Umbriel

Template:Short description {{#invoke:other uses|otheruses}} Template:Featured article Template:Use mdy dates {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Umbriel | image = {{#invoke:InfoboxImage|InfoboxImage|image=PIA00040 Umbrielx2.47.jpg|upright={{#if:||1.1}}|alt=A round spherical body with its left half illuminated. The surface is dark and has a low contrast. There are only a few bright patches. The terminator is slightly to the right from the center and runs from the top to bottom. A large crater named Wunda with a bright ring on its floor can be seen at the top of the image near the terminator. A pair of large craters with bright central peaks can be seen along the terminator in the upper part of the body. The illuminated surface is covered by a large number of craters.}} | caption = Grayscale image of Umbriel from Voyager 2, January 1986. Umbriel's surface is heavily battered; the bright crater Wunda can be seen at the top of the image. | headerstyle = {{#if:|background-color:|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

| header10 = {{#if:|Designations|Designations}}

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}<ref name="orbit" />

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| header60 = Proper orbital elements

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| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = | label72 = Template:Longitem | data72 = Template:Val km<ref name="Thomas 1988" /> | label73 = Template:Longitem | data73 = Template:Val (0.092 Earths)<ref name="Thomas 1988" /> | label74 = Template:Longitem | data74 = | label75 = Template:Longitem | data75 = | label76 = Flattening | data76 = | label77 = Circumference | data77 = | label78 = Template:Longitem | data78 = Template:Val (0.008 Earths)Template:Efn | label79 = Volume | data79 = Template:Val (0.0008 Earths)Template:Efn | label80 = Mass | data80 = Template:Val<ref>Jacobson (2023), as cited in French et al. (2024)<ref name="French et al. 2024"/></ref> | label81 = Template:Longitem | data81 = Template:Val (calculated) | label82 = Template:Longitem | data82 = Template:Gr m/s² (~0.0257 g)Template:Efn | label83 = Template:Longitem | data83 = | label84 = Template:Longitem | data84 = Template:V2 km/sTemplate:Efn | label85 = Template:Longitem | data85 = presumed synchronous<ref name="Smith Soderblom et al. 1986" /> | label86 = Template:Longitem | data86 = | label87 = Template:Longitem | data87 = | label88 = Template:Longitem | data88 = 0<ref name="Smith Soderblom et al. 1986" /> | label89 = Template:Longitem | data89 = | label90 = Template:Longitem | data90 = | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if: |Template:Longitem |Albedo}} | data93 = Template:Plainlist | label94 = Temperature | data94 =

| data100 = {{#if:solstice<ref name="Grundy Young et al. 2006" />|

Surface temp.	min	mean	max
solstice<ref name="Grundy Young et al. 2006" />	?	≈ 75 K	85 K
{{{temp_name2}}}
{{{temp_name3}}}
{{{temp_name4}}}

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| label101 = Surface absorbed dose rate | data101 = | label102 = Surface equivalent dose rate | data102 = | label103 = Template:Longitem | data103 = | label104 = Template:Longitem | data104 = | label105 = Template:Longitem | data105 = 15.1<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> | label106 = Template:Longitem | data106 = | label107 = Template:Longitem | data107 =

| header110 = Atmosphere

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Umbriel (Template:IPAc-en) 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 Template:Convert 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 nameEdit

Umbriel, along with another Uranian satellite, Ariel, was discovered by William Lassell on October 24, 1851.<ref>The Times 24 October 2024 page 25 "On this day"</ref><ref name="Lassell 1851" /><ref name="Lassell, letter 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,<ref name="Herschel 1798" /> his observations were not confirmed and those four objects are now thought to be spurious.<ref name="Struve 1848" />

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,<ref name="Lassell 1852" /> though it is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission.<ref name=podcast>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Umbriel is the "dusky melancholy sprite" in Alexander Pope's The Rape of the Lock,<ref name="Kuiper 1949" /> and the name suggests the Latin {{#invoke:Lang|lang}}, meaning Template:Gloss. The moon is also designated Uranus II.<ref name="Lassell, letter 1851" />

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 symbols, proposed a U (the initial of Umbriel) combined with the low globe of Jérôme Lalande's Uranus symbol as the symbol of Umbriel (File:Umbriel symbol (fixed width).svg). This symbol is not widely used.<ref name=moons>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

OrbitEdit

Umbriel orbits Uranus at the distance of about Template:Convert, being the third farthest from the planet among its five major moons.Template:Efn Umbriel's orbit has a small eccentricity and is inclined very little relative to the equator of Uranus.<ref name="orbit" /> 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.<ref name="Smith Soderblom et al. 1986" /> Umbriel's orbit lies completely inside the Uranian magnetosphere.<ref name="Grundy Young et al. 2006" /> 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.<ref name="Ness Acuña et al. 1986" /> This bombardment may lead to the darkening of the trailing hemispheres, which is observed for all Uranian moons except Oberon (see below).<ref name="Grundy Young et al. 2006" /> 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.<ref name="Krimigis Armstrong et al. 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.<ref name="Grundy Young et al. 2006" /> 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.<ref name="Miller Chanover 2009" /><ref name="Arlot Dumas et al. 2008" />

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.<ref name="Tittemore Wisdom 1990" /> 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.<ref name="Tittemore Wisdom 1989" /><ref name="Malhotra Dermott 1990" />

Composition and internal structureEdit

File:Earth Moon Umbriel size.png

Size comparison of Earth, the Moon, and Umbriel

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.<ref name="Hussmann Sohl et al. 2006"/> The latter could be made of rock and carbonaceous material including heavy organic compounds known as tholins.<ref name="Smith Soderblom et al. 1986" /> The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon.<ref name="Grundy Young et al. 2006" /> Water ice absorption bands are stronger on Umbriel's leading hemisphere than on the trailing hemisphere.<ref name="Grundy Young et al. 2006" /> 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).<ref name="Grundy Young et al. 2006" /> 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.<ref name="Grundy Young et al. 2006" />

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.<ref name="Grundy Young et al. 2006" /> 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.<ref name="Grundy Young et al. 2006" />

Umbriel may be differentiated into a rocky core surrounded by an icy mantle.<ref name="Hussmann Sohl et al. 2006" /> 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).<ref name="Hussmann Sohl et al. 2006" /> The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.<ref name="Hussmann Sohl et al. 2006" />

Surface featuresEdit

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.<ref name="JPLSSD" /> Umbriel has a very low Bond albedo of only about 10% as compared to 23% for Ariel.<ref name="Karkoschka 2001, Hubble" /> 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,<ref name="Bell McCord 1991" /> while fresh bright impact deposits (in Wunda crater, for instance)<ref name="Plescia 1987" /> are even bluer. There may be an asymmetry between the leading and trailing hemispheres; the former appears to be redder than the latter.<ref name="Buratti Mosher 1991" /> The reddening of the surfaces probably results from space weathering from bombardment by charged particles and micrometeorites over the age of the Solar System.<ref name="Bell McCord 1991" /> 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.<ref name="Buratti Mosher 1991" /> The surface of Umbriel is relatively homogeneous—it does not demonstrate strong variation in either albedo or color.<ref name="Bell McCord 1991" />

Scientists have so far recognized only one class of geological feature on Umbriel—craters.<ref name="usgs" /> The surface of Umbriel has far more and larger craters than do Ariel and Titania. It shows the least geological activity.<ref name="Plescia 1987" /> 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.<ref name="Plescia 1987" /><ref name="usgs" /> All recognized craters on Umbriel have central peaks,<ref name="Plescia 1987" /> but no crater has rays.<ref name="Smith Soderblom et al. 1986" />

Near Umbriel's equator lies the most prominent surface feature: Wunda crater, which has a diameter of about 131 km.<ref name="usgsWunda" /><ref name="hunt" /> Wunda has a large ring of bright material on its floor, which may be an impact deposit<ref name="Plescia 1987" /> 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.<ref name=Sori2017/> Nearby, seen along the terminator, are the craters Vuver and Skynd, which lack bright rims but possess bright central peaks.<ref name="Smith Soderblom et al. 1986" /><ref name="hunt" /> 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.<ref name="Moore Schenk et al. 2004" />

Much like other moons of Uranus, the surface of Umbriel is cut by a system of canyons trending northeast–southwest.<ref name="Croft1989" /> They are not officially recognized due to the poor imaging resolution and generally bland appearance of this moon, which hinders geological maping.<ref name="Plescia 1987" />

Umbriel's heavily cratered surface has probably been stable since the Late Heavy Bombardment.<ref name="Plescia 1987" /> 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.<ref name="Helfenstein Thomas et al. 1989" /> 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.<ref name="Helfenstein Thomas et al. 1989" /> 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.Template:Efn<ref name="Buratti Mosher 1991" /> 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.<ref name="Smith Soderblom et al. 1986" />

Named craters on Umbriel<ref name="usgs" />Template:Efn
Crater	Coordinates	Diameter (km)	Approved	Named after	Ref
Alberich	Template:Coord	52.0	1988	Alberich (Norse)	Template:WGPSN
Fin	Template:Coord	43.0	1988	Fin (Danish)	Template:WGPSN
Gob	Template:Coord	88.0	1988	Gob (Pagan)	Template:WGPSN
Kanaloa	Template:Coord	86.0	1988	Kanaloa (Polynesian)	Template:WGPSN
Malingee	Template:Coord	164.0	1988	Malingee (Australian Aboriginal mythology)	Template:WGPSN
Minepa	Template:Coord	58.0	1988	Minepa (Makua people of Mozambique)	Template:WGPSN
Peri	Template:Coord	61.0	1988	Peri (Persian)	Template:WGPSN
Setibos	Template:Coord	50.0	1988	Setebos (Tehuelche)	Template:WGPSN
Skynd	Template:Coord	72.0	1988	Skynd (Danish)	Template:WGPSN
Vuver	Template:Coord	98.0	1988	Vuver (Finnish)	Template:WGPSN
Wokolo	Template:Coord	208.0	1988	Wokolo (Bambara people of West Africa)	Template:WGPSN
Wunda	Template:Coord	131.0	1988	Wunda (Australian Aboriginal mythology)	Template:WGPSN
Zlyden	Template:Coord	44.0	1988	Zlyden (Slavic)	Template:WGPSN

Origin and evolutionEdit

File:Umbriel usgsx2.jpg

False color image of Umbriel showing polygons

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.<ref name="Mousis 2004" /> 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.Template:Efn<ref name="Smith Soderblom et al. 1986" /> 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.<ref name="Mousis 2004" /> 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.<ref name="Smith Soderblom et al. 1986" />

Umbriel's accretion probably lasted for several thousand years.<ref name="Mousis 2004" /> The impacts that accompanied accretion caused heating of the moon's outer layer.<ref name="Squyres Reynolds et al. 1988" /> The maximum temperature of around 180 K was reached at the depth of about 3 km.<ref name="Squyres Reynolds et al. 1988" /> After the end of formation, the subsurface layer cooled, while the interior of Umbriel heated due to decay of radioactive elements in its rocks.<ref name="Smith Soderblom et al. 1986" /> 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.<ref name="Hillier & Squyres 1991" /> This process probably lasted for about 200 million years, implying that any endogenous activity ceased billions of years ago.<ref name="Smith Soderblom et al. 1986" />

The initial accretional heating together with continued decay of radioactive elements may have led to melting of the ice<ref name="Squyres Reynolds et al. 1988" /> if an antifreeze like ammonia (in the form of ammonia hydrate) or some salt was present.<ref name="Hussmann Sohl et al. 2006" /> The melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle.<ref name="Plescia 1987" /> 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.<ref name="Hussmann Sohl et al. 2006" /> Among Uranian moons Umbriel was least subjected to endogenic resurfacing processes,<ref name="Plescia 1987" /> although it may, like other Uranian moons, have experienced a very early resurfacing event.<ref name="Helfenstein Thomas et al. 1989" />

ExplorationEdit

File:Voyager spacecraft.jpg

The Voyager 2 spacecraft

Template:Further

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 Template:Convert,<ref name="Stone 1987" /> the best images of this moon have a spatial resolution of about 5.2 km.<ref name="Plescia 1987" /> The images cover about 40% of the surface, but only 20% was photographed with enough quality for geological mapping.<ref name="Plescia 1987" /> 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.<ref name="Smith Soderblom et al. 1986" />