View source for Ariel (moon)

{{Short description|Fourth-largest moon of Uranus}}
{{Featured article}}
{{Infobox planet
| name = Ariel
| image = Ariel in monochrome.jpg
| image_alt = The dark face of Ariel, cut by valleys and marked by craters, appears half in sunlight and half in shadow
| caption = South polar region of Ariel in greyscale as imaged by ''Voyager 2'' in January 1986. {{efn|Many extensive grabens are visible, including the [[Kachina Chasmata]] in the upper part of the image.}} 
| background = Thistle
| discoverer = [[William Lassell]]
| discovered = 24 October 1851
| alt_names = 
| mpc_name = Uranus I
| pronounced = {{IPAc-en|ˈ|ɛər|i|ə|l}} or {{IPAc-en|ˈ|ær|i|ə|l}}<ref>{{MW|Ariel}}</ref>
| adjectives = Arielian {{IPAc-en|ær|i|ˈ|iː|l|i|ə|n}}<ref>DeKoven (1991) ''Rich and strange: gender, history, modernism''</ref>
| orbit_ref = <ref name="orbit" />
| periapsis = {{gaps |190 |670}}&nbsp;km
| apoapsis = {{gaps |191 |129}}&nbsp;km
| semimajor = {{val|190900|u=km}}
| eccentricity = {{val|0.0012}}
| period = {{val|2.520|ul=d}}
| avg_speed = 5.51 km/s{{efn|name=calculated|Calculated on the basis of other parameters.}}
| inclination = {{val|0.260|u=°}} (to Uranus's equator)
| satellite_of = [[Uranus]]
| mean_radius = {{val|578.9|0.6|u=km}} ({{val|0.0908|u=Earths}})<ref name="Thomas 1988" />
| dimensions = 1162.2 × 1155.8 × 1155.4&nbsp;km<ref name="Thomas 1988" />
| surface_area = {{val|4211300|u=km2}}{{efn|name=surface area}}
| volume = {{val|812600000|u=km3}}{{efn|name=volume}}
| mass = {{val|1.2331|0.0180|e=21|u=kg}}<ref name="French et al. 2024"/>
| density = {{val|1.517|u=g/cm3}} (calculated)
| surface_grav = {{Gr|1.2331|578.9|3}} m/s<sup>2</sup>{{efn|name=surface gravity}}
| escape_velocity = {{V2|1.2331|578.9|3}} km/s{{efn|name=escape velocity}}
| rotation = [[Synchronous rotation|synchronous]]
| albedo = {{plainlist |
* 0.53 (geometrical)
* 0.23 (Bond)<ref name="Karkoschka 2001, Hubble" />
  }}
| magnitude = 14.8 (R-band)<ref name="Arlot Sicardy 2008" />
| temp_name1 = solstice<ref name="Grundy Young et al. 2006" /><ref name="Hanel Conrath et al. 1986" />
| mean_temp_1 = ≈ 60 [[Kelvin|K]]
| max_temp_1 = 84 ± 1 K
| min_temp_1 = ?
| note = no
}}

'''Ariel''' is the fourth-largest [[moons of Uranus|moon of Uranus]]. Ariel orbits and rotates in [[Uranus]]'s equatorial plane, which is almost perpendicular to the planet's orbit, giving the moon an extreme seasonal cycle.

It was discovered on 24 October 1851 by [[William Lassell]] and named for a character in two different pieces of literature. As of 2019, much of the detailed knowledge of Ariel derives from a single [[flyby (spaceflight)|flyby]] of Uranus performed by the [[space probe]] ''[[Voyager 2]]'' in 1986, which managed to image around 35% of the moon's surface. There are no active plans at present to return to study the moon in more detail, although various concepts such as a [[Uranus Orbiter and Probe]] have been proposed.

After [[Miranda (moon)|Miranda]], Ariel is the second-closest of Uranus's five [[Major moon|major rounded satellites]]. Among the smallest of the Solar System's [[Major moon|19 known spherical moons]] (it ranks 14th among them in diameter), it is believed to be composed of roughly equal parts ice and rocky material. Its mass is approximately equal in magnitude to Earth's [[hydrosphere]].

Like all of Uranus's moons, Ariel probably formed from an [[accretion disc]] that surrounded the planet shortly after its formation, and, like other large moons, it is likely [[Planetary differentiation|differentiated]], with an inner core of rock surrounded by a [[mantle (geology)|mantle]] of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of [[fault scarp|scarp]]s, [[canyon]]s, [[graben|grabens]] and [[ridge]]s. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to [[tidal heating]].

== Discovery and name ==
Both Ariel and the slightly larger Uranian satellite [[Umbriel]] were discovered by [[William Lassell]] on 24 October 1851.<ref name="Lassell 1851" /><ref name="Lassell, letter 1851" /> Although [[William Herschel]], who discovered Uranus's two largest moons [[Titania (moon)|Titania]] and [[Oberon (moon)|Oberon]] in 1787, claimed to have observed four additional moons,<ref name="Herschel 1798" /> this was never confirmed and those four objects are now thought to be spurious.<ref name="Struve 1848" /><ref name="Holden 1874" /><ref name="Lassell 1874" />

All of [[moons of Uranus|Uranus's moons]] are named after characters from the works of [[William Shakespeare]] or [[Alexander Pope]]'s ''[[The Rape of the Lock]]''. The names of all four satellites of Uranus then known were suggested by [[John Herschel]] 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>{{cite web |url=https://www.folger.edu/podcasts/shakespeare-unlimited/shakespearean-moons-uranus/ |title=The Shakespearean Moons of Uranus |last=Paul |first=Richard |date=2014 |website=folger.edu |publisher=Folger Shakespeare Library |access-date=25 February 2024 |quote=}}</ref> Ariel is named after the leading [[sylph]] in ''[[The Rape of the Lock]]''.<ref name="Harrington 2011" /> It is also the name of [[Ariel (Shakespeare)|the spirit who serves Prospero]] in Shakespeare's ''[[The Tempest]]''.<ref name="Kuiper 1949" /> The moon is also designated '''Uranus&nbsp;I'''.<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 an A (the initial of Ariel) combined with the low globe of [[Jérôme Lalande]]'s Uranus symbol as the symbol of Ariel ([[File:Ariel symbol (fixed width).svg|16px]]). This symbol is not widely used.<ref name=moons>{{cite web |url=https://www.unicode.org/L2/L2025/25079-phobos-and-deimos.pdf |title=Phobos and Deimos symbols |last1=Bala |first1=Gavin Jared |last2=Miller |first2=Kirk |date=7 March 2025 |website=unicode.org |publisher=The Unicode Consortium |access-date=14 March 2025 |quote=}}</ref>

== Orbit ==
Among Uranus's five [[major moon]]s, Ariel is the second closest to the planet, orbiting at the distance of about 190,000&nbsp;km.{{efn |The five major moons are [[Miranda (moon)|Miranda]], Ariel, [[Umbriel]], Titania and Oberon. }} Its orbit has a small [[orbital eccentricity|eccentricity]] and is [[orbital inclination|inclined]] very little relative to the [[equator]] of Uranus.<ref name="orbit" /> Its [[orbital period]] is around 2.5&nbsp;Earth days, coincident with its [[rotational period]]. This means that one side of the moon always faces the planet; a condition known as [[tidal lock]].<ref name="Smith Soderblom et al. 1986" /> Ariel's orbit lies completely inside the [[Magnetosphere of Uranus|Uranian magnetosphere]].<ref name="Grundy Young et al. 2006" /> The trailing [[Sphere#Hemisphere|hemispheres]] (those facing away from their directions of orbit) of airless satellites orbiting inside a magnetosphere like Ariel are struck by magnetospheric [[Plasma (physics)|plasma]] co-rotating with the planet.<ref name="Ness Acuña et al. 1986" /> This bombardment may lead to the darkening of the trailing hemispheres observed for all Uranian moons except Oberon (see below).<ref name="Grundy Young et al. 2006" /> Ariel also captures magnetospheric charged particles, producing a pronounced dip in energetic particle count near the moon's orbit observed by ''Voyager 2'' in 1986.<ref name="Krimigis Armstrong et al. 1986" />

Because Ariel, like Uranus, orbits the [[Sun]] [[Uranus#Axial tilt|almost on its side]] relative to its rotation, its northern and southern hemispheres face either directly towards or directly away from the Sun at the [[solstice]]s. This means it is subject to an extreme seasonal cycle; just as Earth's poles see [[polar night|permanent night]] or [[midnight sun|daylight]] around the solstices, Ariel's poles see permanent night or daylight for half a Uranian year (42 Earth years), 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 1986 southern summer solstice, when nearly the entire northern hemisphere was dark. 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. A number of such events occurred in 2007–2008, including an occultation of Ariel by Umbriel on 19 August 2007.<ref name="occultations" />

Currently Ariel is not involved in any [[orbital resonance]] with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with [[Miranda (moon)|Miranda]], which could have been partially responsible for the heating of that moon (although the maximum heating attributable to a former 1:3 resonance of Umbriel with Miranda was likely about three times greater).<ref name="Tittemore Wisdom 1990" /> Ariel may have once been locked in the 4:1 resonance with Titania, from which it later escaped.<ref name="Tittemore 1990" /> Escape from a mean motion resonance is much easier for the moons of Uranus than for those of [[Jupiter]] or [[Saturn]], due to Uranus's lesser degree of [[Oblate spheroid|oblate]]ness.<ref name="Tittemore 1990" /> This resonance, which was likely encountered about 3.8&nbsp;billion years ago, would have increased Ariel's [[orbital eccentricity]], resulting in tidal friction due to time-varying [[tidal force]]s from Uranus. This would have caused warming of the moon's interior by as much as 20&nbsp;[[Kelvins|K]].<ref name="Tittemore 1990" />

== Composition and internal structure ==
[[File:Ariel Moon Earth.png|thumb|left|250px|Size comparison of [[Earth]], the [[Moon]], and Ariel.]]

Ariel is the fourth-largest of the Uranian moons by size and [[mass]]. It is also the 14th-largest moon in the [[Solar System]]. The moon's density is 1.52&nbsp;g/cm<sup>3</sup>, which indicates that it consists of roughly equal parts [[Ice|water ice]] and a dense non-ice component.<ref name="Hussmann Sohl et al. 2006" /> The latter could consist of [[rock (geology)|rock]] and [[carbon]]aceous material including heavy [[organic compound]]s known as [[tholin]]s.<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, which is porous and thus transmits little solar heat to layers below.<ref name="Grundy Young et al. 2006" /><ref name="NASA 2017" /> Water ice [[absorption band]]s are stronger on Ariel's leading hemisphere than on its trailing hemisphere.<ref name="Grundy Young et al. 2006" /> The cause of this asymmetry is not known, but it may be related to bombardment by charged particles from [[magnetosphere of Uranus|Uranus's magnetosphere]], 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 [[sputtering|sputter]] water ice, decompose [[methane]] trapped in ice as [[clathrate hydrate]] and darken other organics, leaving a dark, carbon-rich [[residue (chemistry)|residue]] behind.<ref name="Grundy Young et al. 2006" />

Except for water, two other compounds have been identified on the surface of Ariel by [[infrared spectroscopy]]. The first is [[carbon dioxide]] (CO<sub>2</sub>), which is concentrated mainly on its trailing hemisphere. Ariel shows the strongest spectroscopic evidence for CO<sub>2</sub> of any Uranian satellite,<ref name="Grundy Young et al. 2006" /> and was the first Uranian satellite on which this compound was discovered.<ref name="Grundy Young et al. 2006" /> 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 Uranus's magnetosphere or 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 [[Primordial element|primordial]] CO<sub>2</sub> trapped by water ice in Ariel's interior. The escape of CO<sub>2</sub> from the interior may be related to past geological activity on this moon.<ref name="Grundy Young et al. 2006" /> 

The second compound identified by its feature at wavelength of 2.2&nbsp;μm on Ariel is [[ammonia]], which is distributed more or less homogeneously over the surface. The presence of ammonia may indicate that Ariel was geologically active in recent past.<ref name="Beddingfield2020"/>

Given its size, rock/ice composition and the possible presence of salt or [[ammonia]] in solution to lower the freezing point of water, Ariel's interior may be [[planetary differentiation|differentiated]] into a rocky [[core (geology)|core]] surrounded by an icy [[mantle (geology)|mantle]].<ref name="Hussmann Sohl et al. 2006" /> If this is the case, the radius of the core (372&nbsp;km) is about 64% of the radius of the moon, and its mass is around 56% of the moon's mass—the parameters are dictated by the moon's composition. The pressure in the center of Ariel is about 0.3&nbsp;[[Pascal (unit)|GPa]] (3&nbsp;[[Bar (unit)|kbar]]).<ref name="Hussmann Sohl et al. 2006" /> The current state of the icy mantle is unclear. The existence of a subsurface ocean is currently considered possible,<ref name='Sci Am 2017'>[https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/ Overlooked Ocean Worlds Fill the Outer Solar System].  John Wenz, ''Scientific American''. 4 October 2017.</ref> though a 2006 study suggests that radiogenic heating alone would not be enough to allow for one.<ref name="Hussmann Sohl et al. 2006" /> More scientific research concluded that an active underwater ocean is possible for the 4 largest moons of Uranus.<ref>{{Cite web|url=https://weather.com/en-IN/india/space/news/2023-05-10-four-of-uranus-large-moons-may-be-hosting-oceans-nasa-study|title=Four of Uranus' Large Moons May Be Hosting Oceans Under Their Icy Crust {{pipe}} Weather.com|website=The Weather Channel}}</ref><ref name="auto">{{cite web | url=https://www.nasa.gov/feature/jpl/new-study-of-uranus-large-moons-shows-4-may-hold-water | title=New Study of Uranus' Large Moons Shows 4 May Hold Water | date=4 May 2023 }}</ref><ref>{{cite web | url=https://www.space.com/uranus-four-biggest-moons-buried-oceans | title=Uranus' 4 biggest moons may have buried oceans of salty water | website=[[Space.com]] | date=5 May 2023 }}</ref>

In April 2025, a published study attempted to constrain the actual depth of Ariel's possible subsurface ocean using clues gleaned from all the visible fractures and grabens on Ariel's surface, as well as clues from Ariel's inferred past orbital eccentricity. The study claimed that in the past, Ariel's orbital eccentricity could have been as high as 0.04, causing extreme tidal stresses that provided the heat necessary to maintain Ariel's subsurface ocean in a liquid state. The study also pointed out the fact that [[Volatile_(astrogeology)|volatile materials]], particularly ammonia, were tentatively detected on Ariel. Since these compounds dissipate rapidly in space and are easily broken down by interaction with Uranus' magnetic field, they consider it as an indication that something is replenishing them on Ariel, suggesting that the moon's subsurface ocean was still active in the recent past. The study concluded that Ariel's subsurface ocean might have been as deep as {{convert|170|km|mi|abbr=on}}.<ref>{{cite web | title=Constraining ocean and ice shell thickness on Ariel from surface geologic structures and stress mapping | year=2025 | publisher= Science Direct Icarus | url=https://www.sciencedirect.com/science/article/abs/pii/S0019103525003707?via%3Dihub| access-date=2025-10-09 }}</ref>

== Surface ==
[[File:Ariel HiRes.jpg|thumb|The highest-resolution ''Voyager 2'' color image of Ariel. Canyons with floors covered by smooth plains are visible at lower right. The bright crater Laica is at lower left.|alt=the bottom hemisphere of Ariel is seen, reddish and dark, with cracks and craters lining the edge]]

=== Albedo and color ===
Ariel is the most reflective of Uranus's moons.<ref name="Karkoschka 2001, Hubble" /> Its surface shows an [[opposition surge]]: the reflectivity decreases from 53% at a phase angle of 0° ([[albedo|geometrical albedo]]) to 35% at an angle of about 1°. The [[Bond albedo]] of Ariel is about 23%—the highest among Uranian satellites.<ref name="Karkoschka 2001, Hubble" /> The surface of Ariel is generally neutral in color.<ref name="Bell McCord 1991" /> There may be an asymmetry between the leading and trailing hemispheres;<ref name="Buratti Mosher 1991" /> the latter appears to be redder than the former by 2%.{{efn | The color is determined by the ratio of albedos viewed through the green (0.52–0.59&nbsp;μm) and violet (0.38–0.45&nbsp;μm) Voyager filters.<ref name="Bell McCord 1991" /><ref name="Buratti Mosher 1991" /> }} Ariel's surface generally does not demonstrate any correlation between albedo and geology on one hand and color on the other hand. For instance, canyons have the same color as the cratered terrain. However, bright impact deposits around some fresh craters are slightly bluer in color.<ref name="Bell McCord 1991" /><ref name="Buratti Mosher 1991" /> There are also some slightly blue spots, which do not correspond to any known surface features.<ref name="Buratti Mosher 1991" />

=== Possible geological activity ===
In February 2025, planetary scientists at [[Johns Hopkins University]] published their findings on the possibility that Ariel might be geologically active using images taken by Voyager 2 back in 1986. According to the study, the numerous grooves and grabens running across Ariel's surface might be the result of liquid water oozing out from the moon's possible [[subsurface ocean]]. As the liquid water flows out, it pushes aside the solid icy surface of the moon, splitting apart Ariel's surface similar to how [[lava]] flowing out of the [[Earth's mantle]] pushes apart the [[oceanic crust]] on Earth's [[Mid-Atlantic Ridge]]; but instead of lava, it is liquid water for Ariel. The freshly ejected water then freezes once it reaches the surface, creating a new icy crust and the cycle of splitting apart the crust and generating fresh crust continues. If this study's conclusions are correct, then the fresh materials at the center of the grooves and gravens might be exposed samples of what a subsurface ocean might contain, making them attractive places to study and even get samples from to learn more about the nature of subsurface oceans. The study suggests that the grooves are good sources of [[carbon oxides]] samples.<ref>{{cite web | url=https://www.jhuapl.edu/news/news-releases/250203-uranus-moon-ariel-medial-grooves-window-to-interior-subsurface-ocean | title=New Study Suggests Trench-Like Features on Uranus' Moon Ariel May be Windows to its Interior &#124; Johns Hopkins University Applied Physics Laboratory }}</ref>

=== Surface features ===
{{See also|List of geological features on Ariel}}

The observed surface of Ariel can be divided into three terrain types: cratered terrain, ridged terrain, and plains.<ref name="Plescia ('Ariel') 1987" /> The main surface features are [[impact craters]], [[canyon]]s, [[fault scarp]]s, ridges, and [[trough (geology)|trough]]s.<ref name="USGS" />

[[File:Ariel's transecting valleys.jpg|thumb|left|[[Graben]] (chasmata) near Ariel's [[Terminator (solar)|terminator]]. Their floors are covered by smooth material, possibly extruded from beneath via [[cryovolcanism]]. Several are cut by [[sinuous]] central grooves, e.g. Sprite and Leprechaun valles above and below the triangular [[Horst (geology)|horst]] near the bottom.|alt=dark, angular features cut by smooth ravines into triangles, cast into high contrast by sunlight]]

The cratered terrain, a rolling surface covered by numerous impact craters and centered on Ariel's south pole, is the moon's oldest and most geographically extensive [[geological unit]].<ref name="Plescia ('Ariel') 1987" /> It is intersected by a network of scarps, canyons (graben), and narrow ridges mainly occurring in Ariel's mid-southern latitudes.<ref name="Plescia ('Ariel') 1987" /> The canyons, known as ''[[chasmata]]'',<ref name="schenk" /> probably represent [[graben]] formed by [[Extensional tectonics|extensional fault]]ing, which resulted from global tensional stresses caused by the freezing of water (or aqueous ammonia) in the moon's interior (see below).<ref name="Smith Soderblom et al. 1986" /><ref name="Plescia ('Ariel') 1987" /> They are 15–50&nbsp;km wide and trend mainly in an east- or northeasterly direction.<ref name="Plescia ('Ariel') 1987" /> The floors of many canyons are convex; rising up by 1–2&nbsp;km.<ref name="schenk" /> Sometimes the floors are separated from the walls of canyons by grooves (troughs) about 1&nbsp;km wide.<ref name="schenk" /> The widest graben have grooves running along the crests of their convex floors, which are called ''[[Vallis (planetary geology)|valles]]''.<ref name="Smith Soderblom et al. 1986" /> The longest canyon is [[Kachina Chasma]], at over 620&nbsp;km in length (the feature extends into the hemisphere of Ariel that ''Voyager 2'' did not see illuminated).<ref name="USGS" /><ref name="Stryk 2008" />

The second main terrain type—ridged terrain—comprises bands of ridges and troughs hundreds of kilometers in extent. It bounds the cratered terrain and cuts it into polygons. Within each band, which can be up to 25 to 70&nbsp;km wide, are individual ridges and troughs up to 200&nbsp;km long and between 10 and 35&nbsp;km apart. The bands of ridged terrain often form continuations of canyons, suggesting that they may be a modified form of the graben or the result of a different reaction of the crust to the same extensional stresses, such as brittle failure.<ref name="Plescia ('Ariel') 1987" />

[[File:Ariel USGS.png|thumb|False-color map of Ariel. The prominent noncircular [[impact crater|crater]] below and left of center is [[Yangoor (crater)|Yangoor]]. Part of it was erased during formation of ridged [[terrain]] via [[extensional tectonics]].|alt=a patch of observed surface is lit in light blue, against a blank disc representing the moon's entire diameter]]

The youngest terrain observed on Ariel are the plains: relatively low-lying smooth areas that must have formed over a long period of time, judging by their varying [[Crater counting|levels of cratering]].<ref name="Plescia ('Ariel') 1987" /> The plains are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain.<ref name="Smith Soderblom et al. 1986" /> In the latter case they are separated from the cratered terrain by sharp boundaries, which in some cases have a lobate pattern.<ref name="Plescia ('Ariel') 1987" /> The most likely origin for the plains is through volcanic processes; their linear vent geometry, resembling terrestrial [[shield volcano]]es, and distinct topographic margins suggest that the erupted liquid was very viscous, possibly a supercooled water/ammonia solution, with solid ice volcanism also a possibility.<ref name="schenk" /> The thickness of these hypothetical cryolava flows is estimated at 1–3&nbsp;km.<ref name="schenk" /> The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel.<ref name="Plescia ('Ariel') 1987" /> A few of these areas appear to be less than 100&nbsp;million years old, suggesting that Ariel may still be geologically active in spite of its relatively small size and lack of current tidal heating.<ref>{{cite journal|last1=Desch|first1=S. J.|last2=Cook|first2=J. C.|last3=Hawley|first3=W.|last4=Doggett|first4=T. C.|title=Cryovolcanism on Charon and other Kuiper belt objects|journal=Lunar and Planetary Science|date=2007|volume=38|issue=1338|page=1901|url=http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1901.pdf|bibcode=2007LPI....38.1901D}}</ref>

Ariel appears to be fairly evenly cratered compared to other moons of Uranus;<ref name="Smith Soderblom et al. 1986" /> the relative paucity of large craters{{efn | The surface density of craters larger than 30&nbsp;km in diameter ranges from 20 to 70 per million km<sup>2</sup> on Ariel, whereas it is about 1800 for Oberon or Umbriel.<ref name="Plescia" /> }} suggests that its surface does not date to the Solar System's formation, which means that Ariel must have been completely resurfaced at some point of its history.<ref name="Plescia ('Ariel') 1987" /> Ariel's past geologic activity is believed to have been driven by [[Tidal acceleration#Tidal heating|tidal heating]] at a time when its orbit was more eccentric than currently.<ref name="Tittemore 1990" /> The largest crater observed on Ariel, [[Yangoor (crater)|Yangoor]], is only 78&nbsp;km across,<ref name="USGS" /> and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few of the craters are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the preexisting crustal structure. In the cratered plains there are a few large (about 100&nbsp;km in diameter) light patches that may be degraded impact craters. If this is the case they would be similar to [[Palimpsest (planetary astronomy)|palimpsest]]s on [[Jupiter]]'s moon [[Ganymede (moon)|Ganymede]].<ref name="Plescia ('Ariel') 1987" /> It has been suggested that a circular depression 245&nbsp;km in diameter located at 10°S 30°E is a large, highly degraded impact structure.<ref name="Moore Schenk et al. 2004" />

== Origin and evolution ==
Ariel 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 [[Axial tilt|obliquity]].<ref name="Mousis 2004" /> 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.{{efn | For instance, [[Tethys (moon)|Tethys]], a Saturnian moon, has the density of 0.97&nbsp;g/cm<sup>3</sup>, which means that it is more than 90% water.<ref name="Grundy Young et al. 2006" /> }}<ref name="Smith Soderblom et al. 1986" /> Significant amounts of [[carbon]] and [[nitrogen]] may have been present in the form of [[carbon monoxide]] (CO) and [[nitrogen|molecular nitrogen]] (N<sub>2</sub>), instead of methane and [[ammonia]].<ref name="Mousis 2004" /> The moons that formed in such a subnebula would contain less water ice (with CO and N<sub>2</sub> trapped as clathrate) and more rock, explaining the higher density.<ref name="Smith Soderblom et al. 1986" />

The accretion process probably lasted for several thousand years before the moon was fully formed.<ref name="Mousis 2004" /> Models suggest that impacts accompanying accretion caused heating of Ariel's outer layer, reaching a maximum temperature of around 195&nbsp;K at a depth of about 31&nbsp;km.<ref name="Squyres Reynolds et al. 1988" /> After the end of formation, the subsurface layer cooled, while the interior of Ariel heated due to decay of [[radioactivity|radioactive elements]] present in its rocks.<ref name="Smith Soderblom et al. 1986" /> The cooling near-surface layer contracted, while the interior expanded. This caused strong [[Stress (mechanics)|extensional stresses]] in the moon's crust reaching estimates of 30 [[Pascal (unit)|MPa]], which may have led to cracking.<ref name="Hillier Squyres 1991" /> Some present-day scarps and canyons may be a result of this process,<ref name="Plescia ('Ariel') 1987" /> which lasted for about 200&nbsp;million years.<ref name="Hillier Squyres 1991" />

The initial [[accretion (astrophysics)|accretional heating]] together with continued decay of radioactive elements and likely tidal heating may have led to melting of the ice if an [[antifreeze]] like ammonia (in the form of [[hydrate|ammonia hydrate]]) or some [[salt (chemistry)|salt]] was present.<ref name="Squyres Reynolds et al. 1988" /> 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="Hussmann Sohl et al. 2006" /> 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&nbsp;K.<ref name="Hussmann Sohl et al. 2006" /> The ocean, however, is likely to have frozen long ago. The freezing of the water likely led to the expansion of the interior, which may have been responsible for the formation of the canyons and obliteration of the ancient surface.<ref name="Plescia ('Ariel') 1987" /> The liquids from the ocean may have been able to erupt to the surface, flooding floors of canyons in the process known as [[cryovolcanism]].<ref name="Squyres Reynolds et al. 1988" /> More recent analysis concluded that an active ocean is probable for the 4 largest moons of Uranus; specifically including Ariel.<ref name="auto"/>

Thermal modeling of [[Saturn]]'s moon [[Dione (moon)|Dione]], which is similar to Ariel in size, density, and surface temperature, suggests that solid state convection could have lasted in Ariel's interior for billions of years, and that temperatures in excess of 173 [[Kelvins|K]] (the melting point of aqueous ammonia) may have persisted near its surface for several hundred million years after formation, and near a billion years closer to the core.<ref name="Plescia ('Ariel') 1987" />

== Observation and exploration ==
{{main|Exploration of Uranus}}
[[File:Arieluranus.jpg|thumb|[[Hubble Space Telescope|HST]] image of Ariel transiting Uranus, complete with shadow|alt=the planet Uranus is seen through the Hubble telescope, its atmosphere defined by bands of electric blue and green. Ariel appears as a white dot floating above it, casting a dark shadow below]]

The [[apparent magnitude]] of Ariel is 14.8;<ref name="Arlot Sicardy 2008" /> similar to that of [[Pluto]] near [[perihelion]]. However, while Pluto can be seen through a telescope of 30&nbsp;cm [[aperture]],<ref name="Singapore Science Centre, Pluto" /> Ariel, due to its proximity to Uranus's glare, is often not visible to telescopes of 40&nbsp;cm aperture.<ref name="Sinnott &amp; Ashford" />

The only close-up images of Ariel were obtained by the ''[[Voyager 2]]'' probe, which photographed the moon during its flyby of Uranus in January 1986. The closest approach of ''Voyager 2'' to Ariel was {{convert|127000|km|mi|abbr=on}}—significantly less than the distances to all other Uranian moons except Miranda.<ref name="Stone 1987" /> The best images of Ariel have a spatial resolution of about 2&nbsp;km.<ref name="Plescia ('Ariel') 1987" /> They cover about 40% of the surface, but only 35% was photographed with the quality required for [[geological mapping]] and crater counting.<ref name="Plescia ('Ariel') 1987" /> At the time of the flyby, the southern hemisphere of Ariel (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" /> No other spacecraft has ever visited the Uranian system.<ref name="NASA Solar System Exploration 2010" /> The possibility of sending the [[Cassini-Huygens|Cassini spacecraft]] to Uranus was evaluated during its mission extension planning phase.<ref name="spiker" />{{Failed verification|date=December 2025|reason=The PDF says nothing about Uranus.}} It would have taken about twenty years to get to the Uranian system after departing Saturn, and these plans were scrapped in favour of remaining at Saturn and eventually destroying the spacecraft in Saturn's atmosphere.<ref name="spiker" />

=== Transits ===
On 26 July 2006, the [[Hubble Space Telescope]] captured a rare transit made by Ariel on Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Such events are rare and only occur around [[equinox]]es, as the moon's orbital plane about Uranus is tilted 98° to Uranus's orbital plane about the Sun.<ref name="transit" /> Another transit, in 2008, was recorded by the [[European Southern Observatory]].<ref name="European Southern Observatory 2008" />

==See also==
*[[List of natural satellites]]
*[[Planetary Science Decadal Survey]]

== Notes ==
{{Notelist
| notes =
{{efn
| name = surface area
| Surface area derived from the radius ''r'' : <math>4\pi r^2</math>.
}}

{{efn
| name = volume
| Volume ''v'' derived from the radius ''r'' : <math>4\pi r^3/3</math>.
}}

{{efn
| name = surface gravity
| Surface gravity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'' : <math>Gm/r^2</math>.
}}

{{efn
| name = escape velocity
| Escape velocity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'' : {{math|{{Radical|2Gm/r}}}}.
}}

}}

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{{cite web
 |publisher   = NASA Solar System Exploration
 |date        = 2014
 |title       = Ariel: Facts and Figures
 |url         = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Ura_Ariel&Display=Facts
 |access-date  = 2014-11-13
 
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}}
</ref>

<ref name="French et al. 2024">{{Cite journal |last1=French |first1=Richard G. |last2=Hedman |first2=Matthew M. |last3=Nicholson |first3=Philip D. |last4=Longaretti |first4=Pierre-Yves |last5=McGhee-French |first5=Colleen A. |date=2024-03-15 |title=The Uranus system from occultation observations (1977–2006): Rings, pole direction, gravity field, and masses of Cressida, Cordelia, and Ophelia |url=https://www.sciencedirect.com/science/article/pii/S0019103524000150 |journal=Icarus |volume=411 |article-number=115957 |doi=10.1016/j.icarus.2024.115957 |issn=0019-1035|arxiv=2401.04634 |bibcode=2024Icar..41115957F |s2cid=266900064 }}
</ref>

<ref name="Beddingfield2020">{{cite journal |last1=Cartwright |first1=Richard J. |last2=Beddingfield |first2=Chloe B. |last3=Nordheim |first3=Tom A. |last4=Roser |first4=Joseph |last5=Grundy |first5=William M. |last6=Hand |first6=Kevin P. |last7=Emery |first7=Joshua P. |last8=Cruikshank |first8=Dale P. |last9=Scipioni |first9=Francesca |title=Evidence for Ammonia-bearing Species on the Uranian Satellite Ariel Supports Recent Geologic Activity |journal=The Astrophysical Journal Letters |date=1 July 2020 |volume=898 |issue=1 |pages=L22 |doi=10.3847/2041-8213/aba27f|doi-access=free |arxiv=2007.02826 |bibcode=2020ApJ...898L..22C }}</ref>
}}

== External links ==
{{Commons category|Ariel (moon)}}
* [https://web.archive.org/web/20070609141610/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Ura_Ariel Ariel profile] at [http://solarsystem.nasa.gov NASA's Solar System Exploration] site
* [http://adsabs.harvard.edu//full/seri/AN.../0033//0000136.000.html AN, '''33''' (1852) 257/258]
* [https://web.archive.org/web/20110721055806/http://laps.noaa.gov/albers/sos/uranus/ariel/ariel_rgb_cyl_www.jpg Ariel basemap derived from Voyager images]
* [http://www.solarviews.com/eng/ariel.htm Ariel page] (including [http://www.solarviews.com/raw/uranus/arielmap.jpg labelled] [http://www.solarviews.com/raw/uranus/arielps.jpg maps] of Ariel) at ''Views of the Solar System''
* [http://photojournal.jpl.nasa.gov/target/Ariel NASA archive of publicly released Ariel images]
* [https://stereomoons.blogspot.com/2009/08/more-ariel.html Paul Schenk's 3D images and flyover videos of Ariel and other outer solar system satellites]
* [https://planetarynames.wr.usgs.gov/Page/ARIEL/target Ariel nomenclature] from the [https://planetarynames.wr.usgs.gov/ USGS Planetary Nomenclature web site]
*[http://www.planetary.org/blog/article/00001362/ Ted Stryk: Revealing the night sides of Uranus' moons]

{{Ariel}}
{{Moons of Uranus}}
{{Solar System moons (compact)}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{Authority control}}


[[Category:Ariel (moon)| ]]
[[Category:Astronomical objects discovered in 1851|18511021]]
[[Category:Moons with a prograde orbit]]