probably formed from an accretion disk that surrounded the planet just after its formation. The Uranian system has been studied up close only once: by the spacecraft Voyager 2 in January 1986. It took several images of Umbriel, which allowed mapping of about 40% of the moon’s surface.

Discovery and name

Umbriel, along with another Uranian satellite Ariel, was discovered by William Lassell on October 24, 1851.[9][10] Although William Herschel, the discoverer of Titania and Oberon, claimed at the end of the 18th century that he had observed four additional moons of Uranus,[11] his observations were not confirmed and those four objects are now thought to be spurious.[12]

All Uranus's moons are named after characters created by William Shakespeare or Alexander Pope. The names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell.[13] Umbriel is the 'dusky melancholy sprite' in Alexander Pope's The Rape of the Lock,[14] and the name suggests the Latin umbra, meaning shadow. The moon is also designated Uranus II.[10]

Orbit

Umbriel orbits Uranus at the distance of about 266,000 km (165,000 mi), being the third farthest from the planet among its five major moons.[note 6] Umbriel's orbit has a small eccentricity and is inclined very little relative to the equator of Uranus.[2] Its orbital period is around 4.1 Earth days, coincident with its rotational period. In other words, Umbriel is a synchronous or tidally locked satellite, with one face always pointing toward the planet.[5] Umbriel's orbit lies completely inside the Uranian magnetosphere.[7] This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Umbriel ) are struck by magnetospheric plasma, which co-rotates with the planet.[15] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).[7] Umbriel also serves as a sink of the magnetospheric charged particles, which creates a pronounced dip in energetic particle count near the moon's orbit as observed by Voyager 2 in 1986.[16]

Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Umbriel) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in complete darkness, and another 42 years in continuous sunlight.[7] Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. In 2007–2008 a number of such events was observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007 as well as of Ariel by Umbriel on August 19, 2007.[17]

Currently Umbriel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in the 3:1 resonance with Miranda, which was partially responsible for the heating of that moon.[18]

Composition and internal structure

Umbriel is the third largest and fourth most massive of Uranian moons.[note 7] The moon's density is 1.39 g/cm3,[4] which indicates that it mainly consists of water ice, while a dense non-ice component constitutes around 40% of its mass.[20] The latter could be made of rock and carbonaceous material including heavy organic compounds known as tholins.[5] The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon.[7] Water ice absorption bands are stronger on Umbriel's leading hemisphere than on the trailing hemisphere.[7] The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere (due to the plasma's co-rotation).[7] The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.[7]

Except for water, the only other compound identified on the surface of Umbriel by the infrared spectroscopy is carbon dioxide, which is concentrated mainly on the trailing hemisphere.[7] The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the energetic charged particles coming from the magnetosphere of Uranus or the solar ultraviolet radiation. This hypothesis would explain the asymmetry in its distribution, as the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the outgassing of the primordial CO2 trapped by water ice in Umbriel's interior. The escape of CO2 from the interior may be related to the past geological activity on this moon.[7]

Umbriel may be differentiated into a rocky core surrounded by an icy mantle.[20] If this is the case, the radius of the core (317 km) is about 54% of the radius of the moon, and its mass is around 40% of the moon’s mass—the parameters are dictated by the moon's composition. The pressure in the center of Umbriel is about 0.24 GPa (2.4 kbar).[20] The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.[20]

Surface features

A spherical blueish body with its surface covered by craters and polygons. The lower right part is smooth.
The map of Umbriel showing polygons

Umbriel's surface is the darkest of the Uranian moons, and reflects less than half as much light as Ariel, a sister satellite of similar size.[19] Umbriel has a very low bond albedo of only about 10% as compared to 23% for Ariel.[6] The reflectivity of the moon's surface decreases from 26% at a phase angle of 0° (geometrical albedo) to 19% at an angle of about 1°. This phenomenon is called opposition surge. Opposite to what is observed for another dark Uranian moon, Oberon, the surface of Umbriel is slightly blue in color,[21] while fresh bright impact deposits (in Wunda crater, for instance)[22] are even bluer. There may be an asymmetry between the leading and trailing hemispheres; the former appears to be redder than the latter.[23] The reddening of the surfaces probably results from space weathering from bombardment by charged particles and micrometeorites over the age of the Solar System.[21] On the other hand, the color asymmetry is more likely caused by accretion of a reddish material coming from outer parts of the Uranian system, possibly, from irregular satellites, which would occur predominately on the leading hemisphere.[23] The surface of Umbriel is relatively homogeneous—it does not demonstrate strong variation in either albedo or color.[21]

Scientists have so far recognized only one class of geological feature on Umbriel—craters.[24] The surface of Umbriel has far more and larger craters than do Ariel and Titania and is also the least geologically active.[22] In fact only Oberon has more impact craters than Umbriel. The observed crater diameters range from a few kilometers at the low end to 210 kilometers for the largest known crater, Wokolo.[22][24] All recognized craters on Umbriel have central peaks,[22] but no crater has rays.[5]

Named craters on Umbriel[24] [24] (Surface features on Umbriel are named for evil or dark spirits taken from various mythologies)[25]
Crater Named after Coordinate Diameter (km)
Alberich Alberich (Norse) 33°36′S 42°12′E / 33.6°S 42.2°E / -33.6; 42.2 52.0
Fin Fin (Danish) 37°24′S 44°18′E / 37.4°S 44.3°E / -37.4; 44.3 43.0
Gob Gob (Pagan) 12°42′S 27°48′E / 12.7°S 27.8°E / -12.7; 27.8 88.0
Kanaloa Kanaloa (Polynesian) 10°48′S 345°42′E / 10.8°S 345.7°E / -10.8; 345.7 86.0
Malingee Malingee(Australian Aboriginal mythology) 22°54′S 13°54′E / 22.9°S 13.9°E / -22.9; 13.9 164.0
Minepa Minepa (Makua people of Mozambique) 42°42′S 8°12′E / 42.7°S 8.2°E / -42.7; 8.2 58.0
Peri Peri (Persian) 9°12′S 4°18′E / 9.2°S 4.3°E / -9.2; 4.3 61.0
Setibos Setibos (Patagonian) 30°48′S 346°18′E / 30.8°S 346.3°E / -30.8; 346.3 50.0
Skynd Skynd (Danish) 1°48′S 331°42′E / 1.8°S 331.7°E / -1.8; 331.7 72.0
Vuver Vuver (Finnish) 4°42′S 311°36′E / 4.7°S 311.6°E / -4.7; 311.6 98.0
Wokolo Wokolo (Bambara people of West Africa) 30°00′S 1°48′E / 30°S 1.8°E / -30; 1.8 208.0
Wunda Wunda (Australian Aboriginal mythology) 7°54′S 273°36′E / 7.9°S 273.6°E / -7.9; 273.6 131.0
Zlyden Zlyden (Slavic) 23°18′S 326°12′E / 23.3°S 326.2°E / -23.3; 326.2 44.0

Near Umbriel's equator lies the most prominent surface feature: Wunda crater, which has a diameter of about 131 km.[26][27] Wunda has a large ring of bright material on its floor, which appears to be an impact deposit.[22] Nearby, seen along the terminator, are the craters Vuver and Skynd, which lack bright rims but possess bright central peaks.[5][27] Study of limb profiles of Umbriel revealed a possible very large impact feature having the diameter of about 400 km and depth of approximately 5 km.[28]

Much like other moons of Uranus, the surface of Umbriel is cut by a system of canyons trending northeast–southwest.[29] They are not, however, officially recognized due to the poor resolution of images and generally bland appearance of this moon, which hinders geological mapping.[22]

Umbriel's heavily cratered surface has probably been stable since the Late Heavy Bombardment.[22] The only signs of the ancient internal activity are canyons and dark polygons—dark patches with complex shapes measuring from tens to hundreds kilometers across.[30] The polygons were identified from the precise photometry of Voyager's images and are distributed more or less uniformly on the surface of Umbriel trending northeast–southwest. Some polygons correspond to depressions of a few kilometers deep and may have been created during an early episode of tectonic activity.[30] Currently there is no explanation of why Umbriel is so dark and uniform in appearance. Its surface may be covered by a relatively thin layer of dark material (so called umbral material) excavated by an impact or expelled in an explosive volcanic eruption.[note 8][23] Alternatively, Umbriel's crust may be entirely composed of the dark material, which prevented formation of bright features like crater rays. However, the presence of the bright Wunda feature seems to contradict this hypothesis.[5]

Origin and evolution

Umbriel is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity.[31] The precise composition of the subnebula is not known; however, the higher density of Uranian moons compared to the moons of Saturn indicates that it may have been relatively water-poor.[note 9][5] Significant amounts of nitrogen and carbon may have been present in the form of carbon monoxide (CO) and molecular nitrogen (N2) instead of ammonia and methane.[31] The moons that formed in such a subnebula would contain less water ice (with CO and N2 trapped as clathrate) and more rock, explaining the higher density.[5]

Umbriel's accretion probably lasted for several thousand years.[31] The impacts that accompanied accretion caused heating of the moon's outer layer.[32] The maximum temperature of around 180 K was reached at the depth of about 3 km.[32] After the end of formation, the subsurface layer cooled, while the interior of Umbriel heated due to decay of radioactive elements present in its rocks.[5] The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust, which may have led to cracking.[33] The present-day canyons may be a result of this process,[29] which lasted for about 200 million years, implying that any endogenous activity ceased billions years ago.[5]

The initial accretional heating together with continued decay of radioactive elements may have led to melting of the ice[32] if an antifreeze like ammonia (in the form of ammonia hydrate) or some salt was present.[20] The melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle.[22] A layer of liquid water (ocean) rich in dissolved ammonia may have formed at the core–mantle boundary. The eutectic temperature of this mixture is 176 K. The ocean, however, is likely to have frozen long ago.[20] Among Uranian moons Umbriel was least subjected to endogenic resurfacing processes,[22] although it may like other Uranian moons have experienced a very early resurfacing event.[30]

Exploration

Main article: Exploration of Uranus

So far the only close-up images of Umbriel have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest distance between Voyager 2 and Umbriel was 325,000 km (202,000 mi),[34] the best images of this moon have a spatial resolution of about 5.2 km.[22] The images cover about 40% of the surface, but only 20% was photographed with the quality required for geological mapping.[22] At the time of the flyby the southern hemisphere of Umbriel (like those of the other moons) was pointed towards the Sun, so the northern (dark) hemisphere could not be studied.[5] No other spacecraft has ever visited Uranus (and Umbriel), and no mission to this planet is planned.

Notes

  1. ^ In US dictionary transcription, Template:USdict.[1]
  2. ^ Surface area derived from the radius r: .
  3. ^ Volume v derived from the radius r: .
  4. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: .
  5. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: √2Gm/r.
  6. ^ The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.
  7. ^ Due to the current observational error, it is not yet known for certain whether Ariel is more massive than Umbriel.[19]
  8. ^ While a co-orbiting population of dust particles is another possible source of the dark material, this is considered less likely because other satellites were not affected.[5]
  9. ^ For instance, Tethys, a Saturnian moon, has the density of 0.97 g/cm3, which means that it contains more than 90% of water.[7]

References

  1. ^ "Umbriel". Dictionary.com. Retrieved 2010-01-14.
  2. ^ a b c d e Cite error: The named reference orbit was invoked but never defined (see the help page).
  3. ^ Cite error: The named reference Thomas1988 was invoked but never defined (see the help page).
  4. ^ a b c Cite error: The named reference Jacobson1992 was invoked but never defined (see the help page).
  5. ^ a b c d e f g h i j k l m Cite error: The named reference Smith1986 was invoked but never defined (see the help page).
  6. ^ a b Cite error: The named reference Karkoschka2001a was invoked but never defined (see the help page).
  7. ^ a b c d e f g h i j k Cite error: The named reference Grundy2006 was invoked but never defined (see the help page).
  8. ^ Cite error: The named reference Arlot2008 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference Lassell1851 was invoked but never defined (see the help page).
  10. ^ a b Cite error: The named reference Lassell1851b was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference Herschel3 was invoked but never defined (see the help page).
  12. ^ Cite error: The named reference Struve1848 was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference Lassell5 was invoked but never defined (see the help page).
  14. ^ Cite error: The named reference Kuiper1949 was invoked but never defined (see the help page).
  15. ^ Cite error: The named reference Ness1986 was invoked but never defined (see the help page).
  16. ^ Cite error: The named reference Krimigis1986 was invoked but never defined (see the help page).
  17. ^ Cite error: The named reference occultations was invoked but never defined (see the help page).
  18. ^ Cite error: The named reference Tittemore1990 was invoked but never defined (see the help page).
  19. ^ a b Cite error: The named reference JPLSSD was invoked but never defined (see the help page).
  20. ^ a b c d e f Cite error: The named reference Hassmann2006 was invoked but never defined (see the help page).
  21. ^ a b c Cite error: The named reference Bell1991 was invoked but never defined (see the help page).
  22. ^ a b c d e f g h i j k Cite error: The named reference Plescia1987 was invoked but never defined (see the help page).
  23. ^ a b c Cite error: The named reference Buratti1991 was invoked but never defined (see the help page).
  24. ^ a b c d Cite error: The named reference usgs was invoked but never defined (see the help page).
  25. ^ Cite error: The named reference Strobell1987 was invoked but never defined (see the help page).
  26. ^ Cite error: The named reference usgsWunda was invoked but never defined (see the help page).
  27. ^ a b Cite error: The named reference hunt was invoked but never defined (see the help page).
  28. ^ Cite error: The named reference Moore2004 was invoked but never defined (see the help page).
  29. ^ a b Cite error: The named reference Croft1989 was invoked but never defined (see the help page).
  30. ^ a b c Cite error: The named reference Helfenstein1989 was invoked but never defined (see the help page).
  31. ^ a b c Cite error: The named reference Mousis2004 was invoked but never defined (see the help page).
  32. ^ a b c Cite error: The named reference Squyres1988 was invoked but never defined (see the help page).
  33. ^ Cite error: The named reference Hillier1991 was invoked but never defined (see the help page).
  34. ^ Cite error: The named reference Stone1987 was invoked but never defined (see the help page).

External links



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