Ariel (moon)

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Ariel
Ariel
Ariel as seen by Voyager 2 in 1986
Discovery
Discovered byWilliam Lassell
Discovery dateOctober 24, 1851
Designations
PronunciationTemplate:Pron-en[note 1]
Uranus I
AdjectivesArielian
Orbital characteristics[2]
191,020 km
Mean orbit radius
190,900 km
Eccentricity0.0012
2.520 d
Inclination0.260° (to Uranus's equator)
Satellite ofUranus
Physical characteristics
Dimensions1162.2 × 1155.8 × 1155.4 km[3]
Mean radius
578.9 ± 0.6 km (0.0908 Earths)[3]
4,211,300 km²[note 2]
Volume812,600,000 km³[note 3]
Mass1.353 ± 0.120×1021 kg (2.26×10−4 Earths)[4]
Mean density
1.66 ± 0.15 g/cm³[4]
0.269 m/s2[note 4]
0.559 km/s[note 5]
synchronous
Albedo0.53 (geometrical),
0.23 (Bond)[5]
Surface temp. min mean max
solstice[6] ? ~60 K ~70 K
14.4[7]
Atmosphere
Surface pressure
zero

Ariel (Template:Pron-en[note 1]) is the fourth-largest moon of the planet Uranus, and the second-smallest of its five round satellites, after Miranda. Ariel was discovered on 24 October 1851 by William Lassell. Its name derives from a sky spirit in English literature, particularly Shakespeare's The Tempest. Among the smaller of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), Ariel is believed to be compsed of roughly equal parts ice and rocky material. Like all of Uranus's moons, Ariel probably formed from an accretion disk that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel's surface shows signs of more recent geologcal activity than the larger Uranian moons, and its surface, crossed by valleys and canyons, suggests a past geological history affected by tidal heating.

All of our current knowledge of Ariel's surface and structure derives from a single flyby of Uranus performed by the spacecraft Voyager 2 in 1986, which managed to capture only 35% of the moon's surface. There are no plans at present to return to study the system in more detail.

Discovery and name

Both Ariel and the slightly larger Uranian satellite Umbriel were discovered by William Lassell on October 24, 1851.[8][9] Although William Herschel, the discoverer of Uranus's two largest moons Titania and Oberon, claimed at the end of the 18th century that he had observed four additional moons of Uranus,[10] his observations were not confirmed and those four objects are now thought to be spurious.[11]

All 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.[12] Ariel is named after the leading sylph in Alexander Pope's poem The Rape of the Lock. It is also the name of the spirit who serves Prospero in Shakespeare's The Tempest.[13] The moon is also designated Uranus I.[9]

Orbit

Ariel orbits Uranus at the distance of about 190,000 km (120,000 mi), being the second farthest from the planet among its five major moons.[note 6] Ariel's orbit has a small eccentricity and is inclined very little relative to the equator of Uranus.[2] Its orbital period is around 2.5 Earth days, coincident with its rotational period. In other words, Ariel is a synchronous or tidally locked satellite, with one face always pointing toward the planet.[14] Ariel's orbit lies completely inside the Uranian magnetosphere.[6] This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Ariel) are struck by magnetospheric plasma, which co-rotates with the planet.[15] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).[6] Ariel 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.[16]

Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Ariel) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in a 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.[6] 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 a number of such events was observed including an occultation of Ariel by Umbriel on August 19, 2007.[17]

Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in the 5:3 resonance with Miranda, which was partially responsible for the heating of that moon.[18] Ariel may have also been in a 2:1 resonance with Umbriel[18] and 4:1 resonance with Titania. These resonances may have caused heating of the Ariel's interior by a s much as 20 K.[19]

Composition and internal structure

Ariel is the fourth largest and third most massive of Uranian moons.[note 7] The moon's density is 1.66 g/cm3,[4] which indicates that it consists of roughly equal proportions of water ice and a dense non-ice component[21] The latter could be made of rock and carbonaceous material including heavy organic compounds known as tholins.[14] The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon.[6] Water ice absorption bands are stronger on Ariel's leading hemisphere than on the trailing hemisphere.[6] 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).[6] 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.[6]

Except for water, the only other compound identified on the surface of Ariel by the infrared spectroscopy is carbon dioxide, which is concentrated mainly on the trailing hemisphere.[6] It demonstrates the strongest carbon dioxide abortion bands among satellites of Uranus and was, in fact, the first Uranian satellite where this compound was discovered.[6] 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 Ariel's interior. The escape of CO2 from the interior may be related to the past geological activity on this moon.[6]

Ariel may be differentiated into a rocky core surrounded by an icy mantle.[21] If this is the case, the radius of the core (372 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 GPa (3 kbar).[21] The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.[21]

Surface

Albedo and color

Ariel is the brightest of Uranus's moons.[5] Its surface shows an opposition surge: the reflectivity decreases from 53% at a phase angle of 0° (geometrical albedo) to 35% at an angle of about 1°. The Bond albedo of Ariel is about 23%—the highest among Uranian satellites.[5] The surface of Ariel is generally neurtral in color.[22] There may be an asymmetry between the leading and trailing hemispheres;[23] the latter appears to be redder than the former by 2%.[note 8] Ariel's surface generally does not demonstrate any correlation between albedo and geology on the 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.[22][23] The are also some slightly blue spots, which do not correspond to any known surface features.[23]

Surface features

See also: List of geological features on Ariel
Graben on the surface of Ariel. The floor is covered by the smooth material extruded from beneath.

The observed surface of Ariel can be divided into three terrain type: cratered terrain, ridged terrain and plains.[24] The cratered terrain is the most aerially extensive unit. It has rolling surface covered by numerous impact craters. It is the stratigraphically the oldest part of the surface.[24] Younger than the cratered terrain are the regions of ridged terrain, which comprise bands of ridges and troughs hundreds of kilometers in extent. Within each band, which can be up to 25 to 70 km wide, are individual ridges and troughs up to 200 km long and between 10 and 35 km apart. The bands of ridged terrain cut the cratered terrain into polygons or bound it and often form continuations of the graben, suggesting that they may be a modified form of the graben or the result of a different reaction in the crust to the same stresses, such as brittle failure.[24]

The youngest geological features observed on Ariel were the plains: relatively low-lying smooth areas that must have formed over a long period of time, judging by their various levels of cratering.[24] The plains are found on the floors of graben and in a few irregular depressions in the middle of the cratered terrain.[14] The edges of some plains have a lobate pattern. The most likely origin for the plains is erasure of underlying features through volcanic processes; their linear vent geometry (inside graben) and distinct topographic margins suggest that the erupted liquid was very viscous, possibly a supercooled water/ammonia solution. Solid ice volcanism is also a possibility.[25] The graben must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel.[24]

Ariel appears to be fairly evenly cratered compared to other moons of Uranus, and its relative paucity of large craters (221 observed, vs. ~1800 for Oberon or Umbriel) suggest that none date as far back as the Late Heavy Bombardment, which means Ariel's surface must have been completely resurfaced at some point after that event.[26] The largest crater observed on Ariel, Yangoor, is only 78 km across,[27] and shows signs of subsequent deformation. Ariel appears to have regions of fresh frost, particularly the ejecta radiating from young impact craters. The oldest and most extensive geologic unit observed on Ariel by Voyager 2 was a vast area of cratered plains centered near Ariel's south pole.[14]

Voyager 2 also observed a network of faults, canyons, and icy outflows running along Ariel's mid-southern latitudes, breaking up the cratered plains region. The canyons, known as chasmata,[25] probably represent grabens formed by extensional faulting. Grabens on Ariel trend in an east- or northeasterly direction,[26] and are likely the result of global tensional stresses caused by the freezing of water (or aqueous ammonia) in the moon's interior.[26]

Ariel's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently. Early in its history, Ariel was apparently captured in a 4:1 orbital resonance with Titania, from which it subsequently escaped.[19] The resonance would have increased Ariel's orbital eccentricity, resulting tidal friction due to time-varying tidal forces from Uranus, which would have caused warming of the moon's interior. 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 oblateness, and its satellites' larger relative size.

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 obliquity.[28] 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.[note 9][14] 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.[28] 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.[14]

Ariel's accretion probably lasted for several thousand years.[28] The impacts that accompanied accretion caused heating of the moon's outer layer.[29] The maximum temperature of around 195 K was reached at the depth of about 31 km.[29] After the end of formation, the subsurface layer cooled, while the interior of Ariel heated due to decay of radioactive elements present in its rocks.[14] The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust reaching up to 30 MPa, which may have led to cracking.[30] Some present-day scarps and canyons may be a result of this process,[24] which lasted for about 200 million years.[30]

The initial 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 ammonia hydrate) or some salt was present.[29] The melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle.[21] 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 173 K. The ocean, however, is likely to have frozen long ago.[21] The freezing of the water likely led to the expansion of the interior, which may have been responsible for the formation of the majority of canyons and obliteration of the ancient surface.[24] The liquids from the ocean may have been able to erupt to the surface flooding floors of canyons in the process known as cryovolcanism.[29]

Thermal modelling of Saturn's moon Dione, which has a similar size, density and surface temperature to Ariel, suggests that solid state convection could have lasted in Ariel's interior for billions of years, and that temperatures in excess of 173 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.[26]

Exploration

Main article: Exploration of Uranus
Ariel transiting Uranus, complete with shadow
Ariel transiting Uranus, complete with shadow

So far the only close-up images of Ariel 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 Ariel was 127,000 km (79,000 mi)—significantly less that the distances to all other Uranian moons except Miranda.[31] The best images of Ariel have a spatial resolution of about 2 km.[24] They cover about 40% of the surface, but only 35% was photographed with the quality required for geological mapping and crater counting.[24] 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.[14] No other spacecraft has ever visited Uranus (and Ariel), and no mission to Uranus and its moons is planned.

Transits

On 26 July 2006, the Hubble Space Telescope captured a rare transit made by Ariel across the face of Uranus, during which the satellite cast a shadow that could be seen on the Uranian cloudtops. Such events are rare and only occur around equinoxes, as the moon's orbital plane about Uranus is tilted 98° to Uranus's orbital plane about the Sun.[32] Another transit, in 2008, was recorded by the European Southern Observatory.[33]

See also

Notes

  1. ^ a b In US dictionary transcription, Template:USdict.[1]
  2. ^ Surface area derived from the radius r : .
  3. ^ Volume v derived from the radius r : .
  4. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r : .
  5. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r : √2Gm/r.
  6. ^ The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.
  7. ^ Due to the current observational error, it is not yet known for certain whether Ariel is more massive than Umbriel.[20]
  8. ^ The color is determined by the ratio of albedos viewed through the green (0.52–0.59 μm) and violet (0.38–0.45 μm) Voyager filters.[22][23]
  9. ^ For instance, Tethys, a Saturnian moon, has the density of 0.97 g/cm3, which means that it contains more than 90% of water.[6]

Notes and references

  1. ^ "Ariel". Dictionary.com. Retrieved 2010-09-21.
  2. ^ a b "Planetary Satellite Mean Orbital Parameters". Jet Propulsion Laboratory, California Institute of Technology.
  3. ^ a b Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/0019-1035(88)90054-1, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/0019-1035(88)90054-1 instead.
  4. ^ a b c Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1086/116211, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1086/116211 instead.
  5. ^ a b c Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1006/icar.2001.6596, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1006/icar.2001.6596 instead.
  6. ^ a b c d e f g h i j k l Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/j.icarus.2006.04.016, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/j.icarus.2006.04.016 instead.
  7. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/j.pss.2008.02.034, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/j.pss.2008.02.034 instead.
  8. ^ Lassell, W. (1851). "On the interior satellites of Uranus". Monthly Notices of the Royal Astronomical Society. 12: 15–17.
  9. ^ a b Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1086/100198, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1086/100198 instead.
  10. ^ Herschel, William (1798). "On the Discovery of Four Additional Satellites of the Georgium Sidus; The Retrograde Motion of Its Old Satellites Announced; And the Cause of Their Disappearance at Certain Distances from the Planet Explained". Philosophical Transactions of the Royal Society of London. 88: 47–79. doi:10.1098/rstl.1798.0005.
  11. ^ Struve, O. (1848). "Note on the Satellites of Uranus". Monthly Notices of the Royal Astronomical Society. 8 (3): 44–47.
  12. ^ Lassell, W. (1852). "Beobachtungen der Uranus-Satelliten". Astronomische Nachrichten (in German). 34: 325. Retrieved 2008-12-18.
  13. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1086/126146, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1086/126146 instead.
  14. ^ a b c d e f g h Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1126/science.233.4759.43, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1126/science.233.4759.43 instead.
  15. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1126/science.233.4759.85, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1126/science.233.4759.85 instead.
  16. ^ Krimigis, S.M. (1986). "The Magnetosphere of Uranus: Hot Plasma and radiation Environment". Science. 233 (4759): 97–102. doi:10.1126/science.233.4759.97. PMID 17812897. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  17. ^ Miller, C. (2009). "Resolving dynamic parameters of the August 2007 Titania and Ariel occultations by Umbriel". Icarus. 200 (1): 343–6. doi:10.1016/j.icarus.2008.12.010. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ a b Tittemore, W. C. (1990). "Tidal evolution of the Uranian satellites III. Evolution through the Miranda-Umbriel 3:1, Miranda-Ariel 5:3, and Ariel-Umbriel 2:1 mean-motion commensurabilities". Icarus. 85 (2): 394–443. doi:10.1016/0019-1035(90)90125-S. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  19. ^ a b Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/0019-1035(90)90024-4, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/0019-1035(90)90024-4 instead.
  20. ^ "Planetary Satellite Physical Parameters". Jet Propulsion Laboratory (Solar System Dynamics). Retrieved 2009-05-28.
  21. ^ a b c d e f Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/j.icarus.2006.06.005, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/j.icarus.2006.06.005 instead.
  22. ^ a b c Bell III, J.F. (1991). A search for spectral units on the Uranian satellites using color ratio images (Conference Proceedings). Lunar and Planetary Science Conference, 21st, Mar. 12-16, 1990. Houston, TX, United States: Lunar and Planetary Sciences Institute. pp. 473–489. {{cite conference}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ a b c d Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/0019-1035(91)90064-Z, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/0019-1035(91)90064-Z instead.
  24. ^ a b c d e f g h i Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1038/327201a0, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1038/327201a0 instead.
  25. ^ a b Schenk, Paul M. (1991). "Fluid Volcanism on Miranda and Ariel: Flow Morphology and Composition". Journal of Geophysical Research. 96 (B2): 1887–1906. doi:10.1029/90JB01604.
  26. ^ a b c d Plescia, J. B. (1987). "Geology and Cratering History of Ariel". Abstracts of the Lunar and Planetary Science Conference. p. 788. Retrieved 2010-11-26.
  27. ^ "Uranus System Nomenclature Table Of Contents". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 2009-01-05.
  28. ^ a b c Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1051/0004-6361:20031515, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1051/0004-6361:20031515 instead.
  29. ^ a b c d Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1029/JB093iB08p08779, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1029/JB093iB08p08779 instead.
  30. ^ a b Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1029/91JE01401, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1029/91JE01401 instead.
  31. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1029/JA092iA13p14873, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1029/JA092iA13p14873 instead.
  32. ^ "Uranus and Ariel". Hubblesite (News Release 72 of 674). 2006-07-26. Retrieved 2006-12-14.
  33. ^ "Uranus and satellites". European Southern Observatory. 2008. Retrieved 2010-11-27.

External links

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