======================================================================
= Umbriel (moon) =
======================================================================
Introduction
======================================================================
Umbriel () is the third-largest moon of Uranus. It was discovered on
October 24, 1851, by William Lassell at the same time as neighboring
moon Ariel. It was named after a character in Alexander Pope's 1712
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, but the
presence of canyons suggests early internal 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 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 regular
moons of Uranus, probably formed from an accretion disk that
surrounded the planet just after its formation. Umbriel 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. 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, his
observations were not confirmed and those four objects are now thought
to be spurious.
All of 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 (son of William) in
1852 at the request of Lassell, though it is uncertain if Herschel
devised the names, or if Lassell did so and then sought Herschel's
permission. Umbriel is the "dusky melancholy sprite" in Alexander
Pope's 'The Rape of the Lock', and the name suggests the Latin ,
meaning . The moon is also designated 'Uranus II'.
Orbit
======================================================================
Umbriel orbits Uranus at the distance of about 266000 km, being the
third farthest from the planet among its five major moons. Umbriel's
orbit has a small eccentricity and is inclined very little relative to
the equator of Uranus. Its orbital period is around 4.1 Earth days,
coincident with its rotational period, making it a synchronous or
tidally locked satellite, with one face always pointing toward its
parent planet. Umbriel's orbit lies completely inside the Uranian
magnetosphere. 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.
This bombardment may lead to the darkening of the trailing
hemispheres, which is observed for all Uranian moons except Oberon
(see below). 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.
Because Uranus orbits the Sun 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 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. 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,
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.
Currently, Umbriel is not involved in any orbital resonance with other
Uranian satellites. Early in its history however, it may have been in
a 1:3 resonance with Miranda. 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. Due to Uranus's lower 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.
Composition and internal structure
======================================================================
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 of water ice, with a dense non-ice
component constituting around 40% of its mass. The latter could be
made of rock and carbonaceous material including heavy organic
compounds known as tholins. The presence of water ice is supported by
infrared spectroscopic observations, which have revealed crystalline
water ice on the surface of the moon. Water ice absorption bands are
stronger on Umbriel's leading hemisphere than on the trailing
hemisphere. 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). 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.
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. 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 a result of past geological activity
on this moon.
Umbriel may be differentiated into a rocky core surrounded by an icy
mantle. 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). The
current state of the icy mantle is unclear, although the existence of
a subsurface ocean is considered unlikely.
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. Umbriel has a very low Bond albedo of only about 10% as compared
to 23% for Ariel. 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 called opposition surge. The surface
of Umbriel is slightly blue in color, while fresh bright impact
deposits (in Wunda crater, for instance) are even bluer. There may be
an asymmetry between the leading and trailing hemispheres; the former
appears to be redder than the latter. The reddening of the surfaces
probably results from space weathering from bombardment by charged
particles and micrometeorites over the age of the Solar System.
However, the color asymmetry of Umbriel is 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. The surface of Umbriel is relatively
homogeneous—it does not demonstrate strong variation in either albedo
or color.
Scientists have so far recognized only one class of geological feature
on Umbriel—craters. The surface of Umbriel has far more and larger
craters than do Ariel and Titania. It shows the least geological
activity. 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. All recognized craters on Umbriel have central peaks,
but no crater has rays.
Near Umbriel's equator lies the most prominent surface feature: Wunda
crater, which has a diameter of about 131 km. Wunda has a large ring
of bright material on its floor, which may be an impact deposit 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.
Nearby, seen along the terminator, are the craters Vuver and Skynd,
which lack bright rims but possess bright central peaks. 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.
Much like other moons of Uranus, the surface of Umbriel is cut by a
system of canyons trending northeast-southwest. They are not
officially recognized due to the poor imaging resolution and generally
bland appearance of this moon, which hinders geological mapping.
Umbriel's heavily cratered surface has probably been stable since the
Late Heavy Bombardment. 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. The
polygons were identified from precise photometry of 'Voyager 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. 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
called 'umbral material') excavated by an impact or expelled in an
explosive volcanic eruption. 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.
Named craters on Umbriel Crater Coordinates Diameter (km)
Approved Named after Ref
Alberich 52.0 1988 Alberich (Norse)
Fin 43.0 1988 Fin (Danish)
Gob 88.0 1988 Gob (Pagan)
Kanaloa 86.0 1988 Kanaloa (Polynesian)
Malingee 164.0 1988 Malingee (Australian Aboriginal mythology)
Minepa 58.0 1988 Minepa (Makua people of Mozambique)
Peri 61.0 1988 Peri (Persian)
Setibos 50.0 1988 Setibos (Puerto Rican)
Skynd 72.0 1988 Skynd (Danish)
Vuver 98.0 1988 Vuver (Finnish)
Wokolo 208.0 1988 Wokolo (Bambara people of West Africa)
Wunda 131.0 1988 Wunda (Australian Aboriginal mythology)
Zlyden 44.0 1988 Zlyden (Slavic)
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. The precise composition of the
subnebula is not known, but the higher density of Uranian moons
compared to the moons of Saturn indicates that it may have been
relatively water-poor. 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. 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.
Umbriel's accretion probably lasted for several thousand years. The
impacts that accompanied accretion caused heating of the moon's outer
layer. The maximum temperature of around 180 K was reached at the
depth of about 3 km. After the end of formation, the subsurface layer
cooled, while the interior of Umbriel heated due to decay of
radioactive elements in its rocks. 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. This process probably lasted for about 200 million years,
implying that any endogenous activity ceased billions of years ago.
The initial accretional heating together with continued decay of
radioactive elements may have led to melting of the ice if an
antifreeze like ammonia (in the form of ammonia hydrate) or some salt
was present. The melting may have led to the separation of ice from
rocks and formation of a rocky core surrounded by an icy mantle. 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 is likely to have frozen long ago. Among
Uranian moons Umbriel was least subjected to endogenic resurfacing
processes, although it may, like other Uranian moons, have experienced
a very early resurfacing event.
Exploration
======================================================================
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 325000 km, the best images of this moon have a spatial
resolution of about 5.2 km. The images cover about 40% of the surface,
but only 20% was photographed with enough quality for geological
mapping. 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.
See also
======================================================================
* List of natural satellites
References
======================================================================
{{Reflist
| colwidth = 30em
| refs =
}}
External links
======================================================================
*
*
*
* [http://www.solarviews.com/eng/umbriel.htm Umbriel page] (including
a [http://www.solarviews.com/raw/uranus/umbmap1.jpg labelled map of
Umbriel]) at 'Views of the Solar System'
* [http://planetarynames.wr.usgs.gov/Page/UMBRIEL/target Umbriel
Nomenclature] from the [http://planetarynames.wr.usgs.gov/ USGS
Planetary Nomenclature web site]
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/Umbriel_(moon)