MOONS OF URANUS

Uranus and its six largest moons compared at their proper relative sizes and relative positions. From left to right: Puck, Miranda, Ariel, Umbriel, Titania and Oberon

Uranus, the seventh planet of the Solar System, has 27 known moons, all of which are named after characters from the works of William Shakespeare and Alexander Pope. William Herschel discovered the first two moons, Titania and Oberon, in 1787, and the other ellipsoidal moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda). These have planetary mass, and so would be considered (dwarf) planets if they were in direct orbit about the Sun. The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced Earth-based telescopes.

Uranian moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner moons are small dark bodies that share common properties and origins with the planet's rings. The five major moons are massive enough to have achieved hydrostatic equilibrium, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces.[3] The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, and about 20 times less massive than Earth's Moon. Uranus's irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at great distances from the planet.

DISCOVERY The first two moons to be discovered, Titania

and Oberon, were spotted by Sir William Herschel on January 11, 1787, six years after he had discovered the planet itself. Later, Herschel thought he had discovered up to six moons (see below) and perhaps even a ring. For nearly 50 years, Herschel's instrument was the only one with which the moons had been seen.[4] In the 1840s, better instruments and a more favorable position of Uranus in the sky led to sporadic indications of satellites additional to Titania and Oberon. Eventually, the next two moons, Ariel and Umbriel, were discovered by William Lassell in 1851.[5]

The Roman numbering scheme of Uranus's moons was in a state of flux for a considerable time and publications hesitated between Herschel's designations (where Titania and Oberon are Uranus II and IV) and William Lassell's (where they are sometimes I and II).[6] With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck.[7] In 1852, Herschel's son John Herschel gave the four then-known moons their names.[8]

No other discoveries were made for almost another century. In 1948, Gerard Kuiper at the McDonald Observatory discovered the smallest and the last of the five large, spherical moons, Miranda.[8][9] Decades later, the flyby of the Voyager 2 space probe in January 1986 led to the discovery of ten further inner moons.[3] Another satellite, Perdita, was retroactively discovered in 1999[10] after studying old Voyager photographs.[11]

Uranus- was the last giant planet without any known irregular satellites, but since 1997 nine distant irregular moons have been identified using ground-based telescopes.[1] Two more small inner moons, Cupid and Mab, were discovered using the Hubble Space Telescope in 2003.[12] The moon Margaret was the last Uranian moon discovered as of 2008, and its findings were published in October 2003.[13]

SPURIOUS MOONS After Herschel discovered Titania and Oberon on January

11, 1787, he subsequently believed that he observed four other moons; two on January 18 and February 9, 1790, and two more on February 28 and March 26, 1794. It was thus believed for many decades thereafter that Uranus had a system of six satellites, though the four latter moons were never confirmed by any other astronomer. Lassell's observations of 1851, in which he discovered Ariel and Umbriel, however, failed to support Herschel's observations; Ariel and Umbriel, which Herschel certainly ought to have seen if he had seen any satellites beside Titania and Oberon, did not correspond to any of Herschel's four additional satellites in orbital characteristics. Herschel's four spurious satellites were thought to have sidereal periods of 5.89 days (interior to Titania), 10.96 days (between Titania and Oberon), 38.08 and 107.69 days (exterior to Oberon).[14] It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of faint stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell.[15]

: NAMING OF MOONS

The first two Uranian moons, discovered in 1787, did not receive names until 1852, a year after two more moons had been discovered. The responsibility for naming was taken by John Herschel, son of the discoverer of Uranus. Herschel, instead of assigning names from Greek mythology, named the moons after magical spirits in English literature: the fairies Oberon and Titania from William Shakespeare's A Midsummer Night's Dream, and the sylphs Ariel and Umbriel from Alexander Pope's The Rape of the Lock (Ariel is also a sprite in Shakespeare's The Tempest). The reasoning was presumably that Uranus, as god of the sky and air, would be attended by spirits of the air.[16]

Subsequent names, rather than continuing the airy spirits theme (only Puck and Mab continued the trend), have focused on Herschel's source material. In 1949, the fifth moon, Miranda, was named by its discoverer Gerard Kuiper after a thoroughly mortal character in Shakespeare's The Tempest. The current IAU practice is to name moons after characters from Shakespeare's plays and The Rape of the Lock (although at present only Ariel, Umbriel, and Belinda have names drawn from the latter poem; all the rest are from Shakespeare). At first, the outermost moons were all named after characters from one play, The Tempest; but with Margaret being named from Much Ado About Nothing that trend has ended.[8]

] It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of faint stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell.[

THE RELATIVE MASSES OF THE URANIAN MOONS. THE FIVE ROUNDED MOONS VARY FROM MIRANDA AT 0.7% TO TITANIA AT ALMOST 40% OF THE TOTAL MASS. THE OTHER MOONS COLLECTIVELY CONSTITUTE 0.1%, AND ARE BARELY VISIBLE AT THIS SCALE.

Some asteroids share names with moons of Uranus: 171 Ophelia, 218 Bianca, 593 Titania, 666 Desdemona, 763 Cupido and 2758 Cordelia.

CHARACTERISTICS AND GROUPS SCHEMATIC OF THE URANIAN MOON-RING SYSTEM

The Uranian satellite system is the least massive among those of the gas giants; indeed, the combined mass of the five major satellites would be less than half that of Triton (the seventh-largest moon in the Solar System) alone.[note 1] The largest of the satellites, Titania, has a radius of 788.9 km,[18] or less than half that of the Earth's Moon, but slightly more than that of Rhea, the second largest moon of Saturn, making Titania the eighth-largest moon in the Solar System. Uranus is about 10,000 times more massive than its moons.[note 2]

INNER MOONSSEE ALSO: INNER MOON AND RINGS OF URANUS

As of 2008, Uranus is known to possess 13 inner moons.[12] Their orbits lie inside that of Miranda. All inner moons are intimately connected to the rings of Uranus, which probably resulted from the fragmentation of one or several small inner moons.[19] The two innermost moons (Cordelia and Ophelia) serve as shepherds of Uranus's ε ring, while small moon Mab is a source of Uranus's outermost μ ring.[12]

Puck, at 162 km, is the largest of the inner moons of Uranus and the only one imaged by Voyager 2 in any detail. Puck and Mab are the 2 outermost inner satellites of Uranus. All inner moons are dark objects; their geometrical albedo does not exceed 10%.[20] They are made of water ice contaminated with a dark material—probably radiation processed organics.[21]

The small inner moons constantly perturb each other. The system is chaotic and apparently unstable. Simulations show that the moons may perturb each other into crossing orbits, which may eventually result in collisions between the moons.[12] Desdemona may collide with either Cressida or Juliet within the next 100 million years.[22]

THE FIVE LARGEST MOONS OF URANUS COMPARED AT THEIR PROPER RELATIVE SIZES AND BRIGHTNESSES. FROM LEFT TO RIGHT (IN ORDER OF INCREASING DISTANCE FROM URANUS): MIRANDA, ARIEL, UMBRIEL, TITANIA AND OBERON

LARGE MOONS Uranus has five major moons: Miranda, Ariel,

Umbriel, Titania and Oberon. They range in diameter from 472 km for Miranda to 1578 km for Titania.[18] All large moons are relatively dark objects: their geometrical albedo varies in the range of 30–50%, while bond albedo is within the range of 10–23%.[20] Umbriel is the darkest moon and Ariel is the brightest. The masses of the moons range from 6.7 × 1019 kg (Miranda) to 3.5 × 1021 kg (Titania)—for comparison, Earth's Moon has mass of 7.5 × 1022 kg.[23] The major moons of Uranus are believed to have formed in the accretion disc, which existed around Uranus for some time after its formation or resulted from the large impact suffered by Uranus early in its history.[24][25]

ARTIST'S CONCEPTION OF THE SUN'S PATH IN THE SUMMER SKY OF A MAJOR MOON OF URANUS (WHICH SHARES URANUS' AXIAL TILT)

All major moons comprise approximately equal amounts rock and ice, except Miranda, which is made primarily of ice.[26] The ice component may include ammonia and carbon dioxide.[27] Their surfaces are heavily cratered, though all of them (except Umbriel) show signs of endogenic resurfacing in the form of lineaments (canyons) and, in the case of Miranda, ovoid race-track like structures called coronae.[3] Extensional processes associated with upwelling diapirs are likely responsible for the origin of the coronae.[28] Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.[3] A past 3:1 orbital resonance between Miranda and Umbriel and a past 4:1 resonance between Ariel and Titania are thought to be responsible for the heating that caused substantial endogenic activity on Miranda and Ariel.[29][30

] One piece of evidence for such a past resonance is Miranda's unusually high orbital inclination (4.34°) for a body so close to the planet.[31][32] The largest Uranian moons may be internally differentiated, with rocky cores at their centers surrounded by ice mantles.[26] Titania and Oberon may harbor liquid water oceans at the core/mantle boundary.[26] The major moons of Uranus are airless bodies. For instance, Titania was shown to possess no atmosphere at a pressure larger than 10–20 nanobar.[33]

ORBITS OF URANUS' FIVE MAIN MOONS (IN GREEN). URANUS' ORBIT AROUND THE SUN IS SHOWN IN RED.

The path of the Sun in the local sky over the course of a local day during Uranus' and its major moons' summer solstice is quite different from that seen on most other Solar System worlds. The major moons have almost exactly the same rotational axial tilt as Uranus' (their axes are parallel to that of Uranus).[3] The Sun would appear to follow a circular path around Uranus' celestial pole in the sky, at the closest about 7 degrees away from it.[note 3] Near the equator, it would be seen nearly due north or due south (depending on the season). At latitudes higher than 7°, the Sun would trace a circular path about 15 degrees diameter in the sky, and never set.

IRREGULAR MOONS

As of 2005 Uranus is known to have nine irregular moons, which circle the planet at a distance much greater than that of Oberon, the furthest of the large moons. All the irregular moons are probably captured objects that were trapped by Uranus soon after its formation.[1] The diagram illustrates the orbits of those irregular moons discovered so far. The moons above the X axis are prograde, those beneath are retrograde. The radius of the Uranus' Hill sphere is approximately 73 million km.[1]

Uranus's irregular moons range in size from about 150 km (Sycorax) to 18 km (Trinculo).[1] Unlike Jupiter's irregulars, Uranus's show no correlation axis versus inclination. Instead, the retrograde moons can be divided into two groups based on axis/orbital eccentricity. The inner group includes those satellites closer to Uranus (a < 0.15 rH) and moderately eccentric (~0.2), namely Francisco, Caliban, Stephano and Trinculo.[1] The outer group (a > 0.15 rH) includes satellites with high eccentricity (~0.5): Sycorax, Prospero, Setebos and Ferdinand.[1]

The intermediate inclinations 60° < i < 140° are devoid of known moons due to the Kozai instability.[1] In this instability region, solar perturbations at apoapse cause the moons to acquire large eccentricities that lead to collisions with inner satellites or ejection. The lifetime of moons in the instability region is from 10 million to a billion years.[1]

Margaret is the only known irregular prograde moon of Uranus, and it currently has the most eccentric orbit of any moon in the solar system, though Neptune's moon Nereid has a higher mean eccentricity. As of 2008, Margaret's eccentricity is 0.7979.[34]

HUBBLE SPACE TELESCOPE

The Hubble Space Telescope (HST) is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter (7.9 ft) aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared. The telescope is named after the astronomer Edwin Hubble.

Hubble's orbit outside the distortion of Earth's atmosphere allows it to take extremely sharp images with almost no background light. Hubble's Ultra-Deep Field image, for instance, is the most detailed visible-light image ever made of the universe's most distant objects. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.

Although not the first space telescope, Hubble is one of the largest and most versatile, and is well known as both a vital research tool and a public relations boon for astronomy. The HST was built by the United States space agency NASA, with contributions from the European Space Agency, and is operated by the Space Telescope Science Institute. The HST is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.[5]

Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, scientists found that the main mirror had been ground incorrectly, significantly compromising the telescope's capabilities. However, after a servicing mission in 1993, the telescope was restored to its intended quality.

Hubble is the only telescope designed to be serviced in space by astronauts. Between 1993 and 2002, four missions repaired, upgraded, and replaced systems on the telescope, but a fifth mission was canceled on safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved one final servicing mission, completed in 2009. The telescope is now expected to function until at least 2014. Its scientific successor, the James Webb Space Telescope (JWST), is to be launched in 2018 or possibly later.

HUBBLE SPACE TELESCOPE

PLANET URANUS Equatorial Diameter Mass (Earth = 1 Volume (Earth = 1) Gravity (Earth = 1) Temperature Axial Rotation Period Axial Tilt Orbital Period Average Distance From

The Sun Inclination of orbit to

ecliptic Mean Density Number of satellites

51,118 km / 31,763 miles 14.53 64 0.793 -357° F 17.9 hours 97.86° 84.01 years 2,870,990,000 km /

1,783,950,000 miles 0.77° 1.29 g/cm³ 21

URANUS, A MYSTERIOUS BLUE-GREEN BALL

Uranus is the seventh planet in our solar system, located in between Saturn and Neptune. Its average distance from the Sun is about one and three-quarters billion miles, or about twenty times the distance from the Sun to Earth. The orbit Uranus follows around the Sun is an ellipse, or stretched out circle, which means that Uranus' distance from the Sun varies from about 1.7 billion (1,700,000,000) miles at its closest to about 1.87 billion (1,870,000,00) miles at its furthest away. A year on Uranus is 84 Earth years. A day on Uranus is just a little over seventeen Earth hours. One of the many odd facts about Uranus is that it is "lying on its side" as it faces the Sun. Earth faces the sun standing almost straight up, with the north and south poles at the top and bottom as it looks at the Sun. For some reason,

Uranus has rolled over, so what we would think of as the south pole is facing the Sun. Scientists don't know why the planet does this, but it may be the result of a collision with some other body in space. Also, the planet rotates, or spins, from East to West which is the exact opposite of the way that Earth spins

If you live in a place where the skies are dark and you can see to the southern horizon, you might be able to pick out Uranus with a pair of binoculars. The planet will appear as a faint blue-green light. Even through a telescope, Uranus will only be a small blue-green disc.

No matter how close we get to Uranus, or how much we enhance the pictures with computers, about all we get to see is a blue-green ball. All the mysteries that surround the planet will not be solved easily.

Uranus is about four times the size of Earth, just a little over 30 thousand miles in diameter, compared to Earth's diameter of around 7,600 miles. Even though Uranus is much larger than our Earth, it is dwarfed when compared to mighty Jupiter, which is over 85 thousand miles in diameter.

 

HUBBLE TELESCOPE IMAGE OF URANUS

The Voyager spacecraft showed us the rings, which we cannot see at all from Earth. The only way that Earthbound astronomers can even get a hint of the rings is when they occasionally block the light of a star behind them. Even the Hubble Space Telescope cannot get a very good view of the rings. While the rings of Saturn are made up of fairly small pieces of bright white ices, the rings of Uranus for the most part are made of larger chunks of very dark, rocky material. The darkness of the chunks that make up the rings help explain why we cannot see them from Earth.

Uranus was the first planet to be discovered by an astronomer. It was discovered, accidentally, by British astronomer William Herschel in 1781. This meant that someone had to come up with a name for the new planet. Herschel named it "Georgium Sidus", after the King of England at that time, George III (the King that Americans rebelled against). This seemed fair to Herschel, since the King was paying for his research. Grownups being what they are, others called the planet Herschel, in honor of the discoverer. Another astronomer suggested the name Uranus, an ancient Greek god who was the father of Saturn, so the new planet would have a name from mythology like the rest of the planets at that time, and that name was finally agreed on by everybody in the mid 1800's.

Uranus is another member of the family of gas planets that live in our solar system, but it is quite a bit different from Jupiter and Saturn. First, it has methane gas mixed in with the hydrogen and helium that make up most of the giant planets. Methane is what gives Uranus its unusual color. Second, Uranus appears to have a core, or center, of melted rock, which changes into a dirty ocean made of water, ammonia and other elements the further from the center you go. Finally, the dirty ocean changes into the blue-green cover of clouds that we see in the pictures. Scientists believe that the layer of the planet are not separate, like those of an onion, but gradually blend with one another.

Since Uranus is a gas planet, it doesn't have a solid surface like we have here on Earth. The top layer of gas that we see is far from quiet. By carefully studying the pictures sent back by the Voyager spacecraft, scientists were able to see that there are winds blowing at over four hundred miles an hour!

HUBBLE IMAGE OF URANUS AND ITSSYSTEM OF DELICATE RINGS(NASA, ESA, AND M. SHOWALTER)

False color Hubble view of Uranus showing cloud details (NASA and Erich Karkoschka, University of Arizona)

Wife of Cronus The next planetary encounter on our tour brings us to the

domain of the ice giants. These planets differ from the gas giants because they are composed primarily of frozen water, ammonia, and methane rather than hydrogen and helium. Here we find a large, blue planet with little or no surface features. It shines like a blue green gem in the heavens. This is the planet Uranus, and it is the seventh planet from the Sun. Uranus was named after the god of the heavens in Greek mythology. According to myth, Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans. The planet Uranus was the first planet to be discovered in modern times. The British astronomer Sir William Herschel found it with his telescope in 1781. Before this, it had been assumed to be a star. Herschel also discovered Uranus' two largest moons, Oberon and Titania. We now know of 27 moons in orbit around Uranus. They represent a wide range of sizes and shapes. Uranus has only been visited by one spacecraft. This was the extended Voyager 2 mission. That encounter took place on January 24, 1986. Nearly everything we know about Uranus was learned at this time.

RECENT HUBBLE IMAGE OFURANUS AND IT RINGS(ERICH KARKOSCHKA, UNIVERSITY OFARIZONA, AND NASA)

Voyager 2 image of Uranus as seen from behind(NASA/JPL)

The Sideways Planet Uranus is different from any other planet in the Solar

System in that its axis of rotation is tilted 98 degrees. It is nearly parallel to the ecliptic. Uranus appears to be tipped on its side. This has some interesting effects on the planet. The Sun shines alternately on the poles and equatorial regions of the planet rather than constantly shining on its equatorial regions, as on Earth. Astronomers are not sure what accounts for this strange position. One theory suggests that a massive collision with another planet-sized body may have knocked Uranus on its side. This collision is believed to have taken place very early in the Solar System's history. Another strange fact about Uranus is that its magnetic field is tilted at a 60-degree angle to its axis of rotation. This angle is only 12 degrees on Earth. An interesting effect of Uranus' sideways tilt is that the tail of the planet's magnetic field is twisted like a corkscrew.

Features of Uranus When Voyager 2 arrived at Uranus, astronomers

were expecting to find complex cloud structures similar to those on Jupiter and Saturn. But when the images finally arrived, nothing was visible but a featureless blue-green sphere. False-color images of the planet later showed that banded structures of clouds were present but were extremely faint. They are partially obscured by the overlying layer of methane. Uranus is the third of the gas giants from the Sun. It is believed to be composed of about 83% hydrogen, 15% helium, 2% methane, and trace amounts of acetylene and other hydrocarbons. It is the methane in the planet's upper atmosphere that gives it a blue-green color. This is because the methane absorbs red light. High winds in the middle latitudes of Uranus blow in the direction of the planet's rotation at up to 360 miles (580 km) per hour.

Studies have shown somewhat slower winds blowing in the opposite direction at the planet's equator. Astronomers once thought that beneath these layers of clouds was a massive ocean of super-pressurized liquid ammonia and water. But data sent back from Voyager 2 seem to indicate that this is probably not the case. The internal structure of Uranus is probably very similar to that of Jupiter and Saturn, but without the rocky core and liquid metallic hydrogen center. Instead, there is probably a less-dense collection of rocky material at the core. In 1977, Earth-based astronomers discovered that Uranus, like all of the other gas giant planets, has a system of rings. A total of nine rings were identified from Earth. When Voyager 2 visited Uranus in 1986, two more rings were discovered. Two of Uranus' moons, Cordelia and Ophelia, act as shepherding moons for one of the rings. Voyager 2 also discovered that Uranus has a magnetic field

. It is tilted 60 degrees from the planet's rotational axis, and is about the same intensity as the magnetic field on Earth. The magnetic field on Earth and the other terrestrial planets is probably caused by electrical currents produced in the planets' molten core. Since Uranus has no molten core, astronomers are not sure what is responsible for creating the magnetic field.

STATISTICS FOR URANUS Discovered by Year of Discovery Diameter Known Natural Satellites Mean Distance from the Sun Rotational Period Orbital Period Orbital Eccentricity Orbital Inclination Inclination of Axis Mean Cloud Temperature Main Atmospheric

Component Atmospheric Pressure Apparent Magnitude

William Herschel 1781 31,755 miles (51,118 km) 27 1,783,487,000 miles

(2,870,990,000 km) 17.3 hours 84 years 0.047 0.774 degrees 97.86 degrees -315° F (-193° C) Hydrogen 1.2 bars 5.52

VOYAGER 2 PHOTO OF URANUS'UNUSUAL MOON MIRANDA(NASA/JPL) Daughter of Prospero

Miranda [mih-RAN-dah] is the eleventh of Uranus' moons and is the fifth largest. It is the smallest and innermost of the planet's five major moons and is without a doubt the most interesting. Miranda was named after the daughter of the magician Prospero in Shakespeare's The Tempest. Unlike the other moons in the Solar System, which are named after Greek an Roman mythological figures, the moons of Uranus are named after characters from classic literature. Miranda was discovered by Gerard Kuiper in 1948. Nearly everything we know about this moon was learned from the Voyager 2 encounter in 1986.

Voyager 2 close-up of Miranda showing surface details(NASA/JPL)

OBERON Mysterious Dark Craters Oberon is very similar to Umbriel in

composition, although Oberon is about 35% larger. As with all of Uranus' moons, it is believed to be composed of about half water ice and half rocky material. The surface of Oberon is heavily cratered. The lack of other surface features indicates that it has probably been stable since its formation. An unidentified substance covers the floors of many of Oberon's craters. Astronomers are not yet sure where this substance originates. One theory suggests that it could be "dirty water" that upwelled into the crater after impact.

 

Features of Oberon The most notable features of Oberon are its

craters. These craters are more numerous and much larger than those on Ariel and Titania. Some of these craters exhibit bright rays of ejecta material similar to those on Jupiter's moon, Callisto. Several images of Oberon show a lone mountain peak rising 4 miles (6 km) above the surface. In the southern hemisphere, large faults can be seen crossing the surface. This may be evidence of some type of geologic activity early in the moon's history. Oberon has no detectable atmosphere and no magnetic field.

Discovered by Year of Discovery Diameter Mean Distance from

Uranus Rotational Period Orbital Period Orbital Eccentricity Orbital Inclination Main Atmospheric

Component Apparent Magnitude

William Herschel 1787 946 miles (1,523 km) 362,507 miles

(582,600 km) 13.46 days 13.46 days 0.0008 0.1 degrees none 13.94

ARIEL The Mischievous Spirit Ariel [AIR-ee-al]  is the twelfth

of Uranus' moons and is the fourth largest. Ariel was named after a mischievous airy spirit in Shakespeare's play, The Tempest. Ariel was discovered in 1851 by the British astronomer, William Lassell. Nearly everything we now know about this moon was discovered by the Voyager 2 spacecraft in 1986.

 

he Valley World Ariel is believed to be composed of about half

water ice and half rocky material. This is a somewhat larger fraction of rock than Saturn's moon, Rhea. Ariel is very similar to Titania, although Titania is about 35% larger. The most striking feature of Ariel is the complex network of deep rift valleys crisscrossing the entire surface of the moon. These canyons are very similar to those on Mars. They have extremely smooth floors, which indicates that they may have been eroded by some form of liquid. We know it could not have been water, because water is as hard as steel at these extremely low temperatures. Astronomers believe that these canyons may have been carved out by liquid ammonia, methane, or carbon monoxide.

 

Features of Ariel Ariel is the brightest moon in the Uranian system.

The most prominent feature of Ariel is the network of interconnected rift valleys. They cross the entire surface of the moon for hundreds of miles (or kilometers), and reach depths of  6 miles (10 km). These valleys are similar to those on Titania, but on a much larger scale. Some of these valleys have ridges, which are believed to have been formed by upwellings of ice from the moon's interior. The surface of Ariel is also littered with craters. Many of these craters appear to be partially submerged. This indicates that the surface is relatively young. Astronomers believe that some sort of process has reshaped the surface of this moon. Ariel has no detectable atmosphere and no magnetic field.