Ariel (moon)

William Lassell

The moon Ariel ( Uranus I) is the fifteenth and brightest of the 27 known and the fourth-largest and second of the five large moons of the planet Uranus. He owns all of Uranus 's moons the largest surface reflectivity ( albedo).

  • 3.1 size
  • 3.2 Internal structure
  • 3.3 Surface 3.3.1 levels
  • 3.3.2 ridges and grooves
  • 3.3.3 crater terrain

Discovery and designation

Ariel was discovered on October 24, 1851 along with Umbriel as third and fourth moons of Uranus by the British astronomer William Lassell with a 60cm reflecting telescope on a home-built observatory in Liverpool.

The moon was named after a sylph in Alexander Pope's epic poem The Rape of the Lock. Since all the moons of Uranus except Ariel, Umbriel and Belinda are named after characters from William Shakespeare, is often mistakenly assumed that it was to the spirit of the air the same name from the play The Storm.

The names of the first four moons discovered Uranus ( Oberon, Titania, Ariel and Umbriel ) were proposed by John Herschel, the son of William Herschel. William Herschel was the discoverer of Oberon, Titania and Uranus itself

Web properties

Orbit

Ariel orbits Uranus on a prograde, nearly perfectly circular orbit at an average distance of about 190,900 kilometers (about 7,469 Uranus radii ) from the center, or around 165,300 km on whose cloud tops. The orbital eccentricity is 0.0012, the orbit is inclined 0.041 degrees from the equator of Uranus.

The orbit of the next inner moon Miranda is on average 61,000 kilometers away from Ariel's orbit, that of the next outer moon Umbriel about 75,000 km.

Ariel orbits Uranus in 2 days 12 hours 29 minutes and 21 seconds.

Ariel's orbit lies entirely in the magnetosphere of Uranus. The following hemispheres of airless moons like Ariel are thus under constant attack from magnetosphärischem plasma which co-rotates with the planet. This can lead to a darkening of the following hemisphere, which so far could be observed for all Uranian moons except Oberon. Ariel also catches a magneto- spherical charged particles, which leads to an increased number of these particles in Ariel's orbit and could be observed by the Voyager 2 spacecraft.

Since Ariel as Uranus orbits the sun practically relative to the rotation on the side, its northern or southern hemisphere away at the time of the solstice, either directly to the sun or away from it, leading to extreme seasonal effects. This means that the poles of Ariel are illuminated by the sun are in permanent darkness for half a Uranus year of 42 years or so. During the solstice, the sun is therefore close to the zenith over the poles. During the Voyager 2 flyby in 1986, which occurred almost at the solstice, which showed Südhemisphären of Uranus and its moons towards the sun, while the northern hemispheres were in complete darkness. During the equinox, in which the equatorial plane intersects with the direction to the earth and which also happens every 42 years, mutual occultations and eclipses of the moons of Uranus Uranus are possible. A number of these rare events took place last from 2007 to 2008; Ariel was covered by Umbriel on August 19, 2007, taken by the Hubble Space Telescope. Another transit was documented by the European Southern Observatory 2008.

Ariel currently owns no orbital resonance with other moons. In his history he was but possibly in a 5:3 resonance with Miranda, who was possibly responsible for the internal heating of this moon. Also Ariel could have been in a 4:1 resonance with Titania, from which he escaped later, which was favored by the lower oblateness of Uranus and the relatively larger moons compared to Jupiter and Saturn. This resonance, which probably took place 3.8 billion years ago, would the eccentricity of Ariel have increased orbit, and due to Uranus ' tidal forces and the temporal variation, which was created by the increased eccentricity, for heating up to 20 K.

Rotation

The rotation period is equal to the orbital period and Ariel has with how the Earth's moon, a synchronous rotation, which therefore also takes place within 2 days 12 hours 29 minutes and 21 seconds. Its rotation axis is exactly perpendicular to its orbital plane.

Physical Properties

Size

Ariel is slightly irregular in shape with dimensions of 1162.2 × 1155.8 × 1155.4 km. He is the fourth largest moon Uranus and slightly smaller than the third-largest moon Umbriel, but he seems to be more massive than this.

From the size Ariel is most likely with Umbriel, compare Saturn's moon Dione or the Pluto Charon. Of the entire moon has been able by Voyager 2 only 35%, mainly the southern hemisphere - as with all Uranian moons - are explored in more detail.

The total area of ​​4.211 million km2 Ariel is about, this is roughly equivalent to just under the surface of the European Union.

Internal construction

Ariel has an average density of 1.66 g / cm ³. Starting from the high albedo of 0.39 and low density, it is assumed that Ariel is composed of about 50% water ice, 30% siliceous rock and 20 % carbon compounds such as methane and the organic heavy Tholin. The presence of water ice is supported by infrared spectroscopic investigations that brought crystalline water ice on Ariel's surface to light. This seems to be more present on Ariel's leading hemisphere. The reason for this is unknown, but it seems from the bombardment of charged particles of Uranus ' magnetosphere to stem that has a stronger presence on the trailing hemisphere by the co- rotation of the plasma. These energetic particles tend to sputtering of water ice, the decomposition of methane trapped in ice as clathrate hydrate and the darkening of other organic material, resulting in carbon-rich deposits on the surface.

Apart from the water ice previously could only carbon dioxide ( CO2) can be spectroscopically proved beyond doubt and this compound is mainly focused on the following hemisphere. Ariel was the first Uranus moon, in which CO2 could be found, and it is also the most highly represented on it. Its origin is not yet well known. It could be produced locally from carbonates or organic material through the influence of the charged particles of Uranus ' magnetosphere, or by solar ultraviolet radiation. The former hypothesis would explain the asymmetry in the distribution because the following hemisphere is under stronger influence of the magnetosphere. Another possible source is the outgassing of primordial CO2 that is trapped in water ice in Ariel's heart. The release of CO2 from the inside is possibly connected with the past geological activity of the moon.

The size, the water ice - rock mixture and the possible presence of salt or ammonia - which lower the freezing point of water - point out that Ariel is a differentiated body, with a rocky core and a shell of water ice. If this is the case, the diameter of the core would be 744 km, which corresponds to 64 % of the total diameter, and a core mass of 56 % of the total mass - these parameters are determined by the composition of the moon. The pressure in the center of Ariel is about 3 kbar. That could exist in the ice mantle of Ariel an underground ocean as on the Jovian moon Europa, is considered by previous studies as unlikely.

Surface

Ariel's surface has larger regions where only few impact craters are visible. It shows a network of faults and canyons. Some ice fields appear to be relatively freshly formed. This suggests that Ariel in the past was a scene of intense geological activity.

The moon has a bright surface having a high geometric albedo of 0.39, i.e., 39 % of the incident solar light is reflected. The surface shows depending on the lighting angle high brightness effects; the reflectivity of 0.53 at a phase angle of 0 ° to 0.35 decreases rapidly from about 1 °. The Bond albedo is about 0.23, the highest of all Uranus satellites.

The color of Ariel's surface generally appears in a neutral gray, but it seems to exist a minimal dichotomy of the leading and the trailing hemisphere; the latter appears reddish to about 2%. The albedo and geology of Ariel's surface do not seem to correspond with the color; the canyons, for example, show up in the same color as the surrounding craters. There are slightly bluish deposits of Impaktmaterial relatively fresh craters. There are also slightly bluish local points that are related to any of the known surface structures on Ariel in context.

The maximum surface temperature of -189 ° C is Ariel (84 K); on average there are only about an estimated -213 ° C (60 K).

The previously known surface can be divided into three different types of terrain: In planes in of combs and grooves criss-crossed terrain and older, strewn with craters terrain. The commonly observed surface structures on Ariel are impact craters, canyons, fold mountains, ridges and valleys.

Levels

The planes are the most recent observed surface structures on Ariel, which are relatively low-lying areas of gentle and judging by the number of craters must have formed over a long period of time. The levels are in the bottoms of canyons and in some irregular depressions in the middle of zerkraterten terrains. The latter are separated from the surrounding area by sharp boundaries, which have in some cases, a curved pattern. The most likely cause of these levels are kryovulkanische processes. This is reminiscent of terrestrial shield volcanoes geometry and various topographic boundary zones indicate that the exiting material is very viscous, possibly had to consist of a very cold water -ammonia mixture, or even of solid ice. The thickness of this hypothetical Kryolavaflüsse was estimated to be 1-3 km. The canyons must therefore have been formed in a time when the endogenous surface deformation on Ariel was still active.

Ridges and grooves

This type of terrain contains bands of ridges and troughs with several hundred kilometers in length. They limit the terrain studded with craters and cut it up into polygonal structures. Within these bands, which can be up to 25 to 70 km wide, there are a few individual ridges and grooves, which can be up to 200 km long and 15-35 km wide. These structures often represent continuations of the canyon, indicating that they are a modified form of the trenches or caused by a different reaction to the tectonic processes that shaped the canyon. This may have come through another brittleness of the material existence.

Crater terrain

The zerkraterte site provides the most extensive and oldest surface structure is Ariel and extends from the geographic South Pole radially outward. It is intersected by ridges, valleys and canyons, which are located mainly in the central and southern latitudes.

Chasmata and Valles

The canyons are called Chasma (plural Chasmata ), probably represent grave breaches caused by tectonic extension processes. They are the result of a global pressure, which was caused by the freezing of water or a water- ammonia solution in Ariel's heart. They are usually about 15 to 50 km wide and extend mainly in an easterly or northeasterly direction. The floors of many Chasmata are convex, they are higher in part 1 to 2 km. The widest canyons sometimes have grooves that are called run along the crests of the convex base and Valles. It is noteworthy that these linear valleys partly be invisible when they are crossed by transverse grooves running through the valley. Apparently these valleys were filled up again after their tectonic origin by nachfließendes at a later date kryovulkanisches material leveled at these points and visually merged with the environment.

The largest canyon is Kachina Chasmata, a system of several valleys of 622 km long and 50 km wide, which still, however, can be several times longer. Since the Voyager 2 flyby only was largely the southern hemisphere in sunlight, only the called length could be determined with certainty. Subsequent analyzes of part of the northern hemisphere, which was after all illuminated by Uranus and where you brought by advanced processes some details to light, have revealed that the Kachina Chasmata can be up to 1800-2200 km long and thus Ithaca Chasma similar to Saturn's moon Tethys would.

The surface of Ariel acts moderate and balanced cratered compared to other Uranian moons. The relative flatness and the small number of large craters indicates that they arose only after the formation of the solar system. This means that the surface must have completely renewed in a certain time. The largest crater is observed with only 78 km in diameter Yangoor, and this is showing signs of later deformation on. All the great crater craters have flat floors and central mountains, some of the fresher craters show bright deposits from Impaktmaterial. Many craters have polygonal patterns that give an indication of the fact that her appearance was influenced by the existing structure of the crust.

Add the studded with craters levels, there are some bright spots of about 100 km in diameter, which may be leveled crater. In this case, they would resemble the palimpsests that have also been found on Jupiter's moon Ganymede (see also ghost craters). It is believed, that a 245 km wide rounded structure at 10 ° South and 30 degrees East one of these palimpsests.

Formation

Ariel was probably formed by an accretion disk or by a sub- fog, which was possibly to Uranus during its time of origin or formed by the (still theoretical ) impact that could tip the planet on its side. The exact composition of this sub- nebula is not known, but, the higher densities of Uranus system compared to the lying closer to the Sun Saturn 's moons to a relative lack of water out. Potentially significant amounts of nitrogen (N2) and carbon in the form of carbon monoxide ( CO) and molecular nitrogen were present instead of ammonia (NH3 ) and methane ( CH4). Satellites that emerged from such a sub- fog would contain less water ice and CO and N2 as gas hydrate trapped in ice and more rock include, which would explain the higher densities.

The accretion process probably lasted several thousand years until the formation of Ariel was completed. Models show that the accretion accompanying impacts would cause a heating of the outer shell of the moon with a temperature of up to 195 K at a depth of up to 31 km. After the formation of this outer layer cooled, while Ariel's affairs by the decomposition aufheizte radioactive elements in the rock. Cooling the outer sheath contracted, while the inside expanded. This has caused severe stress in the crust of the moon at a pressure of up to 3 kbar estimated, leading to breakage of the crust. The canyons are probably a result of this process, which lasted about 200 million years.

Akkretionshitze the initial and subsequent decomposition radioactive elements may lead to a melting water ice when a freezing point -lowering substance, such as a salt of ammonia or in the form of ammonium hydroxide was present. This would lead to a separation of ice and rock (differentiation) of the core. In this case, a layer of liquid water would have led rich of dissolved ammonia, the boundary of mantle and core. The eutectic temperature of this mixture is 176 K. This ocean is however probably already frozen over. This freezing presumably led to the extension of the interior, which was probably responsible for the formation of Chasmata and the renewal of the surface was. The liquid water may have been able to erupt from the crust, and to pour over the bottoms of Chasmata ( Kryovulkanismus ).

Thermal models of Saturn's moon Dione, which is similar in size, density, and surface temperature as Ariel, show that a solid convection could have lasted over several billion years, and that temperatures of 173 K near the surface over several hundred million years after the could persist formation of the moon, and closer to the core of even up to a billion years.

Research

Since the discovery in 1851 by William Lassell was about 135 years out of the orbital parameters Ariel not much is known. The moon was too small and too far away to resolve it closer with ground-based telescopes. The apparent brightness of Ariel is 14.4 like, similar to that of Pluto at perihelion. While Pluto can be observed through a telescope with 30 cm aperture, a 40 -cm opening is due to the proximity of Ariel at Uranus and the fact that he is thereby eclipsed by this necessary.

On January 20, 1986 Ariel was relatively close passes by the Voyager 2 probe and photographed and measured. The rotation axis of Uranus and Ariel reported, due to the high axial inclination of the planetary system of 98 °, at which point toward Earth, so that the moons of Uranus could not be served individually as before at Jupiter and Saturn on the equatorial plane, but to their orbits like a target around the planet ruled and the planet had to be made virtually. This meant that all the moons of Uranus in each case only the southern hemisphere was photographed at intervals of about two days - the worst possible position for a flyby. In addition, you had to opt for a moon, as a close flyby at a necessarily large distances to all other conditional.

Since we wanted to draw on Voyager 2 to Neptune, the condition was a close Uranus flyby. This revealed that only the moon Miranda could be happening close. The nearest approach to Ariel was 127,000 km, yet he was the only one besides Miranda Uranus moon, could be sent back to Earth from the relatively high-resolution images. The best resolution of the photos was about 2 km; they show about 40 % of the surface, with only about 35 % could be used for geological mapping and crater counting.

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