Umbriel (moon)

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Umbriel
synchronous[6]
0[6]
Albedo
  • 0.26 (geometrical)
  • 0.10 (Bond)[7]
Surface temp. min mean max
solstice[8] ? ≈ 75 K 85 K
15.1[9]
Atmosphere
Surface pressure
zero (presumed to be extremely low)

Umbriel (

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.[10][11] 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,[12] his observations were not confirmed and those four objects are now thought to be spurious.[13]

All of Uranus's moons are named after characters created by

umbra, meaning shadow. The moon is also designated Uranus II.[11]

Orbit

Umbriel orbits Uranus at the distance of about 266,000 km (165,000 mi), being the third farthest from the planet among its

Uranian magnetosphere.[8] 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.[17] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (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 by Voyager 2 in 1986.[18]

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, with the Sun rising close to the zenith over one of the poles at each solstice.[8] The Voyager 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 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 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.[19][20]

Currently, Umbriel is not involved in any

oblateness and smaller size relative to its satellites, its moons can escape more easily from a mean motion resonance than those of Jupiter or Saturn. 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.[22][23]

Composition and internal structure

Size comparison of Earth, the Moon, and Umbriel.

Umbriel is the third-largest and fourth-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 of

magnetosphere of Uranus, which is stronger on the trailing hemisphere (due to the plasma's co-rotation).[8] 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.[8]

Except for water, the only other compound identified on the surface of Umbriel by the infrared spectroscopy is

primordial 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 rocky

kbar).[24] The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.[24]

Surface features

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.

irregular satellites, which would occur predominately on the leading hemisphere.[28] The surface of Umbriel is relatively homogeneous—it does not demonstrate strong variation in either albedo or color.[26]

Scientists have so far recognized only one class of geological feature on Umbriel—craters.[29] The surface of Umbriel has far more and larger craters than do Ariel and Titania. It shows the least geological activity.[27] 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.[27][29] All recognized craters on Umbriel have central peaks,[27] but no crater has rays.[6]

Near Umbriel's equator lies the most prominent surface feature: Wunda crater, which has a diameter of about 131 km.[30][31] Wunda has a large ring of bright material on its floor, which may be an impact deposit[27] or a deposit of pure carbon 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.[32] Nearby, seen along the terminator, are the craters Vuver and Skynd, which lack bright rims but possess bright central peaks.[6][31] 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.[33]

Much like other moons of Uranus, the surface of Umbriel is cut by a system of canyons trending northeast–southwest.[34] They are not, however, officially recognized due to the poor imaging resolution and generally bland appearance of this moon, which hinders geological mapping.[27]

Umbriel's heavily cratered surface has probably been stable since the

umbral material) excavated by an impact or expelled in an explosive volcanic eruption.[f][28] 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]

Named craters on Umbriel[29][g]
Crater Coordinates Diameter (km) Approved Named after Ref
Alberich 33°36′S 42°12′E / 33.6°S 42.2°E / -33.6; 42.2 52.0 1988 Alberich (Norse) WGPSN
Fin 37°24′S 44°18′E / 37.4°S 44.3°E / -37.4; 44.3 43.0 1988
Fin (Danish
) 		WGPSN
Gob 12°42′S 27°48′E / 12.7°S 27.8°E / -12.7; 27.8 88.0 1988 Gob (Pagan) WGPSN
Kanaloa 10°48′S 345°42′E / 10.8°S 345.7°E / -10.8; 345.7 86.0 1988 Kanaloa (Polynesian) WGPSN
Malingee 22°54′S 13°54′E / 22.9°S 13.9°E / -22.9; 13.9 164.0 1988 Malingee (
Australian Aboriginal mythology
) 		WGPSN
Minepa 42°42′S 8°12′E / 42.7°S 8.2°E / -42.7; 8.2 58.0 1988
Makua people of Mozambique
) 		WGPSN
Peri 9°12′S 4°18′E / 9.2°S 4.3°E / -9.2; 4.3 61.0 1988 Peri (Persian) WGPSN
Setibos 30°48′S 346°18′E / 30.8°S 346.3°E / -30.8; 346.3 50.0 1988
Setebos (Patagonian
) 		WGPSN
Skynd 1°48′S 331°42′E / 1.8°S 331.7°E / -1.8; 331.7 72.0 1988 Skynd (Danish) WGPSN
Vuver 4°42′S 311°36′E / 4.7°S 311.6°E / -4.7; 311.6 98.0 1988 Vuver (Finnish) WGPSN
Wokolo 30°00′S 1°48′E / 30°S 1.8°E / -30; 1.8 208.0 1988 Wokolo (Bambara people of West Africa) WGPSN
Wunda 7°54′S 273°36′E / 7.9°S 273.6°E / -7.9; 273.6 131.0 1988 Wunda (Australian Aboriginal mythology) WGPSN
Zlyden 23°18′S 326°12′E / 23.3°S 326.2°E / -23.3; 326.2 44.0 1988
Slavic
) 		WGPSN

Origin and evolution

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

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.[37] 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.[h][6] 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.[37] 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.

radioactive elements present in its rocks.[6] 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.[39] This process probably lasted for about 200 million years, implying that any endogenous activity ceased billions of years ago.[6]

The initial

eutectic temperature of this mixture is 176 K. The ocean, however, is likely to have frozen long ago.[24] Among Uranian moons Umbriel was least subjected to endogenic resurfacing processes,[27] although it may like other Uranian moons have experienced a very early resurfacing event.[35]

Exploration

The Voyager 2 spacecraft
Further information: Exploration of Uranus

So far, the only close-up images of Umbriel have been from the

geological mapping.[27] 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]
No other spacecraft has ever visited Uranus or its moons.

See also

Notes

  1. ^ Surface area derived from the radius r : .
  2. ^ Volume v derived from the radius r : .
  3. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r : .
  4. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r : .
  5. ^ The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.
  6. ^ 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]
  7. ^ Surface features on Umbriel are named for evil or dark spirits taken from various mythologies.[36]
  8. ^ 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]

References

  1. ^ "Umbriel". Merriam-Webster.com Dictionary.
  2. ^ a b "Planetary Satellite Mean Orbital Parameters". Jet Propulsion Laboratory, California Institute of Technology.
  3. ^ Thomas, P. C. (1988). "Radii, shapes, and topography of the satellites of Uranus from limb coordinates". Icarus. 73 (3): 427–441.
    doi:10.1016/0019-1035(88)90054-1
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  5. ^ Jacobson (2023), as cited in French et al. (2024)[4]
  6. ^ a b c d e f g h i j k l m Smith, B. A.; Soderblom, L. A.; Beebe, A.; Bliss, D.; Boyce, J. M.; Brahic, A.; Briggs, G. A.; Brown, R. H.; Collins, S. A. (July 4, 1986). "Voyager 2 in the Uranian System: Imaging Science Results". Science. 233 (4759): 43–64.
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  7. ^ a b
    doi:10.1006/icar.2001.6596
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  8. ^ a b c d e f g h i j k Grundy, W. M.; Young, L. A.; Spencer, J. R.; Johnson, R. E.; Young, E. F.; Buie, M. W. (October 2006). "Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations". Icarus. 184 (2): 543–555.
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  9. ^ Andy Wilson (December 1, 2021). "Observer's Challenge – The Moons of Uranus". British Astronomical Association. Retrieved March 1, 2023.
  10. ^ Lassell, W. (1851). "On the interior satellites of Uranus".
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  13. ^ Struve, O. (1848). "Note on the Satellites of Uranus". Monthly Notices of the Royal Astronomical Society. 8 (3): 44–47.
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  14. ^ Lassell, W. (1852). "Beobachtungen der Uranus-Satelliten". Astronomische Nachrichten (in German). 34: 325.
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  15. ^ Paul, Richard (2014). "The Shakespearean Moons of Uranus". folger.edu. Folger Shakespeare Library. Retrieved February 25, 2024.
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  19. ^ Miller, C.; Chanover, N. J. (March 2009). "Resolving dynamic parameters of the August 2007 Titania and Ariel occultations by Umbriel". Icarus. 200 (1): 343–346.
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  20. ^ Arlot, J. -E.; Dumas, C.; Sicardy, B. (December 2008). "Observation of an eclipse of U-3 Titania by U-2 Umbriel on December 8, 2007 with ESO-VLT". Astronomy and Astrophysics. 492 (2): 599–602.
    doi:10.1051/0004-6361:200810134
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  21. ^ Tittemore, William C.; Wisdom, Jack (June 1990). "Tidal evolution of the Uranian satellites: III. Evolution through the Miranda-Umbriel 3:1, Miranda-Ariel 5:3, and Ariel-Umbriel 2:1 mean-motion commensurabilities". Icarus. 85 (2): 394–443.
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  22. ^ Tittemore, William C.; Wisdom, Jack (March 1989). "Tidal evolution of the Uranian satellites: II. An explanation of the anomalously high orbital inclination of Miranda". Icarus. 78 (1): 63–89.
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  23. ^ Malhotra, Renu; Dermott, Stanley F. (June 1990). "The role of secondary resonances in the orbital history of Miranda". Icarus. 85 (2): 444–480.
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  24. ^ a b c d e f Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects".
    doi:10.1016/j.icarus.2006.06.005
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  26. ^ a b c Bell, 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.
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  27. ^ a b c d e f g h i j k Plescia, J. B. (December 30, 1987). "Cratering history of the Uranian satellites: Umbriel, Titania and Oberon". Journal of Geophysical Research. 92 (A13): 14, 918–14, 932.
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  28. ^ a b c Buratti, Bonnie J.; Mosher, Joel A. (March 1991). "Comparative global albedo and color maps of the Uranian satellites". Icarus. 90 (1): 1–13.
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  29. ^ a b c "Umbriel Nomenclature Table Of Contents". Gazetteer of Planetary Nomenclature. United States Geological Survey, Astrogeology. Retrieved September 26, 2009.
  30. ^ "Umbriel:Wunda". Gazetteer of Planetary Nomenclature. United States Geological Survey, Astrogeology. Retrieved August 8, 2009.
  31. ^ a b Hunt, Garry E.; Patrick Moore (1989). Atlas of Uranus. Cambridge University Press. p. 82.
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  33. ^ Moore, Jeffrey M.; Schenk, Paul M.; Bruesch, Lindsey S.; Asphaug, Erik; McKinnon, William B. (October 2004). "Large impact features on middle-sized icy satellites" (PDF). Icarus. 171 (2): 421–443.
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  34. ^ 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.
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  35. ^ a b c Helfenstein, P.; Thomas, P. C.; Veverka, J. (March 1989). "Evidence from Voyager II photometry for early resurfacing of Umbriel". Nature. 338 (6213): 324–326.
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  36. ^ 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.
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  38. ^ a b c Squyres, S. W.; Reynolds, Ray T.; Summers, Audrey L.; Shung, Felix (1988). "Accretional Heating of the Satellites of Saturn and Uranus". Journal of Geophysical Research. 93 (B8): 8779–8794.
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  39. ^ Hillier, John; Squyres, Steven W. (August 1991). "Thermal stress tectonics on the satellites of Saturn and Uranus". Journal of Geophysical Research. 96 (E1): 15, 665–15, 674.
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  40. ^ Stone, E. C. (December 30, 1987). "The Voyager 2 Encounter with Uranus" (PDF). Journal of Geophysical Research. 92 (A13): 14, 873–14, 876.
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