Ariel (moon)
| Ariel | |
|---|---|
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| Voyager 2 shot of Ariel | |
| Central body | Uranus |
| Properties of the orbit | |
| Major semi-axis | 190,900 km |
| Periapsis | 190,670 km |
| Apoapsis | 191,130 km |
| eccentricity | 0.0012 |
| Orbit inclination | 0.041 ° |
| Orbital time | 2,520379 d |
| Mean orbital velocity | 5.51 km / s |
| Physical Properties | |
| Albedo | 0.39 |
| Apparent brightness | 13.70 likes |
| Medium diameter | 1157.8 ± 1.2 (1162.2 × 1155.8 × 1155.4) km |
| Dimensions | 1.353 ± 0.120 × 10 21 kg |
| surface | 4,211,000 km 2 |
| Medium density | 1.66 ± 0.15 g / cm 3 |
| Sidereal rotation | 2.520379 days |
| Axis inclination | 0 ° |
| Acceleration of gravity on the surface | 0.27 m / s 2 |
| Escape speed | 558 m / s |
| Surface temperature | −189 ± 1 ° C to −213 ° C / 84 ± 1–60 K |
| discovery | |
| Explorer | |
| Date of discovery | October 24, 1851 |
| Remarks | Brightest Uranus moon |
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| Size comparison between Uranus (left) and its largest moons, from left to right Puck, Miranda, Ariel, Umbriel, Titania and Oberon (photo montage to scale) | |
The moon Ariel (also Uranus I ) is the brightest and fourth largest of the 27 known moons of the planet Uranus . It has - counting from the inside out - the fifteenth orbit and of all Uranus moons has the greatest surface reflectivity ( albedo ).
Discovery and naming
Ariel and Umbriel were discovered on October 24, 1851 as the third and fourth Uranus moons by the British astronomer William Lassell with a 60 cm reflecting telescope at a self-made observatory in Liverpool .
The moon was named after a sylph in Alexander Pope's verse The Robbery . Since all Uranus moons except Ariel, Umbriel and Belinda are named after characters by William Shakespeare , it is often wrongly assumed that it is the aerial spirit of the same name from his play The Tempest .
The names of the four first discovered Uranus moons ( Oberon , Titania , Ariel and Umbriel) suggested John Herschel , the son of Wilhelm Herschel . Wilhelm Herschel himself discovered Oberon, Titania and Uranus.
Track properties
Orbit
Ariel orbits Uranus on a prograde , almost perfectly circular orbit at an average distance of around 190,900 km (approx. 7.469 Uranus radii) from its center, i.e. around 165,300 km above its cloud ceiling. The orbit eccentricity is 0.0012, the orbit is inclined 0.041 ° to the equator of Uranus .
The orbit of the nearest inner moon Miranda is on average 61,000 km 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 is entirely in the magnetosphere of Uranus. The subsequent hemispheres of atmospheric moons like Ariel are therefore under constant bombardment by magnetospheric plasma , which rotates with the planet. This can lead to a darkening of the subsequent hemisphere, which has so far been observed in all Uranus moons except Oberon. Ariel also traps magnetospherically charged particles, resulting in increased numbers of these particles in Ariel's orbit, as observed with the Voyager 2 spacecraft .
Since Ariel, like Uranus, practically orbits the sun on its side relative to the rotation, its northern and southern hemisphere points either directly towards or away from the sun at the time of the solstice , which leads to extreme seasonal effects. This means that the poles of Ariel are in permanent darkness or are illuminated by the sun for half a Uranus year of 42 years. During the solstice, the sun is therefore near the zenith over the poles. During the Voyager 2 flyby in 1986, which occurred almost at the solstice, the southern hemispheres of Uranus and its moons pointed toward the sun, while the northern hemispheres were in complete darkness. During the equinox , when the equatorial plane crosses with the direction towards the earth and which also occurs every 42 years, mutual occultations of the Uranus moons and solar eclipses on Uranus are possible. A number of these rare events last took place between 2007 and 2008; Ariel was covered by Umbriel on August 19, 2007 and was captured by the Hubble Space Telescope . The European Southern Observatory documented another transit in 2008 .
Currently Ariel has no orbital resonance with other moons. However, in its history it may have been in a 5: 3 resonance with Miranda, possibly responsible for the internal heating of this moon. Ariel could also have been in a 4: 1 resonance with Titania , from which he later escaped, which was favored by the lower flattening of Uranus and the relatively larger moons compared to Jupiter and Saturn . This resonance, which probably took place 3.8 billion years ago, would have the eccentricity increased from Ariel's orbit and due to Uranus' tidal forces , for heating by up to 20 and the temporal variation that was created by the increased eccentricity K lead.
rotation
The rotation time is the same as the orbital time and Ariel shows, like the earth's moon , a synchronous rotation , which thus also takes place within 2 days, 12 hours 29 minutes and 21 seconds. Its axis of rotation is exactly perpendicular to its plane .
Physical Properties
size
Ariel is somewhat irregular in shape with dimensions of 1162.2 × 1155.8 × 1155.4 km. It is thus the fourth largest moon of Uranus and slightly smaller than the third largest moon Umbriel, but it seems to be more massive than this.
In terms of size, Ariel can best be compared with Umbriel, the Saturn moon Dione or the Pluto moon Charon . Voyager 2 has so far only been able to investigate 35% of the entire moon , mainly the southern hemisphere - as with all Uranus moons.
The total area of Ariel is about 4,211,000 km 2 , which roughly corresponds to the area of the European Union .
internal structure
Ariel has an average density of 1.66 g / cm³. Based on the high albedo of 0.39 and the low density, it is assumed that Ariel is composed of around 50% water ice , 30% silicate rock and 20% carbon compounds such as methane and the organic heavy tholine . The presence of water ice is supported by infrared spectroscopic studies that revealed crystalline water ice on Ariel's surface. This seems to be more prominent on Ariel's leading hemisphere. The reason for this is unknown, but it appears to have come from the bombardment of charged particles from Uranus' magnetosphere, which is more represented in the following hemisphere due to the co-rotation of the plasma. These energetic particles tend to sputter water ice, decompose methane trapped in ice as gas hydrate , and darken other organic matter, resulting in carbon-rich deposits on the surface.
Apart from the water ice, only carbon dioxide (CO 2 ) could be detected spectroscopically without any doubt and this compound is mainly concentrated on the following hemisphere. Ariel was the first moon of Uranus , in which CO 2 could be found, and it is also the most represented on it. Its origin is not yet sufficiently known. It could be produced locally from carbonates or organic matter by the influence of the charged particles of Uranus' magnetosphere, or by solar ultraviolet radiation . The former hypothesis would explain the asymmetry in the distribution, since the following hemisphere is under greater influence from the magnetosphere. Another possible source is the outgassing of primordial CO 2 trapped in water ice inside Ariel. The release of CO 2 from the interior may be related to 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 - indicate that Ariel is a differentiated body, with a rock core and a coat of water ice. If so, the diameter of the core would be 744 km, which is 64% of the total diameter, and a core mass of 56% of the total mass - these parameters are dictated by the composition of the moon. The pressure in the center of Ariel is about 3 kbar . According to previous studies, it is unlikely that an underground ocean like that on Jupiter's moon Europa could exist in the ice mantle of Ariel .
surface
Ariel's surface has larger regions with few impact craters visible. A network of faults and canyons emerges . Some ice fields appear to be relatively fresh. This suggests that Ariel has been the site of intense geological activity in the past.
The moon has a bright surface with a high geometric albedo of 0.39, i.e. That is, 39% of the incident sunlight is reflected. The surface shows great brightness effects depending on the lighting angle; the reflectivity of 0.53 at a phase angle of 0 ° drops rapidly to 0.35 at around 1 °. The spherical albedo is around 0.23, the highest of all uranium satellites.
The color of Ariel's surface is generally a neutral gray, but there appears to be a minimal dichotomy of the leading and trailing hemispheres; the latter appears about 2% more reddish. The albedo and geology of Ariel's surface do not seem to correspond to its color; the canyons, for example, appear in the same color as the surrounding crater landscape. Slightly bluish deposits of impact material exist around relatively fresh craters. There are also slightly bluish local points that are not associated with any of the known surface structures on Ariel.
The maximum surface temperature of Ariel is −189 ° C (84 K ); on average, however, it is only an estimated −213 ° C (60 K).
The previously known surface can be divided into three different types of terrain: Plains, terrain with ridges and grooves, and older terrain strewn with craters. The most commonly observed surface structures on Ariel are impact craters, canyons, fold mountains , ridges and troughs.
Levels
The plains are the most recent surface structures observed on Ariel, which represent relatively deep-lying, gentle areas and, judging by the number of craters, must have formed over a long period of time. The plains are in the canyon floors and in some irregular depressions in the middle of the crumbled terrain. The latter are separated from the surrounding terrain by sharp boundaries, which in some cases have a curved pattern. The most likely cause of these levels is cryovolcanic processes. This geometry, reminiscent of terrestrial shield volcanoes , and various topographical edge zones indicate that the emerging material must have been very viscous , possibly a very cold water-ammonia mixture, or even solid ice. The thickness of these hypothetical cryolava flows was estimated to be 1–3 km. The canyons must therefore have formed at a time when endogenous surface transformation was still active on Ariel.
Combs and grooves
This type of terrain contains bands of ridges and troughs several hundred kilometers in length. They delimit the crater-strewn terrain and cut it up into polygon- like structures. Within these bands, which can be up to 25 to 70 km wide, there are individual individual ridges and grooves that can be up to 200 km long and 15 to 35 km wide. These structures often represent continuations of the canyons, indicating that they are a modified form of the trenches or that they were created by a different response from the tectonic processes that formed the canyons. This may be due to another fragility of the material.
Crater area
The crumpled terrain represents the most extensive and oldest surface structure on Ariel and extends radially outward from the geographic South Pole. It is cut through by ridges, valleys and canyons, which are mainly located in the mid and southern latitudes.
Chasmata and Valles
The canyons, which are called Chasma (plural Chasmata ), presumably represent rift breaks that have arisen through tectonic expansion processes. They are the result of global pressure caused by the freezing of water or a water-ammonia solution inside Ariel. They are usually about 15 to 50 km wide and run mainly in an easterly or north-easterly direction. The floors of many Chasmata are convex , they are partly 1 to 2 km higher. The widest canyons sometimes have grooves that run along the ridges of the convex floors, called valles . It is noteworthy that these linear valleys become partly invisible when they are crossed by grooves running across the valley. Apparently these valleys were only filled up after their tectonic formation by cryovolcanic material flowing in at a later point in time, leveled at these points and optically merged with the surroundings.
The largest canyon is Kachina Chasmata, a system of several valleys 622 km long and 50 km wide, but which can be several times longer. Since only the southern hemisphere was largely exposed to sunlight during the Voyager 2 flyby, only the length mentioned could be determined with certainty. Later analyzes of a part of the northern hemisphere, which was illuminated by Uranus and where some details were brought to light by advanced processes, have shown that the Kachina Chasmata can be up to 1800 to 2200 km long and thus resemble Ithaca Chasma on the Saturn moon Tethys would.
| Surname | Length (km) | Coordinates | Origin of name |
|---|---|---|---|
| Kachina Chasmata | 622.0 | 33 ° 42′S 246 ° 00′E / 33.7 ° S 246 ° E | Kachina ( Hopi mythology ) |
| Kewpie Chasma | 467.0 | 28 ° 18′S 326 ° 54′E / 28.3 ° S 326.9 ° E | Kewpie ( English folklore ) |
| Korrigan Chasma | 365.0 | 27 ° 36′S 347 ° 30′E / 27.6 ° S 347.5 ° E | Korrigan , ( Breton mythology ) |
| Sylph Chasma | 349.0 | 48 ° 36′S 353 ° 00′E / 48.6 ° S 353 ° E | Sylph , (English folklore) |
| Brownie Chasma | 343.0 | 16 ° 00′S 37 ° 36′E / 16 ° S 37.6 ° E | Brownie (English Folklore) |
| Pixie Chasma | 278.0 | 20 ° 24′S 5 ° 06′E / 20.4 ° S 5.1 ° E | Pixie (English folklore) |
| Kra Chasma | 142.0 | 32 ° 06′S 354 ° 12′E / 32.1 ° S 354.2 ° E | Kra ( Akan belief system ) |
| Surname | Length (km) | Coordinates | Origin of name |
|---|---|---|---|
| Leprechaun Vallis | 328.0 | 10 ° 24′S 10 ° 12′E / 10.4 ° S 10.2 ° E | Leprechaun ( Irish mythology ) |
| Sprite Vallis | 305.0 | 14 ° 54′S 340 ° 00′E / 14.9 ° S 340 ° E | Sprite ( Celtic Mythology ) |
The surface of Ariel appears moderately and balanced cratered compared to other Uranus moons. The relative flatness and the small number of large craters indicate that they only formed after the solar system was formed. This means that the surface must have been completely renewed in a certain time. The largest crater observed is Yangoor , only 78 km in diameter , and it shows signs of later deformation. All large craters have flat crater floors and central mountains , some of the fresher craters show light-colored deposits of impact material. Many craters have polygonal patterns that indicate that their appearance was influenced by the pre-existing structure of the crust.
In the crater-studded plains there are some bright spots about 100 km in diameter that may be leveled craters. In this case they would resemble the palimpsests that were also found on Jupiter's moon Ganymede (see also ghost crater ). It is believed that a 245 km round structure at 10 ° South and 30 ° East represents one of these palimpsests.
| Surname | Diameter (km) | Coordinates | Origin of name |
|---|---|---|---|
| Yangoor | 78.0 | 68 ° 42′S 279 ° 42′E / 68.7 ° S 279.7 ° E | Yangoor , a good spirit who brings the day |
| Domovoy | 71.0 | 71 ° 30′S 339 ° 42′E / 71.5 ° S 339.7 ° E | Domovoi ( Slavic mythology ) |
| Melusine | 50.0 | 52 ° 54′S 8 ° 54′E / 52.9 ° S 8.9 ° E | Melusine ( French literature ) |
| Rima | 41.0 | 18 ° 18′S 260 ° 48′E / 18.3 ° S 260.8 ° E | Rima from William Henry Hudson's Green Mansions |
| Huon | 40.0 | 37 ° 48′S 33 ° 42′E / 37.8 ° S 33.7 ° E | Huon of Bordeaux (French literature) |
| Oonagh | 39.0 | 21 ° 54′S 244 ° 24′E / 21.9 ° S 244.4 ° E | Oonagh ( Irish mythology ) |
| Agape | 34.0 | 46 ° 54′S 336 ° 30′E / 46.9 ° S 336.5 ° E | Agape ( Spenser ) |
| Gwyn | 34.0 | 77 ° 30′S 22 ° 30′E / 77.5 ° S 22.5 ° E | Gwyn ap Nudd (Irish mythology) |
| Mab | 34.0 | 38 ° 48′S 352 ° 12′E / 38.8 ° S 352.2 ° E | Queen Mab (Irish Mythology) |
| Finvara | 31.0 | 15 ° 48′S 19 ° 00′E / 15.8 ° S 19 ° E | Finvarra (Irish mythology) |
| Laica | 30.0 | 21 ° 18′S 44 ° 24′E / 21.3 ° S 44.4 ° E | Laica ( Inca mythology ) |
| Berylune | 29.0 | 22 ° 30′S 327 ° 54′E / 22.5 ° S 327.9 ° E | Bérylune ( Maurice Maeterlinck ) |
| Ataksak | 22.0 | 53 ° 06′S 224 ° 18′E / 53.1 ° S 224.3 ° E | Ataksak ( Inuit mythology ) |
| Djadek | 22.0 | 12 ° 00′S 251 ° 06′E / 12 ° S 251.1 ° E | Djadek ( Czech folklore ) |
| Befana | 21.0 | 17 ° 00′S 31 ° 54′E / 17 ° S 31.9 ° E | Befana ( Italian folklore) |
| Abans | 20.0 | 15 ° 30′S 251 ° 18′E / 15.5 ° S 251.3 ° E | Aban ( Persian mythology ) |
| Deive | 20.0 | 22 ° 18′S 23 ° 00′E / 22.3 ° S 23 ° E | Deivė ( Lithuanian folklore) |
Emergence
Ariel was probably formed by an accretion disk or by a sub-nebula that may have been around Uranus during its formation or that formed after the (still theoretical) impact that caused the planet to tip over. The exact composition of this sub-nebula is not known, but the higher densities of the Uranus system compared to Saturn's moons closer to the Sun indicate a relative scarcity of water. There may have been significant amounts of nitrogen (N 2 ) and carbon in the form of carbon monoxide (CO) , and molecular nitrogen instead of ammonia (NH 3 ) and methane (CH 4 ) . Satellites that emerged from such a sub-nebula should contain less water ice and CO and N 2 than gas hydrate trapped in ice and more rock, which would explain the higher densities.
The accretion process may take several thousand years to complete the formation of Ariel. Models show that the impacts accompanying the accretion should cause the outer shell of the moon to heat up to a temperature of up to 195 K at a depth of up to 31 km. Once formed, this outer layer cooled while Ariel's interior heated up due to the decomposition of radioactive elements in the rock. The cooling outer shell contracted while the interior expanded . This caused strong stresses in the moon's crust with a pressure of up to an estimated 3 kbar , which led to fractures on the crust. The canyons are likely a result of this process, which took about 200 million years.
The initial heat of accretion and the subsequent decomposition of radioactive elements may have melted water ice if a substance that depresses the freezing point, such as salt or ammonia, was present in the form of ammonia water . This should have led to a separation of ice and rock (differentiation) of the core. In this case, a layer of liquid water rich in dissolved ammonia should have formed at the boundary between the mantle and core. The eutectic temperature of this mixture is 176 K. However, this ocean has probably long since frozen over. This freezing over probably led to the expansion of the interior, which was probably responsible for the formation of the chasmata and the renewal of the surface. The liquid water may have been able to erump from the crust and pour over the floors of the Chasmata ( cryovolcanism ).
Thermal models of Saturn's moon Dione, which is similar in size, density, and surface temperature to Ariel, show that solid convection could have lasted for billions of years, and that temperatures of 173K near the surface could have lasted for hundreds of millions of years after formation of the moon could persist, closer to its core even for up to a billion years.
exploration
For about 135 years since William Lassell's discovery in 1851, little was known about Ariel other than orbit parameters. The moon was too small and too far away to be resolved with terrestrial telescopes. The apparent magnitude of Ariel is 14.4 mag, which is similar to that of Pluto in perihelion . While Pluto can be observed through a telescope with a 30 cm aperture, the proximity of Ariel to Uranus and the fact that it is thereby outshone by it requires a 40 cm aperture.
On January 20, 1986, Ariel was passed relatively close by the " Voyager 2 " probe and photographed and measured. As a result of the high inclination of the planetary system of 98 °, the axes of rotation of Uranus and Ariel pointed towards Earth at this point in time, so that the moons of Uranus could not be approached individually on the equatorial plane as was previously the case with Jupiter and Saturn, but rather each other Arranged orbits like a target around the planet and the planet had to be hit. This meant that of all of Uranus' moons, only the southern hemisphere could be photographed every two days - the worst possible position for a flyby. In addition, you had to choose a moon, as a close pass by inevitably required large distances to all others.
Since Voyager 2 was to be steered further to Neptune , the prerequisite for this was a close Uranus flyby. As a result, only the moon Miranda could be passed close. The closest approach to Ariel was 127,000 km, but besides Miranda it was the only Uranus moon from which relatively high-resolution images could be sent back to Earth. The best resolution of the photos was about 2 km; they show about 40% of the surface, whereby only about 35% could be used for geological maps and crater counting.
Web links
- Satellite Viewer Orbit simulation of the Uranus moons
- USGS list of named structures on Ariel