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Umbriel (moon)
Encyclopedia
Umbriel is a moon of Uranus discovered on October 24, 1851, by William Lassell
. It was discovered at the same time as Ariel
and named after a character in Alexander Pope
's 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. 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 crater
s reaching 210 km (130.5 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.
, 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
in 1852 at the request of Lassell. Umbriel is the 'dusky melancholy sprite' in Alexander Pope's The Rape of the Lock
, and the name suggests the Latin
umbra
, meaning shadow. The moon is also designated Uranus II.
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. In other words, Umbriel is 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 actually 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, 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
. 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 occultation
s 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.
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.
, while a dense non-ice component constitutes around 40% of its mass. The latter could be made of rock
and carbonaceous
material including heavy organic compound
s known as tholin
s. 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 band
s 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 carbonate
s 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.
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 satellite
s, 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
and 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 appears to be an impact deposit. 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, however, 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 kilometers across. The polygons were identified from precise photometry of Voyager 2s 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 umbra
l 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.
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; however, 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 present 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, however, 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.
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 (201,946.1 mi), 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 the quality required 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. No other spacecraft has ever visited Uranus (and Umbriel), and no mission to Uranus and its moons is planned.
William Lassell
William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...
. It was discovered at the same time as Ariel
Ariel (moon)
Ariel is the brightest and fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus, and so has an extreme seasonal cycle....
and named after a character in Alexander Pope
Alexander Pope
Alexander Pope was an 18th-century English poet, best known for his satirical verse and for his translation of Homer. He is the third-most frequently quoted writer in The Oxford Dictionary of Quotations, after Shakespeare and Tennyson...
's poem The Rape of the Lock
The Rape of the Lock
The Rape of the Lock is a mock-heroic narrative poem written by Alexander Pope, first published anonymously in Lintot's Miscellany in May 1712 in two cantos , but then revised, expanded and reissued under Pope's name on March 2, 1714, in a much-expanded 5-canto version...
. Umbriel consists mainly of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
with a substantial fraction of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
, and may be differentiated into a rocky core and an icy mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. 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 crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s reaching 210 km (130.5 mi) in diameter, Umbriel is the second most heavily cratered satellite of Uranus after Oberon
Oberon (moon)
Oberon , also designated ', is the outermost major moon of the planet Uranus. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies...
. The most prominent surface feature is a ring of bright material on the floor of Wunda
Wunda (crater)
Wunda is a large crater on the surface of Uranus' moon Umbriel. It is 131 km in diameter and is located near the equator of Umbriel. The crater named after Wunda, a dark spirit of Indigenous Australian aboriginal mythology....
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
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
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, ArielAriel (moon)
Ariel is the brightest and fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus, and so has an extreme seasonal cycle....
, was discovered by William Lassell
William Lassell
William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...
on October 24, 1851. Although William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...
, the discoverer of Titania
Titania (moon)
Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1578 km. Discovered by William Herschel in 1787, Titania is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream...
and Oberon
Oberon (moon)
Oberon , also designated ', is the outermost major moon of the planet Uranus. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies...
, 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
William Shakespeare
William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...
or Alexander Pope
Alexander Pope
Alexander Pope was an 18th-century English poet, best known for his satirical verse and for his translation of Homer. He is the third-most frequently quoted writer in The Oxford Dictionary of Quotations, after Shakespeare and Tennyson...
. The names of all four satellites of Uranus then known were suggested by John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...
in 1852 at the request of Lassell. Umbriel is the 'dusky melancholy sprite' in Alexander Pope's The Rape of the Lock
The Rape of the Lock
The Rape of the Lock is a mock-heroic narrative poem written by Alexander Pope, first published anonymously in Lintot's Miscellany in May 1712 in two cantos , but then revised, expanded and reissued under Pope's name on March 2, 1714, in a much-expanded 5-canto version...
, and the name suggests the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
umbra
Umbra
The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
, meaning shadow. The moon is also designated Uranus II.
Orbit
Umbriel orbits Uranus at the distance of about 266000 km (165,285.1 mi), being the third farthest from the planet among its five major moons. Umbriel's orbit has a small eccentricityOrbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
and is inclined very little relative to the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
of Uranus. Its orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
is around 4.1 Earth days, coincident with its rotational period. In other words, Umbriel is a synchronous
Synchronous orbit
A synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...
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
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, which co-rotates with the planet. This bombardment may lead to the darkening of the trailing hemispheres, which is actually 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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
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
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
over one of the poles at each solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
. 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
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
and its equatorial plane intersects the Earth, mutual occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
s 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.
Currently Umbriel is not involved in any orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with other Uranian satellites. Early in its history, however, it may have been in a 1:3 resonance with Miranda
Miranda (moon)
-External links:* at * at The Nine8 Planets* at Views of the Solar System* * from the...
. 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
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
or Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.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 fourth most massive of Uranian moons. The moon's density is 1.39 g/cm3, which indicates that it mainly consists of water iceIce
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
, while a dense non-ice component constitutes around 40% of its mass. The latter could be made of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
and carbonaceous
Carbonaceous
Carbonaceous is the defining attribute of a substance rich in carbon. Particularly, carbonaceous hydrocarbons are very unsaturated, high-molecular-weight hydrocarbons, having an elevated carbon:hydrogen ratio....
material including heavy organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s known as tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...
s. The presence of water ice is supported by infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption band
Absorption band
An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite a particular transition in a substance...
s 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
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
water ice, decompose methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
trapped in ice as clathrate hydrate
Clathrate hydrate
Clathrate hydrates are crystalline water-based solids physically resembling ice, in which small non-polar molecules or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded water molecules...
and darken other organics, leaving a dark, carbon-rich residue
Residue (chemistry)
In chemistry, residue is the material remaining after a distillation or an evaporation, or to a portion of a larger molecule, such as a methyl group. It may also refer to the undesired byproducts of a reaction....
behind.
Except for water, the only other compound identified on the surface of Umbriel by the infrared spectroscopy is carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, which is concentrated mainly on the trailing hemisphere. The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
s or organic materials under the influence of the energetic charged particles coming from the magnetosphere of Uranus or the solar ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
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
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean 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
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. 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
KBAR
KBAR may refer to:* KBAR-FM, a radio station licensed to Victoria, Texas, United States* KBAR , a radio station licensed to Burley, Idaho, United States...
). 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 albedoBond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...
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
Geometric albedo
The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle to that of an idealized flat, fully reflecting, diffusively scattering disk with the same cross-section....
) to 19% at an angle of about 1°. This phenomenon is called opposition surge
Opposition surge
The opposition surge is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer...
. The surface of Umbriel is slightly blue in color, while fresh bright impact deposits (in Wunda
Wunda (crater)
Wunda is a large crater on the surface of Uranus' moon Umbriel. It is 131 km in diameter and is located near the equator of Umbriel. The crater named after Wunda, a dark spirit of Indigenous Australian aboriginal mythology....
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
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
from bombardment by charged particles and micrometeorites
Micrometeoroid
A micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...
over the age of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. 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 satellite
Irregular satellite
In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....
s, 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
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
. The surface of Umbriel has far more and larger craters than do Ariel and Titania
Titania (moon)
Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1578 km. Discovered by William Herschel in 1787, Titania is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream...
and 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
Ray system
A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking a bit like many thin spokes coming from the hub of a wheel. The rays can extend for lengths up to several times the diameter of their originating crater, and are often accompanied by...
.
| Crater | Named after | Coordinate | Diameter (km) |
|---|---|---|---|
| Alberich | Alberich Alberich Alberich was a legendary sorcerer who originated in the mythology or epic sagas of the Frankish Merovingian Dynasty of the 5th to 8th century AD, and whose name means king of the elves , who possessed the ability to become invisible... (Norse Norse mythology Norse mythology, a subset of Germanic mythology, is the overall term for the myths, legends and beliefs about supernatural beings of Norse pagans. It flourished prior to the Christianization of Scandinavia, during the Early Middle Ages, and passed into Nordic folklore, with some aspects surviving... ) |
33.6°S 42.2°E | 52.0 |
| Fin | Fin Fin (troll) Fin is a troll in a legend from Kalundborg, Zealand, Denmark. The legend also exists in Sweden, but it instead has a giant from Lund, with the name Jätten Finn .-The legend:... (Danish Danish folklore Danish folklore consists of folk tales, legends, songs, music, dancing, popular beliefs and traditions comuminicated by the inhabitants of towns and villages across the country, often passed on from generation to generation by word of mouth. As in neighbouring countries, interest in folklore grew... ) |
37.4°S 44.3°E | 43.0 |
| Gob | Gob (Pagan Paganism Paganism is a blanket term, typically used to refer to non-Abrahamic, indigenous polytheistic religious traditions.... ) |
12.7°S 27.8°E | 88.0 |
| Kanaloa | Kanaloa Kanaloa In the traditions of ancient Hawaii, Kanaloa is symbolized by the squid or by the octopus, and is typically associated with Kāne. It is also the name of an extinct volcano in Hawaii. In legends and chants Kāne and Kanaloa are portrayed as complementary powers... (Polynesian Polynesian mythology Polynesian mythology is the oral traditions of the people of Polynesia, a grouping of Central and South Pacific Ocean island archipelagos in the Polynesian triangle together with the scattered cultures known as the Polynesian outliers... ) |
10.8°S 345.7°E | 86.0 |
| Malingee | Malingee (Australian Aboriginal mythology Australian Aboriginal mythology Australian Aboriginal myths are the stories traditionally performed by Aboriginal peoples within each of the language groups across Australia.... ) |
22.9°S 13.9°E | 164.0 |
| Minepa | Minepa Minepa In the belief system of the Benue–Congo-speaking Makua and Banayi people of Mozambique, Minepa is the evil spirit who opposes Muluku.... (Makua people of Mozambique Mozambique Mozambique, officially the Republic of Mozambique , is a country in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west and Swaziland and South Africa to the southwest... ) |
42.7°S 8.2°E | 58.0 |
| Peri | Peri Peri In Persian mythology, which constitutes the mythology of not just Persians but all Iranian peoples, peris are descended from fallen angels who have been denied paradise until they have done penance. In earlier sources they are described as agents of evil; later, they are benevolent... (Persian Islamic mythology Islamic mythology is the body of traditional narratives associated with Islam from a mythographical perspective. Many Muslims believe that these narratives are historical and sacred and contain profound truths... ) |
9.2°S 4.3°E | 61.0 |
| Setibos | Setibos (Patagonia Patagonia Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean... n) |
30.8°S 346.3°E | 50.0 |
| Skynd Skynd (crater) Skynd is a crater on the surface of Uranus' moon Umbriel. It is estimated to be between 72 and 110 km in diameter. Its center is located at .... |
Skynd Skynd Skynd is a troll in a Danish legend from Skjelverod, near Ringsted. Skynd lived in a hill, and from just one man in the village of Englerup stole three wives. One evening the man was riding home from Ringsted, and as he passed the hill he saw a crowd of Hill-folk dancing round it. But on looking a... (Danish Danish folklore Danish folklore consists of folk tales, legends, songs, music, dancing, popular beliefs and traditions comuminicated by the inhabitants of towns and villages across the country, often passed on from generation to generation by word of mouth. As in neighbouring countries, interest in folklore grew... ) |
1.8°S 331.7°E | 72.0 |
| Vuver Vuver (crater) Vuver is a crater on the surface of Uranus' moon Umbriel. It is estimated to be 98 km in diameter. The longitude and latitude of its center are 311.6° and −4.7°, respectively.... |
Vuver (Finnish Finnish mythology Finnish mythology is the mythology that went with Finnish paganism which was practised by the Finnish people prior to Christianisation. It has many features shared with fellow Finnic Estonian mythology and its non-Finnic neighbours, the Balts and the Scandinavians... ) |
4.7°S 311.6°E | 98.0 |
| Wokolo | Wokolo (Bambara people of West Africa West Africa West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:... ) |
30°S 1.8°E | 208.0 |
| Wunda Wunda (crater) Wunda is a large crater on the surface of Uranus' moon Umbriel. It is 131 km in diameter and is located near the equator of Umbriel. The crater named after Wunda, a dark spirit of Indigenous Australian aboriginal mythology.... |
Wunda (Australian Aboriginal mythology) | 7.9°S 273.6°E | 131.0 |
| Zlyden | Zlyden (Slavic Slavic mythology Slavic mythology is the mythological aspect of the polytheistic religion that was practised by the Slavs before Christianisation.The religion possesses many common traits with other religions descended from the Proto-Indo-European religion.... ) |
23.3°S 326.2°E | 44.0 |
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 appears to be an impact deposit. Nearby, seen along the terminator
Terminator (solar)
A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of a planetary body...
, are the craters Vuver
Vuver (crater)
Vuver is a crater on the surface of Uranus' moon Umbriel. It is estimated to be 98 km in diameter. The longitude and latitude of its center are 311.6° and −4.7°, respectively....
and Skynd
Skynd (crater)
Skynd is a crater on the surface of Uranus' moon Umbriel. It is estimated to be between 72 and 110 km in diameter. Its center is located at ....
, 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, however, officially recognized due to the poor imaging resolution and generally bland appearance of this moon, which hinders geological mapping
Geologic map
A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...
.
Umbriel's heavily cratered surface has probably been stable since the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
. The only signs of the ancient internal activity are canyons and dark polygons—dark patches with complex shapes measuring from tens to hundreds kilometers across. The polygons were identified from precise photometry of Voyager 2s 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 umbra
Umbra
The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
l 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.
Origin and evolution
Umbriel is thought to have formed from an accretion discAccretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
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
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
. 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. Significant amounts of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
may have been present in the form of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO) and molecular nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(N2) instead of ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
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 present 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
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
together with continued decay of radioactive elements may have led to melting of the ice if an antifreeze
Antifreeze
Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....
like ammonia (in the form of ammonia hydrate
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
) or some salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
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, however, 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
So far the only close-up images of Umbriel have been from the Voyager 2Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
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 (201,946.1 mi), 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 the quality required 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. No other spacecraft has ever visited Uranus (and Umbriel), and no mission to Uranus and its moons is planned.
External links
- Umbriel page (including a labelled map of Umbriel) at Views of the Solar System
- Umbriel Nomenclature from the USGS Planetary Nomenclature web site
