2.  Uranus has a relatively featureless
appearance at visible wavelengths. Even
from Voyager 2 at a distance of 80,000
km there were few distinguishable
features. This is believed to be due to
Uranus being further from the Sun than
Jupiter and Saturn, which means its
temperature is lower (only 58 degrees
Kelvin in the upper atmosphere). This
decreases the liklihood of chemical
reactions making the colorful compounds
that give the surface features on Jupiter
and Saturn. In addition, the upper
atmosphere is thought to have a high-
level petrochemical haze that obscures
features lower in the atmosphere.

3.  The blue color is because of
methane gas in the
atmosphere, which absorbs red
and orange light strongly, leaving
more blue light to be scattered to
the observer. The clouds are
thought to be mostly methane
ice, with a temperature at the
cloud tops of about -221 degrees
Celsius.

4.  Voyager 2 confirmed the suspicion that
Uranus had a magnetic field. The field is
about 50 times stronger than that of the
Earth and is tilted about 60 degrees with
respect to the rotation axis. As a result, the
magnetic field moves like a corkscrew as
Uranus rotates, as illustrated in the following
movie (5 MB). One hypothesis for this
behavior of the magnetic field is that it
originates in a thin conducting shell outside
the core of the planet rather than deep in
the core as for the Earth or Jupiter. The
pressure would not be high enough for the
relevant conducting material to be metallic
hydrogen. A mixture of water, methane, and
ammonia under sufficient pressure could
provide the requisite electrical conductor.

5.  The magnetosphere contains belts
of charged particles similar to
those of the Earth. The rings and
most of the moons orbit within the
magnetososphere and thus are
protected from the Solar wind.

8.  The atmosphere of Uranus, like those
of the larger gas giants Jupiter and
Saturn, is composed primarily of
hydrogen and helium. At depth it is
significantly enriched in volatiles
(dubbed "ices") such as water, ammonia
and methane. The opposite is true for
the upper atmosphere, which contains
very few gases heavier than hydrogen
and helium due to its low temperature.
Uranus's atmosphere is the coldest of
all the planets, with its temperature
reaching as low as 49 K.

9.  The Uranian atmosphere can be
divided into three layers: the
troposphere, between altitudes of
−300[a] and 50 km and pressures from
100 to 0.1 bar; the
stratosphere, spanning altitudes
between 50 and 4000 km and pressures
of between 0.1 and 10−10 bar; and the
hot thermosphere (or exosphere)
extending from an altitude of 4,000 km
to several Uranian radii from the
nominal surface at 1 bar pressure.[1]
Unlike Earth's, Uranus's atmosphere has
no mesosphere.

10.  The troposphere hosts four cloud layers:
methane clouds at about 1.2 bar, hydrogen
sulfide/ammonia clouds in at 3–
10 bar, ammonium hydrosulfide clouds at
20–40 bar, and finally water clouds below
50 bar. Only the upper two cloud layers
have been observed directly—the deeper
clouds remain speculative. Above the
clouds lie several tenuous layers of
photochemical haze. Discrete bright
tropospheric clouds are rare on
Uranus, probably due to sluggish
convection in the planet's interior.
Nevertheless observations of such clouds
were used to measure the planet's zonal
winds, which are remarkably fast with
speeds up to 240 m/s.

11.  Little
is known about the Uranian
atmosphere as to date only one
spacecraft, Voyager 2, which passed
by the planet in 1986, has studied it
in detail. No other missions to
Uranus are currently scheduled.

13.  Although there is no well-defined
solid surface within Uranus's
interior, the outermost part of
Uranus's gaseous envelope (the
region accessible to remote sensing)
is called its atmosphere.[1] Remote
sensing capability extends down to
roughly 300 km below the 1 bar
level, with a corresponding pressure
around 100 bar and temperature of
320 K.[2]

14.  The observational history of the Uranian
atmosphere is long and full of errors and
frustrations. Uranus is a relatively faint
object, and its visible angular diameter is smaller
than 4″.[3] The first spectra of Uranus were
observed through a prism in 1869 and 1871 by
Angelo Secchi and William Huggins, who found a
number of broad dark bands, which they were
unable to identify.[3] They also failed to detect any
solar Fraunhofer lines—the fact later interpreted
by Norman Lockyer as indicating that Uranus
emitted its own light as opposed to reflecting light
from the Sun. In 1889 however, astronomers
observed solar Fraunhofer lines in photographic
ultraviolet spectra of the planet, proving once and
for all that Uranus was shining by reflected light.
The nature of the broad dark bands in its visible
spectrum remained unknown until the fourth
decade of the twentieth century.

15.  The key to deciphering Uranus's spectrum was found in the
1930s by Rupert Wildt and Vesto Slipher,[6] who found that
the dark bands at 543, 619, 925, 865 and 890 nm belonged
to gaseous methane.[3] They had never been observed
before because they were very weak and required a long
path length to be detected.[6] This meant that the
atmosphere of Uranus was transparent to a much greater
depth compared to those of other giant planets. In
1950, Gerard Kuiper noticed another diffuse dark band in
the spectrum of Uranus at 827 nm, which he failed to
identify.[7] In 1952 Gerhard Herzberg, a future Nobel Prize
winner, showed that this band was caused by the weak
quadrapole absorption of molecular hydrogen, which thus
became the second compound detected on Uranus.[8] Until
1986 only two gases, methane and hydrogen, were known
in the Uranian atmosphere.[3] The far-infrared
spectroscopic observation beginning from 1967 consistently
showed the atmosphere of Uranus was in approximate
thermal balance with incoming solar radiation (in other
words, it radiated as much heat as it received from the
Sun), and no internal heat source was required to explain
observed temperatures.[9] No discrete features had been
observed on Uranus prior to the Voyager 2 visit in 1986.[10]

16.  InJanuary 1986, the Voyager 2
spacecraft flew by Uranus at a minimal
distance of 107,100 km providing the
first close-up images and spectra of its
atmosphere. They generally confirmed
that the atmosphere was made of
mainly hydrogen and helium with around
2% methane.The atmosphere appeared
highly transparent and lacking thick
stratospheric and tropospheric hazes.
Only a limited number of discrete clouds
were observed.

17.  Inthe 1990s and 2000s, observations by
the Hubble Space Telescope and by ground
based telescopes equipped with adaptive
optics systems (the Keck telescope and
NASA Infrared Telescope Facility, for
instance) made it possible for the first
time to observe discrete cloud features
from Earth.[14] Tracking them has allowed
astronomers to re-measure windspeeds on
Uranus, known before only from the
Voyager 2 observations, and to study the
dynamics of the Uranian atmopshere.[1

18. 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.

20.  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.

21.  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.

22.  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.

23.  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.

26. Overview
In 1985, Howard B. Keck of the W. M. Keck
Foundation gave $70 million to fund the design and
construction of the Keck I Telescope. The key
advance that allowed the construction of the Keck's
large telescopes was the ability to operate smaller
mirror segments as a single, contiguous mirror. In the
case of the Keck each of the primary mirrors is
composed of 36 hexagonal segments that work
together as a single unit. The mirrors were made
from Zerodur glass-ceramic by the German
company Schott AG . On the telescope, each
segment is kept stable by a system of active optics,
which uses extremely rigid support structures in
combination with adjustable warping harnesses.

27. During observation, a computer-controlled system of sensors
and actuators adjusts the position of each segment, relative to
its neighbors, to an accuracy of four nanometers. This twice-
per-second adjustment counters the effect of gravity as the
telescope moves, in addition to other environmental effects that
can affect the mirror shape.
Each Keck telescope sits on an altazimuth mount. During the
design stage, computer analysis determined that this mounting
style provides the greatest strength and stiffness for the least
amount of steel, which totals about 270 tons per telescope. The
weight of each telescope is about 300 tons.

28. •The telescopes are equipped with a
suite of
instruments, both cameras and spectro
meters that allow observations across
much of the visible and near infrared
spectrum

30.  Thecomposition of the Uranian
atmosphere is different from that of
Uranus as a whole, consisting mainly
of molecular hydrogen and helium.
The helium molar fraction, i.e. the
number of helium atoms per
molecule of hydrogen/helium, was
determined from the analysis of
Voyager 2 far infrared and radio
occultation observations.

31.  Knowledge of the isotopic composition of
Uranus's atmosphere is very limited. To
date the only known isotope abundance
ratio is that of deuterium to light
hydrogen: 5.5+3.5
−1.5 × 10−5, which was measured by the
Infrared Space Observatory (ISO) in the
1990s. It appears to be higher than the
protosolar value of 2.25 ± 0.35×10−5
measured in Jupiter.The deuterium is
found almost exclusively in hydrogen
deuteride molecules which it forms with
normal hydrogen atoms.

32.  Structure The Uranian atmosphere can be
divided into three layers: the troposphere,
between altitudes of −300 and 50 km and
pressures from 100 to 0.1 bar; the
stratosphere, spanning altitudes between 50
and 4000 km and pressures between 0.1 and
10−10 bar; and the thermosphere/exosphere
extending from 4000 km to as high as a few
Uranus radii from the surface. There is no
mesosphere.

33. Temperature profile of the Uranian
troposphere and lower stratosphere.
Cloud and haze layers are also indicated.

35. but keep in mind that the core
of Jupiter is more like 24,000 K –
much hotter. The core of Uranus
has a density of about 9
g/cm3, which makes it about
twice as dense as the average
density of the Earth.

36.  For astronomers, Uranus has an unusually
low temperature; and that’s a mystery.
One ideas is that the same impact that
knocked Uranus off its rotational axis
might have also caused it to expel much
of its primordial heat. With the heat
gone, Uranus was able to cool down
significantly further than the other
planets. Another idea is that there’s some
kind of barrier in Uranus’ upper
atmosphere that prevents heat from the
core to reach the surface.

37. We have written many
stories about Uranus on
Universe Today. Here’s an
article about a dark spot in
the clouds on Uranus, and
here’s an article about the
composition of Uranus.

38. Uranus has a mass of roughly
14.5 times that of
Earth, which makes it the
least massive of the giant
planets. Astronomers know
that it’s mostly made of
various ices, like
water, ammonia and
methane. And they theorize
that Uranus probably has a

39.  Thecore of Uranus probably only
accounts for 20% of the radius of
Uranus, and only about 0.55 Earth
masses. With gravity of all the outer
mantle and atmosphere, regions in
the core experience a pressure of
about 8 million bars, and have a
temperature of 5,000 Kelvin. That
sounds hot, like as hot as the
surface of the Sun.

40.  Uranus's Moons
 1. Cordelia
2. Ophelia
3. Bianca
4. Cressida
5. Desdemona
6. Juliet
7. Portia
8. Rosalind
9. Mab
10. Belinda
11. Perdita
12. Puck
13. Cupid
14. Miranda
15. Francisco

41.  16.Ariel
17. Umbriel
18. Titania
19. Oberon
20. Caliban
21. Stephano
22. Trinculo
23. Sycorax
24. Margaret
25. Prospero
26. Setebos
27. Ferdinand

42.  Sweet Moon," William Shakespeare wrote in "A
Midsummer Night's Dream," "I thank thee for thy
sunny beams; I thank thee, Moon, for shining
now so bright." Centuries later, the moons of
Uranus pay homage to the famous playwright.
 The Hubble Space Telescope captured this false-
color image of Uranus and its moons.
 While most of the satellites orbiting other
planets take their names from Greek
mythology, Uranus' moons are unique in being
named for Shakespearean characters, along with
a couple of the moons being named for
characters from the works of Alexander Pope.

43.  Oberon and Titania are the largest
Uranian moons, and were first to be
discovered -- by William Herschel in
1787. William Lassell, who had been
first to see a moon orbiting
Neptune, discovered the next
two, Ariel and Umbriel. Nearly a
century passed before Gerard Kuiper
found Miranda in 1948. And that was
it until a NASA robot made it to
distant Uranus.

45.  Oberon is the second largest moon of Uranus.
Discovered in 1787, little was known about
this moon until Voyager 2 passed it during its
flyby of Uranus in January 1986. Oberon is
heavily cratered -- similar to Umbriel --
especially when compared to three other
moons of Uranus: Ariel, Titania and Miranda.
Like all of Uranus' large moons, Oberon is
composed of roughly half ice and half rock.
Oberon has at least one large mountain that
rises about 6 km off the surface.

46. Discovery:
Oberon was discovered
in January 1787 by
William Herschel.

47.  How Oberon Got its Name:
Oberon is named for the king of the
fairies in Shakespeare's "A Midsummer
Night's Dream."
 Moons of Uranus are named for
characters in William Shakespeare's
plays and from Alexander Pope's
"Rape of the Lock

49.  Uranus is the only giant planet
whose equator is nearly at right
angles to its orbit. A collision with
an Earth-sized object may explain
Uranus' unique tilt. Nearly a twin in
size to Neptune, Uranus has more
methane in its mainly hydrogen and
helium atmosphere than Jupiter or
Saturn. Methane gives Uranus its
blue tint.

50. Featured Mission: Voyager 2
Most of what we know about
Uranus came from Voyager
2's flyby in 1986. The
spacecraft discovered 10
additional moons and several
rings before heading on to
Neptune.

51.  The largest ring is twice the diameter of
the planet's previously known rings. The
rings are so far from the planet, they are
being called Uranus' "second ring system."
One of the new moons shares its orbit
with one of the rings. Analysis of the
Hubble data also reveals the orbits of
Uranus' family of inner moons have
changed significantly over the past
decade.

52.  sincedust orbiting Uranus is
expected to be depleted by spiraling
away, the planet's rings must be
continually replenished with fresh
material. "The new discoveries
demonstrate that Uranus has a
youthful and dynamic system of
rings and moons," said Mark
Showalter of the SETI
Institute, Mountainview, California.

53. Showalter and Jack Lissauer of
NASA's Ames Research
Center, Moffet
Field, Calif., propose that the
outermost ring is replenished by
a 12-mile-wide newly discovered
moon, named Mab, which they
first observed using Hubble in
2003.

54.  Hubble uncovered the rings in
August 2004 during a series of
80, four-minute exposures of
Uranus. The team later recognized
the faint new rings in 24 similar
images taken a year earlier. Images
from September 2005 reveal the
rings even more clearly.

57.  The image is a color composite made from
short exposures, showing the disk of Uranus
with some cloud features. Just to the left
and right of the color image of the disk are a
combination of deeper, panchromatic images
showing Uranus's inner rings; the brightest is
the Epsilon Ring. The satellite Mab is visible
as eight dots adjacent to the outer ring on
the right side.