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 MOONS
SEE 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.

PLANET URANUS
 Equatorial Diameter  51,118 km / 31,763 miles
 Mass (Earth = 1  14.53
 Volume (Earth = 1)  64
 Gravity (Earth = 1)  0.793
 Temperature  -357° F
 Axial Rotation Period  17.9 hours
 Axial Tilt  97.86°
 Orbital Period  84.01 years
 Average Distance From The  2,870,990,000 km /
Sun 1,783,950,000 miles
 Inclination of orbit to ecliptic  0.77°
 Mean Density  1.29 g/cm³
 Number of satellites  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 ITS
False color Hubble view of
SYSTEM OF DELICATE Uranus showing cloud
RINGS details (NASA and Erich
Karkoschka, University of
(NASA, ESA, AND M. Arizona)
SHOWALTER)

 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 OF
URANUS AND IT RINGS Voyager 2 image of
(ERICH KARKOSCHKA, UNIVERSITY Uranus as seen from
OF behind
ARIZONA, AND NASA) (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  William Herschel
 Year of Discovery  1781
 Diameter  31,755 miles (51,118 km)
 Known Natural Satellites  27
 Mean Distance from the Sun  1,783,487,000 miles
 Rotational Period (2,870,990,000 km)
 Orbital Period  17.3 hours
 Orbital Eccentricity  84 years
 Orbital Inclination  0.047
 Inclination of Axis  0.774 degrees
 Mean Cloud Temperature  97.86 degrees
 Main Atmospheric Component  -315° F (-193° C)
 Atmospheric Pressure  Hydrogen
 Apparent Magnitude  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  William Herschel
 Year of Discovery  1787
 Diameter  946 miles (1,523 km)
 Mean Distance from  362,507 miles (582,600
Uranus km)
 Rotational Period  13.46 days
 Orbital Period  13.46 days
 Orbital Eccentricity  0.0008
 Orbital Inclination  0.1 degrees
 Main Atmospheric  none
Component  13.94
 Apparent Magnitude

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.

Moons of uranus

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