Editing Umbriel (section)

== 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.<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)|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 [[Micrometeoroid|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 satellite]]s, 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—[[Impact crater|craters]].<ref name="usgs" /> The surface of Umbriel has far more and larger craters than do Ariel and [[Titania (moon)|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&nbsp;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 [[Ray system|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&nbsp;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 [[Dry ice|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 (solar)|terminator]], are the craters [[Vuver (crater)|Vuver]] and [[Skynd (crater)|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&nbsp;km and depth of approximately 5&nbsp;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 map]]ing.<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''<nowiki/>'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 ''[[umbra]]l material'') excavated by an impact or expelled in an explosive volcanic eruption.{{efn|While a co-orbiting population of dust particles is another possible source of the dark material, this is considered less likely because other satellites were not affected.<ref name="Smith Soderblom et al. 1986" />}}<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" />

{| class="wikitable plainrowheaders sortable"
|+ Named craters on Umbriel<ref name="usgs" />{{efn|name=spirits|Surface features on Umbriel are named for evil or dark spirits taken from various mythologies.<ref name="Strobell & Masursky 1987" />}}
! scope="col" | Crater
! scope="col" class="unsortable" | Coordinates
! scope="col" | Diameter (km)
! scope="col" | Approved
! scope="col" | Named after
! scope="col" | Ref
|-
! scope="row" | Alberich
| {{coord|33.6|S|42.2|E|dim:52.0km_globe:umbriel_type:landmark}}
| 52.0
| 1988
| [[Alberich]] ([[Norse mythology|Norse]])
| {{WGPSN|163}}
|-
! scope="row" | Fin
| {{coord|37.4|S|44.3|E|dim:43.0km_globe:umbriel_type:landmark}}
| 43.0
| 1988
| [[Fin (troll)|Fin]] ([[Danish folklore|Danish]])
| {{WGPSN|1955}}
|-
! scope="row" | Gob
| {{coord|12.7|S|27.8|E|dim:88.0km_globe:umbriel_type:landmark}}
| 88.0
| 1988
| Gob ([[Paganism|Pagan]])
| {{WGPSN|2196}}
|-
! scope="row" | Kanaloa
| {{coord|10.8|S|345.7|E|dim:86.0km_globe:umbriel_type:landmark}}
| 86.0
| 1988
| [[Kanaloa]] ([[Polynesian mythology|Polynesian]])
| {{WGPSN|2912}}
|-
! scope="row" | Malingee
| {{coord|22.9|S|13.9|E|dim:164.0km_globe:umbriel_type:landmark}}
| 164.0
| 1988
| Malingee ([[Australian Aboriginal mythology]])
| {{WGPSN|3608}}
|-
! scope="row" | Minepa
| {{coord|42.7|S|8.2|E|dim:58.0km_globe:umbriel_type:landmark}}
| 58.0
| 1988
| [[Minepa]] ([[Makua (people)|Makua]] people of [[Mozambique]])
| {{WGPSN|3909}}
|-
! scope="row" | Peri
| {{coord|9.2|S|4.3|E|dim:61.0km_globe:umbriel_type:landmark}}
| 61.0
| 1988
| [[Peri]] ([[Islamic mythology|Persian]])
| {{WGPSN|4657}}
|-
! scope="row" | Setibos
| {{coord|30.8|S|346.3|E|dim:50.0km_globe:umbriel_type:landmark}}
| 50.0
| 1988
| [[Setebos]] ([[Tehuelche people|Tehuelche]])
| {{WGPSN|5443}}
|-
! scope="row" | [[Skynd (crater)|Skynd]]
| {{coord|1.8|S|331.7|E|dim:72.0km_globe:umbriel_type:landmark}}
| 72.0
| 1988
| [[Skynd (troll)|Skynd]] ([[Danish folklore|Danish]])
| {{WGPSN|5595}}
|-
! scope="row" | [[Vuver (crater)|Vuver]]
| {{coord|4.7|S|311.6|E|dim:98.0km_globe:umbriel_type:landmark}}
| 98.0
| 1988
| [[Vuver]] ([[Finnish mythology|Finnish]])
| {{WGPSN|6460}}
|-
! scope="row" | [[Wokolo (crater)|Wokolo]]
| {{coord|30|S|1.8|E|dim:208.0km_globe:umbriel_type:landmark}}
| 208.0
| 1988
| Wokolo ([[Bambara people]] of West Africa)
| {{WGPSN|6568}}
|-
! scope="row" | [[Wunda (crater)|Wunda]]
| {{coord|7.9|S|273.6|E|dim:131.0km_globe:umbriel_type:landmark}}
| 131.0
| 1988
| Wunda (Australian Aboriginal mythology)
| {{WGPSN|6587}}
|-
! scope="row" | Zlyden
| {{coord|23.3|S|326.2|E|dim:44.0km_globe:umbriel_type:landmark}}
| 44.0
| 1988
| [[Zlyden]] ([[Slavic mythology|Slavic]])
| {{WGPSN|6747}}
|}