Umbriel (moon)

Infobox Planet
name = Umbriel
alt_names = Uranus III
adjectives = Umbrielian


caption = Click image for description
bgcolour = #a0ffa0
discovery = yes
discoverer = William Herschel
discovered = October 24, 1851
semimajor = 266 000 km
orbital_speed = 4.650 km/s
eccentricity = 0.?
period = 4.144 d
inclination = 0.205° (to Uranus' equator)
satellite_of = Uranus
physical_characteristics = yes
mean_radius = 584.7 km (0.092 Earths)cite web|last=Yeomans|first=Donald K.|date=2006-Jul-13|title=Planetary Satellite Physical Parameters|publisher=JPL Solar System Dynamics|url=http://ssd.jpl.nasa.gov/?sat_phys_par|accessdate=2005-06-12]
surface_area = 4 296 000 km² (0.008 Earths)
volume = 837 300 000 km³ (0.0008 Earths)
mass = (1.2 ± 0.1)e|21 kg (2e|-4 Earths)
density = 1.40 ± 0.16 g/cm³
surface_grav = 0.23 m/s² (~0.023 g)
escape_velocity = 0.52 km/s
rotation = presumed synchronous
axial_tilt = 0
albedo = 0.21 ± 0.02
magnitude = 14.81cite web|title=Classic Satellites of the Solar System|url=http://www.oarval.org/ClasSaten.htm|publisher=Observatorio ARVAL|accessdate=2007-09-28]
single_temperature = ~61 K

Umbriel (pronEng|ˈʌmbriəl respell|UM|bree-əl) is a moon of Uranus discovered on October 24, 1851 by William Lassell. It was discovered at the same time as Ariel.

Name

The name "Umbriel" and the names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell. [ [http://adsabs.harvard.edu//full/seri/AN.../0034//0000169.000.html http://adsabs.harvard.edu//full/seri/AN.../0034//0000169.000.html] "Adsabs.harvard.edu" Retrieved on 06-01-07 ] Lassell had earlier endorsed Herschel's 1847 naming scheme for the seven then-known satellites of Saturn and had named his newly-discovered eighth satellite Hyperion in accordance with Herschel's naming scheme in 1848. Umbriel is the 'dusky melancholy sprite' in Alexander Pope's "The Rape of the Lock", and the name suggests the Latin "umbra", shadow. The adjectival form of the name is "Umbrielian".

It is also designated Uranus III.

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 satellite. Umbriel's heavily cratered surface has probably been stable since the Late Heavy Bombardment. It has far more and larger craters than do Ariel and Titania and is also the least geologically active.cite web |last=Arnett |first=William A. |date=2006 Aug 25 |url=http://www.nineplanets.org/umbriel.html |title=Umbriel |publisher=The Nine Planets |accessdate=2007-11-09] It is mostly composed 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.cite web |last=Overbye |first=Roger |date=April 1986 |url=http://findarticles.com/p/articles/mi_m1511/is_v7/ai_5083814/pg_7 |title=Voyager was on target again; in the latest unmanned triumph, Voyager 2 surveyed Uranus and sent back a real bull's-eye |publisher=Discover |accessdate=2007-11-09] Near-IR spectra of Ariel and Umbriel clearly show that water ice dominates the spectra of these objects.cite journal | first=Christophe| last=Dumas | coauthors=Bradford A. Smith, and Richard J. Terrile | title=Hubble Space Telescope NICMOS Multiband Photometry of Proteus and Puck | year=2003 | journal=The Astronomical Journal| volume=126 | issue=2 | pages=1080–1085 | doi=10.1086/375909 | url=http://www.iop.org/EJ/article/1538-3881/126/2/1080/203103.html ]

Umbriel's most prominent feature is Wunda, a large ring of bright material near Umbriel's equator (see picture; the viewpoint is nearly polar). Wunda is presumably some kind of crater, but its exact nature is mysterious. Nearby, seen along the terminator, is the crater Skynd, which lacks a bright rim but possesses a bright central peak.

ee also

*List of craters on Umbriel

External links

* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Ura_Umbriel Umbriel Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://adsabs.harvard.edu//full/seri/AJ.../0002//0000070.000.html William Lassell, "Astronomical Journal" 2 (1851) 70]
* [http://adsabs.harvard.edu//full/seri/AN.../0033//0000136.000.html AN, 33 (1852) 257/258]
* [http://adsabs.harvard.edu//full/seri/AN.../0034//0000169.000.html AN, 34 (1852) 325/326]
* [http://www.eso.org/public/outreach/press-rel/pr-2007/images/phot-37-07-normal.jpgUranus and moons] as seen by the VLT in August 2007

References

Uranus