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Umbriel (moon)

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Umbriel as seen by Voyager 2 in 1986
Discovered byWilliam Lassell
Discovery timeOctober 24, 1851
How to say it/ˈʌmbriəl/ UM-bree-əl[1]
Other namesUranus II
Longest distance from the center of its orbital path
("semi-major axis")
266 000 km
How egg-shaped its orbit is
How long it takes to complete an orbit4.144 d
Angle above the reference plane
0.128° (to Uranus's equator)
What it orbitsUranus
Size and Other Qualities
Average distance from its center to its surface584.7 ± 2.8 km (0.092 Earths)[3]
Area of its surface4 296 000 km2 (0.008 Earths)[a]
Volume inside it837 300 000 km3 (0.0008 Earths)[b]
Mass1.172 ± 0.135 × 1021 kg (2 × 10−4 Earths)[4]
Average density1.39 ± 0.16 g/cm3[4]
Gravity at its surface0.2 m/s2 (~ 0.023 g)[c]
Slowest speed able to escape into space
("escape velocity")
0.52 km/s[d]
How long it takes to turn around one timepresumed synchronous[5]
Angle at which it turns
(in relation to its orbit)
How much light it reflects
  • 0.26 (geometrical)
  • 0.10 (Bond)[6]
Surface temp. Min. Avg. Max.
solstice[8] ? ~75 K 85 K
Seeming brightness
("apparent magnitude")
14.5 (V-band, opposition)[7]

Umbriel is a moon of Uranus found on October 24, 1851 by William Lassell. It was found at the same time as Ariel.


The name "Umbriel" and the names of all four moons of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell.[9] Lassell had earlier supported Herschel's 1847 naming scheme for the seven then-known moons of Saturn and had named his newly-found eighth moon Hyperion in accordance with Herschel's naming scheme in 1848. The adjectival form of the name is Umbrielian.

It is also designated Uranus II.

Physical characteristics

So far the only close-up images of Umbriel are from the Voyager 2 probe, which made observations of the moon during its Uranus flyby in January, 1986. During the flyby the southern hemisphere of the moon was pointed towards the Sun so only it was studied.

Umbriel's surface is the darkest of the Uranian moons, and reflects only about half as much light as Ariel, Uranus' brightest moon. It has far more and bigger craters than do Ariel and Titania and is also the least geologically active.[10] It is mostly made of water ice, with the balance made up of silicate rock, and other ices such as methane. Methane can break down and form reddish-black organic compounds such as tholins when bombarded by high-energy particles.[11] Near-IR spectra of Ariel and Umbriel clearly show that water ice dominates the spectra of these objects.[12]


Craters on Umbriel are named after many different demons from various mythologies.

Crater Named after
Alberich Alberich (Norse)
Fin Fin (Danish)
Gob Gob (Pagan)
Kanaloa Kanaloa (Polynesian)
Malingee Malingee (Australian Aboriginal mythology)
Minepa Minepa (Makua people of Mozambique)
Peri Peri (Persian)
Setibos Setebos (Patagonian)
Skynd Skynd (Danish)
Vuver Vuver (Finnish)
Wokolo Wokolo (Bambara people of West Africa)
Wunda Wunda (Australian Aboriginal mythology)
Zlyden Zlyden (Slavic)

Other websites


  1. Surface area derived from the radius r : [math]4\pi r^2[/math].
  2. Volume v derived from the radius r : [math]4\pi r^3/3[/math].
  3. Surface gravity derived from the mass m, the gravitational constant G and the radius r : [math]Gm/r^2[/math].
  4. Escape velocity derived from the mass m, the gravitational constant G and the radius r : [math]\sqrt{\frac{2Gm}{r}}[/math].


  1. "Umbriel". Retrieved 2010-01-14.
  2. "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 .
  4. 4.0 4.1 Jacobson, R. A.; Campbell, J. K.; Taylor, A. H.; Synnott, S. P. (June 1992). "The masses of Uranus and its major satellites from Voyager tracking data and earth-based Uranian satellite data". The Astronomical Journal 103 (6): 2068–2078. doi:10.1086/116211 .
  5. 5.0 5.1 Smith, B. A.; Soderblom, L. A.; Beebe, A.; Bliss, D.; Boyce, J. M.; Brahic, A.; Briggs, G. A.; Brown, R. H. et al. (4 July 1986). "Voyager 2 in the Uranian System: Imaging Science Results". Science 233 (4759): 43–64. doi:10.1126/science.233.4759.43 . PMID 17812889 .
  6. Karkoschka, Erich (2001). "Comprehensive Photometry of the Rings and 16 Satellites of Uranus with the Hubble Space Telescope". Icarus 151 (1): 51–68. doi:10.1006/icar.2001.6596 .
  7. "Planetary Satellite Physical Parameters". NASA/JPL. Retrieved June 6, 2010.
  8. 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. doi:10.1016/j.icarus.2006.04.016 .
  9. (German) Beobachtungen der Uranus-Satelliten Retrieved on 06-01-07
  10. Arnett, William A. (2006 Aug 25). "Umbriel". The Nine Planets. Retrieved 2007-11-09.
  11. Overbye, Roger (April 1986). "Voyager was on target again; in the latest unmanned triumph, Voyager 2 surveyed Uranus and sent back a real bull's-eye". Discover. Retrieved 2007-11-09.
  12. Dumas, Christophe; Bradford A. Smith, and Richard J. Terrile (2003). "Hubble Space Telescope NICMOS Multiband Photometry of Proteus and Puck". The Astronomical Journal 126 (2003): 1080–1085. doi:10.1086/375909 .