Grayscale image of Umbriel fromVoyager 2, January 1986. Umbriel's surface is heavily battered; the bright crater Wunda can be seen at the top of the image near the moon's equator
Umbriel (/ˈʌmbriəl/) is the third-largestmoon of Uranus. It was discovered on October 24, 1851, byWilliam Lassell at the same time as neighboring moonAriel. It was named after a character inAlexander Pope's 1712 poemThe Rape of the Lock. Umbriel consists mainly ofice with a substantial fraction ofrock, and may bedifferentiated into a rockycore and an icymantle. The surface is the darkest among Uranian moons and appears to have been shaped primarily by impacts, However, the presence of canyons suggests early internal processes. The moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.
Covered by numerousimpact craters reaching 210 km (130 mi) in diameter, Umbriel is the second-most heavily cratered satellite of Uranus afterOberon. The most prominent surface feature is a ring of bright material on the floor ofWunda crater. This moon, like all regular moons of Uranus, probably formed from anaccretion disk that surrounded the planet just after its formation. Umbriel has been studied up close only once, by the spacecraftVoyager 2 in January 1986. It took several images of Umbriel, which allowed mapping of about 40% of the moon's surface.
Umbriel, along with another Uranian satellite,Ariel, was discovered byWilliam Lassell on October 24, 1851.[10][11][12] AlthoughWilliam Herschel, the discoverer ofTitania andOberon, claimed at the end of the 18th century that he had observed four additional moons of Uranus,[13] his observations were not confirmed and those four objects are now thought to be spurious.[14]
All of Uranus's moons are named after characters created byWilliam Shakespeare orAlexander Pope. The names of all four satellites of Uranus then known were suggested byJohn Herschel (son of William) in 1852 at the request of Lassell,[15] though it is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission.[16] Umbriel is the "dusky melancholy sprite" in Alexander Pope'sThe Rape of the Lock,[17] and the name suggests theLatinumbra, meaning'shadow'. The moon is also designatedUranus II.[12]
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a U (the initial of Umbriel) combined with the low globe ofJérôme Lalande's Uranus symbol as the symbol of Umbriel (). This symbol is not widely used.[18]
Umbriel orbits Uranus at the distance of about 266,000 km (165,000 mi), being the third farthest from the planet among itsfive major moons.[e] Umbriel's orbit has a smalleccentricity and isinclined very little relative to theequator of Uranus.[2] Itsorbital period is around 4.1 Earth days, coincident with itsrotational period, making it asynchronous ortidally locked satellite, with one face always pointing toward its parent planet.[6] Umbriel's orbit lies completely inside theUranian magnetosphere.[8] This is important, because the trailinghemispheres of airless satellites orbiting inside a magnetosphere (like Umbriel) are struck by magnetosphericplasma, which co-rotates with the planet.[19] This bombardment may lead to the darkening of the trailing hemispheres, which is observed for all Uranian moons exceptOberon (see below).[8] 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 byVoyager 2 in 1986.[20]
Because Uranus orbits theSun almost on its side, and its moons orbit in the planet's equatorial plane, Umbriel and the other moons are subject to an extreme seasonal cycle. Both northern and southernpoles spend 42 years in complete darkness, and another 42 years in continuous sunlight, with the Sun rising close to thezenith over one of the poles at eachsolstice.[8] TheVoyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was unilluminated. Once every 42 years, when Uranus has anequinox and its equatorial plane intersects the Earth, mutualoccultations of Uranus's moons become possible. In 2007–2008, several such events were 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.[21][22]
Currently, Umbriel is not involved in anyorbital resonance with other Uranian satellites. Early in its history however, it may have been in a 1:3 resonance withMiranda. This would have increased Miranda's orbital eccentricity, contributing to the internal heating and geological activity of that moon, while Umbriel's orbit would have been less affected.[23] Due to Uranus's loweroblateness and smaller size relative to its satellites, its moons can escape more easily from a mean motion resonance than those ofJupiter orSaturn. After Miranda escaped from this resonance (through a mechanism that probably resulted in its anomalously high orbital inclination), its eccentricity would have been damped, turning off the heat source.[24][25]
Umbriel is the third-largest and third-most massive of the Uranian moons. Umbriel is the 13th-largest moon in the Solar System, and it is also the 13th-most massive. The moon's density is 1.54 g/cm3, which indicates that it mainly consists ofwater ice, with a dense non-ice component constituting around 40% of its mass.[26] The latter could be made ofrock andcarbonaceous material including heavyorganic compounds known astholins.[6] The presence of water ice is supported byinfraredspectroscopic observations, which have revealedcrystalline water ice on the surface of the moon.[8] Water iceabsorption bands are stronger on Umbriel's leading hemisphere than on the trailing hemisphere.[8] 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).[8] The energetic particles tend tosputter water ice, decomposemethane trapped in ice asclathrate hydrate and darken other organics, leaving a dark, carbon-richresidue behind.[8]
Except for water, the only other compound identified on the surface of Umbriel by the infrared spectroscopy iscarbon dioxide, which is concentrated mainly on the trailing hemisphere.[8] The origin of the carbon dioxide is not completely clear. It might be produced locally fromcarbonates or organic materials under the influence of the energetic charged particles coming from the magnetosphere of Uranus or the solarultraviolet 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 theoutgassing of theprimordial CO2 trapped by water ice in Umbriel's interior. The escape of CO2 from the interior may be a result of past geological activity on this moon.[8]
Umbriel may be differentiated into a rockycore surrounded by an icymantle.[26] 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).[26] The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.[26]
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.[27] Umbriel has a very lowBond albedo of only about 10% as compared to 23% for Ariel.[7] The reflectivity of the moon's surface decreases from 26% at a phase angle of 0° (geometric albedo) to 19% at an angle of about 1°. This phenomenon is calledopposition surge. The surface of Umbriel is slightly blue in color,[28] while fresh bright impact deposits (inWunda crater, for instance)[29] are even bluer. There may be an asymmetry between the leading and trailing hemispheres; the former appears to be redder than the latter.[30] The reddening of the surfaces probably results fromspace weathering from bombardment by charged particles andmicrometeorites over the age of theSolar System.[28] However, the color asymmetry of Umbriel is likely caused by accretion of a reddish material coming from outer parts of the Uranian system, possibly, fromirregular satellites, which would occur predominately on the leading hemisphere.[30] The surface of Umbriel is relatively homogeneous—it does not demonstrate strong variation in either albedo or color.[28]
Scientists have so far recognized only one class of geological feature on Umbriel—craters.[31] The surface of Umbriel has far more and larger craters than do Ariel andTitania. It shows the least geological activity.[29] In fact, among the Uranian moons 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.[29][31] All recognized craters on Umbriel have central peaks,[29] but no crater hasrays.[6]
Near Umbriel's equator lies the most prominent surface feature: Wunda crater, which has a diameter of about 131 km.[32][33] Wunda has a large ring of bright material on its floor, which may be an impact deposit[29] or a deposit of purecarbon dioxide ice, which formed when the radiolytically formed carbon dioxide migrated from all over the surface of Umbriel and then got trapped in relatively cold Wunda.[34] Nearby, seen along theterminator, are the cratersVuver andSkynd, which lack bright rims but possess bright central peaks.[6][33] 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.[35]
Much like other moons of Uranus, the surface of Umbriel is cut by a system of canyons trending northeast–southwest.[36] They are not officially recognized due to the poor imaging resolution and generally bland appearance of this moon, which hindersgeological maping.[29]
Umbriel's heavily cratered surface has probably been stable since theLate Heavy Bombardment.[29] The only signs of the ancient internal activity are canyons and dark polygons—dark patches with complex shapes measuring from tens to hundreds of kilometers across.[37] The polygons were identified from precise photometry ofVoyager 2'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.[37] Currently there is no explanation for why Umbriel is so dark and uniform in appearance. Its surface may be covered by a relatively thin layer of dark material (so calledumbral material) excavated by an impact or expelled in an explosive volcanic eruption.[f][30] 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 feature within Wunda seems to contradict this hypothesis.[6]
Umbriel is thought to have formed from anaccretion 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 largeobliquity.[39] The precise composition of the subnebula is not known, but the higher density of Uranian moons compared to themoons of Saturn indicates that it may have been relatively water-poor.[h][6] Significant amounts ofnitrogen andcarbon may have been present in the form ofcarbon monoxide (CO) and molecular nitrogen (N2) instead ofammonia and methane.[39] 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.[6]
Umbriel's accretion probably lasted for several thousand years.[39] The impacts that accompanied accretion caused heating of the moon's outer layer.[40] The maximum temperature of around 180 K was reached at the depth of about 3 km.[40] After the end of formation, the subsurface layer cooled, while the interior of Umbriel heated due to decay ofradioactive elements in its rocks.[6] The cooling near-surface layer contracted, while the interior expanded. This caused strongextensional stresses in the moon's crust, which may have led to cracking.[41] This process probably lasted for about 200 million years, implying that any endogenous activity ceased billions of years ago.[6]
The initialaccretional heating together with continued decay of radioactive elements may have led to melting of the ice[40] if anantifreeze like ammonia (in the form ofammonia hydrate) or some salt was present.[26] The melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle.[29] A layer of liquid water (ocean) rich in dissolved ammonia may have formed at the core–mantle boundary. Theeutectic temperature of this mixture is 176 K. The ocean is likely to have frozen long ago.[26] Among Uranian moons Umbriel was least subjected to endogenic resurfacing processes,[29] although it may, like other Uranian moons, have experienced a very early resurfacing event.[37]
The only close-up images of Umbriel have been from theVoyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest distance betweenVoyager 2 and Umbriel was 325,000 km (202,000 mi),[42] the best images of this moon have a spatial resolution of about 5.2 km.[29] The images cover about 40% of the surface, but only 20% was photographed with enough quality forgeological mapping.[29] 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.[6]
^The five major moons areMiranda,Ariel, Umbriel, Titania and Oberon.
^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.[6]
^Surface features on Umbriel are named for evil or dark spirits taken from various mythologies.[38]
^For instance,Tethys, a Saturnian moon, has a density of 0.97 g/cm3, which suggests that over 90% of its composition is water.[8]
^Krimigis, S. M.; Armstrong, T. P.; Axford, W. I.; Cheng, A. F.; Gloeckler, G.; Hamilton, D. C.; Keath, E. P.; Lanzerotti, L. J.; Mauk, B. H. (July 4, 1986). "The Magnetosphere of Uranus: Hot Plasma and Radiation Environment".Science.233 (4759):97–102.Bibcode:1986Sci...233...97K.doi:10.1126/science.233.4759.97.PMID17812897.S2CID46166768.
^abcBell, J. F. III; McCord, T. B. (1991).A search for spectral units on the Uranian satellites using color ratio images. Lunar and Planetary Science Conference, 21st, Mar. 12–16, 1990 (Conference Proceedings). Houston, TX, United States: Lunar and Planetary Sciences Institute. pp. 473–489.Bibcode:1991LPSC...21..473B.
^Sori, Michael M.; Bapst, Jonathan; Bramson, Ali M.; Byrne, Shane; Landis, Margaret E. (2017). "A Wunda-full world? Carbon dioxide ice deposits on Umbriel and other Uranian moons".Icarus.290:1–13.Bibcode:2017Icar..290....1S.doi:10.1016/j.icarus.2017.02.029.
^Croft, S. K. (1989).New geological maps of Uranian satellites Titania, Oberon, Umbriel and Miranda.Proceedings of Lunar and Planetary Sciences. Vol. 20. Lunar and Planetary Sciences Institute, Houston. p. 205C.Bibcode:1989LPI....20..205C.
^Strobell, M. E.; Masursky, H. (March 1987). "New Features Named on the Moon and Uranian Satellites".Abstracts of the Lunar and Planetary Science Conference.18:964–965.Bibcode:1987LPI....18..964S.
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