Umbriel () is the third-largest moon of Uranus. It was discovered on October 24, 1851, by

William Lassell William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer.Ariel. It was named after a character in

Alexander Pope Alexander Pope (21 May 1688 Old Style and New Style dates, O.S. – 30 May 1744) was an English poet, translator, and satirist of the Age of Enlightenment, Enlightenment era who is considered one of the most prominent English poets of the early ...

's 1712 poem '' The Rape of the Lock''. Umbriel consists mainly of

ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

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 crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

s reaching in diameter, Umbriel is the second-most heavily cratered satellite of Uranus after

Oberon Oberon () is a king of the fairy, fairies in Middle Ages, medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania ...

. 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 William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer. Although

William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

, the discoverer of Titania and

, 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 ( 23 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...

. The names of all four satellites of Uranus then known were suggested by

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor and experimental photographer who invented the blueprint and did botanical work. ...

(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 Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

, meaning . The moon is also designated ''Uranus II''. Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...

symbols, proposed a U (the initial of Umbriel) combined with the low globe of

Jérôme Lalande Joseph Jérôme Lefrançois de Lalande (; 11 July 1732 – 4April 1807) was a French astronomer, freemason and writer. He is known for having estimated a precise value of the astronomical unit (the distance from the Earth to the Sun) using measu ...

's Uranus symbol as the symbol of Umbriel (). This symbol is not widely used.

Orbit

Umbriel orbits Uranus at the distance of about , being the third farthest from the planet among its five major moons. Umbriel's orbit has a small

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

and is inclined very little relative to the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

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

(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 Pole or poles may refer to: People *Poles (people), another term for Polish people, from the country of Poland * Pole (surname), including a list of people with the name * Pole (musician) (Stefan Betke, born 1967), German electronic music artist ...

spend 42 years in complete darkness, and another 42 years in continuous sunlight, with the Sun rising close to the

zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...

over one of the poles at each

solstice A solstice is the time when the Sun reaches its most northerly or southerly sun path, excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries ...

. 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 A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...

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 Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

. 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

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

aceous material including heavy

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s known as tholins. The presence of water ice is supported by

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

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 Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

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 Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

, 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 A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

s or organic materials under the influence of the energetic charged particles coming from the magnetosphere of Uranus or the solar

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

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 (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (whic ...

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 The Bond albedo (also called spheric albedo, planetary albedo, and bolometric albedo), named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic radi ...

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 In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle (astronomy), phase angle) to that of an ''idealized'' flat, fully reflecting, diffuse reflection, d ...

) 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 The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. 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 map A geological map or geologic map is a special-purpose map made to show various geological features. Rock (geology), Rock units or stratum, geologic strata are shown by color or symbols. Bed (geology), Bedding planes and structural features such ...

ing. 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.

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 Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

and

may have been present in the form of

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

(CO) and molecular nitrogen (N₂) instead of

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

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 An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allow ...

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 , 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.

Discovery and name

Orbit

Composition and internal structure

Surface features

Origin and evolution

Exploration

See also

Notes

References

External links