Ariel (moon)

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Ariel
synchronous
Albedo
  • 0.53 (geometrical)
  • 0.23 (Bond)[7]
Surface temp. min mean max
solstice[8][9] ? ≈ 60 K 84 ± 1 K
14.8 (R-band)[10]

Ariel is the fourth-largest moon of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus and so has an extreme seasonal cycle.

It was discovered in October 1851 by

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

20 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 differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating
.

Discovery and name

Discovered on 24 October 1851 by

Shakespeare's The Tempest
.

Both Ariel and the slightly larger Uranian satellite Umbriel were discovered by William Lassell on 24 October 1851.[11][12] Although William Herschel, who discovered Uranus's two largest moons Titania and Oberon in 1787, claimed to have observed four additional moons,[13] this was never confirmed and those four objects are now thought to be spurious.[14][15][16]

All of

the spirit who serves Prospero in Shakespeare's The Tempest.[20] The moon is also designated Uranus I.[12]

Orbit

Among Uranus's five

Uranian magnetosphere.[8] The trailing hemispheres (those facing away from their directions of orbit) of airless satellites orbiting inside a magnetosphere like Ariel are struck by magnetospheric plasma co-rotating with the planet.[22] This bombardment may lead to the darkening of the trailing hemispheres observed for all Uranian moons except Oberon (see below).[8] 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.[23]

Because Ariel, like Uranus, orbits the Sun 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 solstices. This means it is subject to an extreme seasonal cycle; just as Earth's poles see permanent night or 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.[8] 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 occultations 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.[24]

Currently Ariel is not involved in any

K.[26]

Composition and internal structure

Size comparison of Earth, the Moon, and Ariel.

Ariel is the fourth-largest of the Uranian moons by size and

Uranus's magnetosphere, which is stronger on the trailing hemisphere (due to the plasma's co-rotation).[8] 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.[8]

Except for water, two other compounds have been identified on the surface of Ariel by

primordial CO2 trapped by water ice in Ariel's interior. The escape of CO2 from the interior may be related to past geological activity on this moon.[8]

The second compound identified by its feature at wavelength of 2.2 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.[29]

Given its size, rock/ice composition and the possible presence of salt or

core surrounded by an icy mantle.[27] 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).[27] The current state of the icy mantle is unclear. The existence of a subsurface ocean is currently considered possible,[30] though a 2006 study suggests that radiogenic heating alone would not be enough to allow for one.[27] More scientific research concluded that an Active underwater ocean is possible for the 4 largest moons of Uranus.[31][32][33]

Surface

the bottom hemisphere of Ariel is seen, reddish and dark, with cracks and craters lining the edge
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.

Albedo and color

Ariel is the most reflective of Uranus's moons.[7] 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.[7] The surface of Ariel is generally neutral in color.[34] There may be an asymmetry between the leading and trailing hemispheres;[35] the latter appears to be redder than the former by 2%.[g] 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.[34][35] There are also some slightly blue spots, which do not correspond to any known surface features.[35]

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.

impact craters, canyons, fault scarps, ridges, and troughs.[37]

sinuous central grooves, e.g. Sprite and Leprechaun valles above and below the triangular horst
near the bottom.

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

Kachina Chasma, at over 620 km in length (the feature extends into the hemisphere of Ariel that Voyager 2 did not see illuminated).[37][39]

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 km wide, are individual ridges and troughs up to 200 km long and between 10 and 35 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.[36]

a patch of observed surface is lit in light blue, against a blank disc representing the moon's entire diameter
False-color map of Ariel. The prominent noncircular crater below and left of center is Yangoor. Part of it was erased during formation of ridged terrain via extensional tectonics.

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 levels of cratering.[36] The plains are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain.[21] In the latter case they are separated from the cratered terrain by sharp boundaries, which in some cases have a lobate pattern.[36] The most likely origin for the plains is through volcanic processes; their linear vent geometry, resembling terrestrial shield volcanoes, 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.[38] The thickness of these hypothetical cryolava flows is estimated at 1–3 km.[38] The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel.[36] A few of these areas appear to be less than 100 million years old, suggesting that Ariel may still be geologically active in spite of its relatively small size and lack of current tidal heating.[40]

Ariel appears to be fairly evenly cratered compared to other moons of Uranus;[21] the relative paucity of large craters[h] 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.[36] 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.[26] The largest crater observed on Ariel, Yangoor, is only 78 km across,[37] 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 km in diameter) light patches that may be degraded impact craters. If this is the case they would be similar to palimpsests on Jupiter's moon Ganymede.[36] It has been suggested that a circular depression 245 km in diameter located at 10°S 30°E is a large, highly degraded impact structure.[42]

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.[43] 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.[i][21] Significant amounts of carbon and nitrogen may have been present in the form of carbon monoxide (CO) and molecular nitrogen (N2), instead of methane and ammonia.[43] 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.[21]

The accretion process probably lasted for several thousand years before the moon was fully formed.

radioactive elements present in its rocks.[21] The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust reaching estimates of 30 MPa, which may have led to cracking.[45] Some present-day scarps and canyons may be a result of this process,[36] which lasted for about 200 million years.[45]

The initial

cryovolcanism.[44] More recent analysis concluded that an active ocean is probable for the 4 largest moons of Uranus; specifically including Ariel.[32]

Thermal modeling of

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.[36]

Observation and exploration

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

The

perihelion. However, while Pluto can be seen through a telescope of 30 cm aperture,[46] Ariel, due to its proximity to Uranus's glare, is often not visible to telescopes of 40 cm aperture.[47]

The only close-up images of Ariel were obtained by the

Cassini spacecraft to Uranus was evaluated during its mission extension planning phase.[50] 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.[50]

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 equinoxes, as the moon's orbital plane about Uranus is tilted 98° to Uranus's orbital plane about the Sun.[51] Another transit, in 2008, was recorded by the European Southern Observatory.[52]

See also

Notes

  1. ^ Calculated on the basis of other parameters.
  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. ^ 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.[34][35]
  8. ^ The surface density of craters larger than 30 km in diameter ranges from 20 to 70 per million km2 on Ariel, whereas it is about 1800 for Oberon or Umbriel.[41]
  9. ^ For instance, Tethys, a Saturnian moon, has the density of 0.97 g/cm3, which means that it is more than 90% water.[8]

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