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Uranus (from the Latin name "Ūranus" for
the Greek god Οὐρανός) is the seventh
planet from the Sun.
It has the third-largest planetary radius
and fourth-largest planetary mass in the Solar
System.
Uranus is similar in composition to Neptune,
and both have bulk chemical compositions which
differ from that of the larger gas giants
Jupiter and Saturn.
For this reason, scientists often classify
Uranus and Neptune as "ice giants" to distinguish
them from the gas giants.
Uranus's atmosphere is similar to Jupiter's
and Saturn's in its primary composition of
hydrogen and helium, but it contains more
"ices" such as water, ammonia, and methane,
along with traces of other hydrocarbons.
It is the coldest planetary atmosphere in
the Solar System, with a minimum temperature
of 49 K (−224 °C; −371 °F), and has
a complex, layered cloud structure with water
thought to make up the lowest clouds and methane
the uppermost layer of clouds.
The interior of Uranus is mainly composed
of ices and rock.Like the other giant planets,
Uranus has a ring system, a magnetosphere,
and numerous moons.
The Uranian system has a unique configuration
among those of the planets because its axis
of rotation is tilted sideways, nearly into
the plane of its solar orbit.
Its north and south poles, therefore, lie
where most other planets have their equators.
In 1986, images from Voyager 2 showed Uranus
as an almost featureless planet in visible
light, without the cloud bands or storms associated
with the other giant planets.
Observations from Earth have shown seasonal
change and increased weather activity as Uranus
approached its equinox in 2007.
Wind speeds can reach 250 metres per second
(900 km/h; 560 mph).Uranus is the only planet
whose name is derived directly from a figure
from Greek mythology, from the Latinised version
of the Greek god of the sky Ouranos.
== History ==
Like the classical planets, Uranus is visible
to the naked eye, but it was never recognised
as a planet by ancient observers because of
its dimness and slow orbit.
Sir William Herschel announced its discovery
on 13 March 1781, expanding the known boundaries
of the Solar System for the first time in
history and making Uranus the first planet
discovered with a telescope.
=== Discovery ===
Uranus had been observed on many occasions
before its recognition as a planet, but it
was generally mistaken for a star.
Possibly the earliest known observation was
by Hipparchos, who in 128 BC might have recorded
it as a star for his star catalogue that was
later incorporated into Ptolemy's Almagest.
The earliest definite sighting was in 1690,
when John Flamsteed observed it at least six
times, cataloguing it as 34 Tauri.
The French astronomer Pierre Charles Le Monnier
observed Uranus at least twelve times between
1750 and 1769, including on four consecutive
nights.
Sir William Herschel observed Uranus on 13
March 1781 from the garden of his house at
19 New King Street in Bath, Somerset, England
(now the Herschel Museum of Astronomy), and
initially reported it (on 26 April 1781) as
a comet.
Herschel "engaged in a series of observations
on the parallax of the fixed stars", using
a telescope of his own design.
Herschel recorded in his journal: "In the
quartile near ζ Tauri … either [a] Nebulous
star or perhaps a comet."
On 17 March he noted: "I looked for the Comet
or Nebulous Star and found that it is a Comet,
for it has changed its place."
When he presented his discovery to the Royal
Society, he continued to assert that he had
found a comet, but also implicitly compared
it to a planet:
The power I had on when I first saw the comet
was 227.
From experience I know that the diameters
of the fixed stars are not proportionally
magnified with higher powers, as planets are;
therefore I now put the powers at 460 and
932, and found that the diameter of the comet
increased in proportion to the power, as it
ought to be, on the supposition of its not
being a fixed star, while the diameters of
the stars to which I compared it were not
increased in the same ratio.
Moreover, the comet being magnified much beyond
what its light would admit of, appeared hazy
and ill-defined with these great powers, while
the stars preserved that lustre and distinctness
which from many thousand observations I knew
they would retain.
The sequel has shown that my surmises were
well-founded, this proving to be the Comet
we have lately observed.
Herschel notified the Astronomer Royal Nevil
Maskelyne of his discovery and received this
flummoxed reply from him on 23 April 1781:
"I don't know what to call it.
It is as likely to be a regular planet moving
in an orbit nearly circular to the sun as
a Comet moving in a very eccentric ellipsis.
I have not yet seen any coma or tail to it."Although
Herschel continued to describe his new object
as a comet, other astronomers had already
begun to suspect otherwise.
Finnish-Swedish astronomer Anders Johan Lexell,
working in Russia, was the first to compute
the orbit of the new object.
Its nearly circular orbit led him to a conclusion
that it was a planet rather than a comet.
Berlin astronomer Johann Elert Bode described
Herschel's discovery as "a moving star that
can be deemed a hitherto unknown planet-like
object circulating beyond the orbit of Saturn".
Bode concluded that its near-circular orbit
was more like a planet than a comet.The object
was soon universally accepted as a new planet.
By 1783, Herschel acknowledged this to Royal
Society president Joseph Banks: "By the observation
of the most eminent Astronomers in Europe
it appears that the new star, which I had
the honour of pointing out to them in March
1781, is a Primary Planet of our Solar System."
In recognition of his achievement, King George
III gave Herschel an annual stipend of £200
on condition that he move to Windsor so that
the Royal Family could look through his telescopes.
=== Name ===
The name of Uranus references the ancient
Greek deity of the sky Uranus (Ancient Greek:
Οὐρανός), the father of Cronus (Saturn)
and grandfather of Zeus (Jupiter), which in
Latin became "Ūranus" (Latin pronunciation:
[ˈuːranʊs]).
It is the only planet whose name is derived
directly from a figure of Greek mythology.
The adjectival form of Uranus is "Uranian".
The pronunciation of the name Uranus preferred
among astronomers is , with stress on the
first syllable as in Latin Ūranus, in contrast
to , with stress on the second syllable and
a long a, though both are considered acceptable.Consensus
on the name was not reached until almost 70
years after the planet's discovery.
During the original discussions following
discovery, Maskelyne asked Herschel to "do
the astronomical world the faver [sic] to
give a name to your planet, which is entirely
your own, [and] which we are so much obliged
to you for the discovery of".
In response to Maskelyne's request, Herschel
decided to name the object Georgium Sidus
(George's Star), or the "Georgian Planet"
in honour of his new patron, King George III.
He explained this decision in a letter to
Joseph Banks:
In the fabulous ages of ancient times the
appellations of Mercury, Venus, Mars, Jupiter
and Saturn were given to the Planets, as being
the names of their principal heroes and divinities.
In the present more philosophical era it would
hardly be allowable to have recourse to the
same method and call it Juno, Pallas, Apollo
or Minerva, for a name to our new heavenly
body.
The first consideration of any particular
event, or remarkable incident, seems to be
its chronology: if in any future age it should
be asked, when this last-found Planet was
discovered?
It would be a very satisfactory answer to
say, 'In the reign of King George the Third'.
Herschel's proposed name was not popular outside
Britain, and alternatives were soon proposed.
Astronomer Jérôme Lalande proposed that
it be named Herschel in honour of its discoverer.
Swedish astronomer Erik Prosperin proposed
the name Neptune, which was supported by other
astronomers who liked the idea to commemorate
the victories of the British Royal Naval fleet
in the course of the American Revolutionary
War by calling the new planet even Neptune
George III or Neptune Great Britain.In a March
1782 treatise, Bode proposed Uranus, the Latinised
version of the Greek god of the sky, Ouranos.
Bode argued that the name should follow the
mythology so as not to stand out as different
from the other planets, and that Uranus was
an appropriate name as the father of the first
generation of the Titans.
He also noted that elegance of the name in
that just as Saturn was the father of Jupiter,
the new planet should be named after the father
of Saturn.
In 1789, Bode's Royal Academy colleague Martin
Klaproth named his newly discovered element
uranium in support of Bode's choice.
Ultimately, Bode's suggestion became the most
widely used, and became universal in 1850
when HM Nautical Almanac Office, the final
holdout, switched from using Georgium Sidus
to Uranus.Uranus has two astronomical symbols.
The first to be proposed, ♅, was suggested
by Lalande in 1784.
In a letter to Herschel, Lalande described
it as "un globe surmonté par la première
lettre de votre nom" ("a globe surmounted
by the first letter of your surname").
A later proposal, ⛢, is a hybrid of the
symbols for Mars and the Sun because Uranus
was the Sky in Greek mythology, which was
thought to be dominated by the combined powers
of the Sun and Mars.Uranus is called by a
variety of translations in other languages.
In Chinese, Japanese, Korean, and Vietnamese,
its name is literally translated as the "sky
king star" (天王星).
In Thai, its official name is Dao Yurenat
(ดาวยูเรนัส), as in English.
Its other name in Thai is Dao Maritayu (ดาวมฤตยู,
Star of Mṛtyu), after the Sanskrit word
for "death", Mrtyu (मृत्यु).
In Mongolian, its name is Tengeriin Van (Тэнгэрийн
ван), translated as "King of the Sky",
reflecting its namesake god's role as the
ruler of the heavens.
In Hawaiian, its name is Hele‘ekala.
In Māori, its name is Whērangi.
== Orbit and rotation ==
Uranus orbits the Sun once every 84 years.
Its average distance from the Sun is roughly
20 AU (3 billion km; 2 billion mi).
The difference between its minimum and maximum
distance from the Sun is 1.8 AU, larger than
that of any other planet, though not as large
as that of dwarf planet Pluto.
The intensity of sunlight varies inversely
with the square of distance, and so on Uranus
(at about 20 times the distance from the Sun
compared to Earth) it is about 1/400 the intensity
of light on Earth.
Its orbital elements were first calculated
in 1783 by Pierre-Simon Laplace.
With time, discrepancies began to appear between
the predicted and observed orbits, and in
1841, John Couch Adams first proposed that
the differences might be due to the gravitational
tug of an unseen planet.
In 1845, Urbain Le Verrier began his own independent
research into Uranus's orbit.
On 23 September 1846, Johann Gottfried Galle
located a new planet, later named Neptune,
at nearly the position predicted by Le Verrier.The
rotational period of the interior of Uranus
is 17 hours, 14 minutes.
As on all the giant planets, its upper atmosphere
experiences strong winds in the direction
of rotation.
At some latitudes, such as about 60 degrees
south, visible features of the atmosphere
move much faster, making a full rotation in
as little as 14 hours.
=== Axial tilt ===
The Uranian axis of rotation is approximately
parallel with the plane of the Solar System,
with an axial tilt of 97.77° (as defined
by prograde rotation).
This gives it seasonal changes completely
unlike those of the other planets.
Near the solstice, one pole faces the Sun
continuously and the other faces away.
Only a narrow strip around the equator experiences
a rapid day–night cycle, but with the Sun
low over the horizon.
At the other side of Uranus's orbit the orientation
of the poles towards the Sun is reversed.
Each pole gets around 42 years of continuous
sunlight, followed by 42 years of darkness.
Near the time of the equinoxes, the Sun faces
the equator of Uranus giving a period of day–night
cycles similar to those seen on most of the
other planets.
Uranus reached its most recent equinox on
7 December 2007.
One result of this axis orientation is that,
averaged over the Uranian year, the polar
regions of Uranus receive a greater energy
input from the Sun than its equatorial regions.
Nevertheless, Uranus is hotter at its equator
than at its poles.
The underlying mechanism that causes this
is unknown.
The reason for Uranus's unusual axial tilt
is also not known with certainty, but the
usual speculation is that during the formation
of the Solar System, an Earth-sized protoplanet
collided with Uranus, causing the skewed orientation.
Uranus's south pole was pointed almost directly
at the Sun at the time of Voyager 2's flyby
in 1986.
The labelling of this pole as "south" uses
the definition currently endorsed by the International
Astronomical Union, namely that the north
pole of a planet or satellite is the pole
that points above the invariable plane of
the Solar System, regardless of the direction
the planet is spinning.
A different convention is sometimes used,
in which a body's north and south poles are
defined according to the right-hand rule in
relation to the direction of rotation.
=== Visibility ===
The mean apparent magnitude of Uranus is 5.68
with a standard deviation of 0.17, while the
extremes are 5.38 and 6.03.
This range of brightness is near the limit
of naked eye visibility.
Much of the variability is dependent upon
the planetary latitudes being illuminated
from the Sun and viewed from the Earth.
Its angular diameter is between 3.4 and 3.7
arcseconds, compared with 16 to 20 arcseconds
for Saturn and 32 to 45 arcseconds for Jupiter.
At opposition, Uranus is visible to the naked
eye in dark skies, and becomes an easy target
even in urban conditions with binoculars.
In larger amateur telescopes with an objective
diameter of between 15 and 23 cm, Uranus appears
as a pale cyan disk with distinct limb darkening.
With a large telescope of 25 cm or wider,
cloud patterns, as well as some of the larger
satellites, such as Titania and Oberon, may
be visible.
== Physical characteristics ==
=== Internal structure ===
Uranus's mass is roughly 14.5 times that of
Earth, making it the least massive of the
giant planets.
Its diameter is slightly larger than Neptune's
at roughly four times that of Earth.
A resulting density of 1.27 g/cm3 makes Uranus
the second least dense planet, after Saturn.
This value indicates that it is made primarily
of various ices, such as water, ammonia, and
methane.
The total mass of ice in Uranus's interior
is not precisely known, because different
figures emerge depending on the model chosen;
it must be between 9.3 and 13.5 Earth masses.
Hydrogen and helium constitute only a small
part of the total, with between 0.5 and 1.5
Earth masses.
The remainder of the non-ice mass (0.5 to
3.7 Earth masses) is accounted for by rocky
material.The standard model of Uranus's structure
is that it consists of three layers: a rocky
(silicate/iron–nickel) core in the centre,
an icy mantle in the middle and an outer gaseous
hydrogen/helium envelope.
The core is relatively small, with a mass
of only 0.55 Earth masses and a radius less
than 20% of Uranus's; the mantle comprises
its bulk, with around 13.4 Earth masses, and
the upper atmosphere is relatively insubstantial,
weighing about 0.5 Earth masses and extending
for the last 20% of Uranus's radius.
Uranus's core density is around 9 g/cm3, with
a pressure in the centre of 8 million bars
(800 GPa) and a temperature of about 5000
K.
The ice mantle is not in fact composed of
ice in the conventional sense, but of a hot
and dense fluid consisting of water, ammonia
and other volatiles.
This fluid, which has a high electrical conductivity,
is sometimes called a water–ammonia ocean.The
extreme pressure and temperature deep within
Uranus may break up the methane molecules,
with the carbon atoms condensing into crystals
of diamond that rain down through the mantle
like hailstones.
Very-high-pressure experiments at the Lawrence
Livermore National Laboratory suggest that
the base of the mantle may comprise an ocean
of liquid diamond, with floating solid 'diamond-bergs'.The
bulk compositions of Uranus and Neptune are
different from those of Jupiter and Saturn,
with ice dominating over gases, hence justifying
their separate classification as ice giants.
There may be a layer of ionic water where
the water molecules break down into a soup
of hydrogen and oxygen ions, and deeper down
superionic water in which the oxygen crystallises
but the hydrogen ions move freely within the
oxygen lattice.Although the model considered
above is reasonably standard, it is not unique;
other models also satisfy observations.
For instance, if substantial amounts of hydrogen
and rocky material are mixed in the ice mantle,
the total mass of ices in the interior will
be lower, and, correspondingly, the total
mass of rocks and hydrogen will be higher.
Presently available data does not allow a
scientific determination which model is correct.
The fluid interior structure of Uranus means
that it has no solid surface.
The gaseous atmosphere gradually transitions
into the internal liquid layers.
For the sake of convenience, a revolving oblate
spheroid set at the point at which atmospheric
pressure equals 1 bar (100 kPa) is conditionally
designated as a "surface".
It has equatorial and polar radii of 25,559
± 4 km (15,881.6 ± 2.5 mi) and 24,973 ± 20
km (15,518 ± 12 mi), respectively.
This surface is used throughout this article
as a zero point for altitudes.
==== Internal heat ====
Uranus's internal heat appears markedly lower
than that of the other giant planets; in astronomical
terms, it has a low thermal flux.
Why Uranus's internal temperature is so low
is still not understood.
Neptune, which is Uranus's near twin in size
and composition, radiates 2.61 times as much
energy into space as it receives from the
Sun, but Uranus radiates hardly any excess
heat at all.
The total power radiated by Uranus in the
far infrared (i.e. heat) part of the spectrum
is 1.06±0.08 times the solar energy absorbed
in its atmosphere.
Uranus's heat flux is only 0.042±0.047 W/m2,
which is lower than the internal heat flux
of Earth of about 0.075 W/m2.
The lowest temperature recorded in Uranus's
tropopause is 49 K (−224.2 °C; −371.5
°F), making Uranus the coldest planet in
the Solar System.One of the hypotheses for
this discrepancy suggests that when Uranus
was hit by a supermassive impactor, which
caused it to expel most of its primordial
heat, it was left with a depleted core temperature.
This impact hypothesis is also used in some
attempts to explain the planet's axial tilt.
Another hypothesis is that some form of barrier
exists in Uranus's upper layers that prevents
the core's heat from reaching the surface.
For example, convection may take place in
a set of compositionally different layers,
which may inhibit the upward heat transport;
perhaps double diffusive convection is a limiting
factor.
=== Atmosphere ===
Although there is no well-defined solid surface
within Uranus's interior, the outermost part
of Uranus's gaseous envelope that is accessible
to remote sensing is called its atmosphere.
Remote-sensing capability extends down to
roughly 300 km below the 1 bar (100 kPa) level,
with a corresponding pressure around 100 bar
(10 MPa) and temperature of 320 K (47 °C;
116 °F).
The tenuous thermosphere extends over two
planetary radii from the nominal surface,
which is defined to lie at a pressure of 1
bar.
The Uranian atmosphere can be divided into
three layers: the troposphere, between altitudes
of −300 and 50 km (−186 and 31 mi) and
pressures from 100 to 0.1 bar (10 MPa to 10
kPa); the stratosphere, spanning altitudes
between 50 and 4,000 km (31 and 2,485 mi)
and pressures of between 0.1 and 10−10 bar
(10 kPa to 10 µPa); and the thermosphere
extending from 4,000 km to as high as 50,000
km from the surface.
There is no mesosphere.
==== Composition ====
The composition of Uranus's atmosphere is
different from its bulk, consisting mainly
of molecular hydrogen and helium.
The helium molar fraction, i.e. the number
of helium atoms per molecule of gas, is 0.15±0.03
in the upper troposphere, which corresponds
to a mass fraction 0.26±0.05.
This value is close to the protosolar helium
mass fraction of 0.275±0.01, indicating that
helium has not settled in its centre as it
has in the gas giants.
The third-most-abundant component of Uranus's
atmosphere is methane (CH4).
Methane has prominent absorption bands in
the visible and near-infrared (IR), making
Uranus aquamarine or cyan in colour.
Methane molecules account for 2.3% of the
atmosphere by molar fraction below the methane
cloud deck at the pressure level of 1.3 bar
(130 kPa); this represents about 20 to 30
times the carbon abundance found in the Sun.
The mixing ratio is much lower in the upper
atmosphere due to its extremely low temperature,
which lowers the saturation level and causes
excess methane to freeze out.
The abundances of less volatile compounds
such as ammonia, water, and hydrogen sulfide
in the deep atmosphere are poorly known.
They are probably also higher than solar values.
Along with methane, trace amounts of various
hydrocarbons are found in the stratosphere
of Uranus, which are thought to be produced
from methane by photolysis induced by the
solar ultraviolet (UV) radiation.
They include ethane (C2H6), acetylene (C2H2),
methylacetylene (CH3C2H), and diacetylene
(C2HC2H).
Spectroscopy has also uncovered traces of
water vapour, carbon monoxide and carbon dioxide
in the upper atmosphere, which can only originate
from an external source such as infalling
dust and comets.
==== Troposphere ====
The troposphere is the lowest and densest
part of the atmosphere and is characterised
by a decrease in temperature with altitude.
The temperature falls from about 320 K (47
°C; 116 °F) at the base of the nominal troposphere
at −300 km to 53 K (−220 °C; −364 °F)
at 50 km.
The temperatures in the coldest upper region
of the troposphere (the tropopause) actually
vary in the range between 49 and 57 K (−224
and −216 °C; −371 and −357 °F) depending
on planetary latitude.
The tropopause region is responsible for the
vast majority of Uranus's thermal far infrared
emissions, thus determining its effective
temperature of 59.1 ± 0.3 K (−214.1 ± 0.3
°C; −353.3 ± 0.5 °F).The troposphere
is thought to have a highly complex cloud
structure; water clouds are hypothesised to
lie in the pressure range of 50 to 100 bar
(5 to 10 MPa), ammonium hydrosulfide clouds
in the range of 20 to 40 bar (2 to 4 MPa),
ammonia or hydrogen sulfide clouds at between
3 and 10 bar (0.3 and 1 MPa) and finally directly
detected thin methane clouds at 1 to 2 bar
(0.1 to 0.2 MPa).
The troposphere is a dynamic part of the atmosphere,
exhibiting strong winds, bright clouds and
seasonal changes.
==== Upper atmosphere ====
The middle layer of the Uranian atmosphere
is the stratosphere, where temperature generally
increases with altitude from 53 K (−220
°C; −364 °F) in the tropopause to between
800 and 850 K (527 and 577 °C; 980 and 1,070
°F) at the base of the thermosphere.
The heating of the stratosphere is caused
by absorption of solar UV and IR radiation
by methane and other hydrocarbons, which form
in this part of the atmosphere as a result
of methane photolysis.
Heat is also conducted from the hot thermosphere.
The hydrocarbons occupy a relatively narrow
layer at altitudes of between 100 and 300
km corresponding to a pressure range of 10
to 0.1 mbar (10.00 to 0.10 hPa) and temperatures
of between 75 and 170 K (−198 and −103
°C; −325 and −154 °F).
The most abundant hydrocarbons are methane,
acetylene and ethane with mixing ratios of
around 10−7 relative to hydrogen.
The mixing ratio of carbon monoxide is similar
at these altitudes.
Heavier hydrocarbons and carbon dioxide have
mixing ratios three orders of magnitude lower.
The abundance ratio of water is around 7×10−9.
Ethane and acetylene tend to condense in the
colder lower part of stratosphere and tropopause
(below 10 mBar level) forming haze layers,
which may be partly responsible for the bland
appearance of Uranus.
The concentration of hydrocarbons in the Uranian
stratosphere above the haze is significantly
lower than in the stratospheres of the other
giant planets.The outermost layer of the Uranian
atmosphere is the thermosphere and corona,
which has a uniform temperature around 800
to 850 K.
The heat sources necessary to sustain such
a high level are not understood, as neither
the solar UV nor the auroral activity can
provide the necessary energy to maintain these
temperatures.
The weak cooling efficiency due to the lack
of hydrocarbons in the stratosphere above
0.1 mBar pressure level may contribute too.
In addition to molecular hydrogen, the thermosphere-corona
contains many free hydrogen atoms.
Their small mass and high temperatures explain
why the corona extends as far as 50,000 km
(31,000 mi), or two Uranian radii, from its
surface.
This extended corona is a unique feature of
Uranus.
Its effects include a drag on small particles
orbiting Uranus, causing a general depletion
of dust in the Uranian rings.
The Uranian thermosphere, together with the
upper part of the stratosphere, corresponds
to the ionosphere of Uranus.
Observations show that the ionosphere occupies
altitudes from 2,000 to 10,000 km (1,200 to
6,200 mi).
The Uranian ionosphere is denser than that
of either Saturn or Neptune, which may arise
from the low concentration of hydrocarbons
in the stratosphere.
The ionosphere is mainly sustained by solar
UV radiation and its density depends on the
solar activity.
Auroral activity is insignificant as compared
to Jupiter and Saturn.
Uranus's atmosphere
=== Magnetosphere ===
Before the arrival of Voyager 2, no measurements
of the Uranian magnetosphere had been taken,
so its nature remained a mystery.
Before 1986, scientists had expected the magnetic
field of Uranus to be in line with the solar
wind, because it would then align with Uranus's
poles that lie in the ecliptic.Voyager's observations
revealed that Uranus's magnetic field is peculiar,
both because it does not originate from its
geometric centre, and because it is tilted
at 59° from the axis of rotation.
In fact the magnetic dipole is shifted from
the Uranus's centre towards the south rotational
pole by as much as one third of the planetary
radius.
This unusual geometry results in a highly
asymmetric magnetosphere, where the magnetic
field strength on the surface in the southern
hemisphere can be as low as 0.1 gauss (10
µT), whereas in the northern hemisphere it
can be as high as 1.1 gauss (110 µT).
The average field at the surface is 0.23 gauss
(23 µT).
Studies of Voyager 2 data in 2017 suggest
that this asymmetry causes Uranus's magnetosphere
to connect with the solar wind once a Uranian
day, opening the planet to the Sun's particles.
In comparison, the magnetic field of Earth
is roughly as strong at either pole, and its
"magnetic equator" is roughly parallel with
its geographical equator.
The dipole moment of Uranus is 50 times that
of Earth.
Neptune has a similarly displaced and tilted
magnetic field, suggesting that this may be
a common feature of ice giants.
One hypothesis is that, unlike the magnetic
fields of the terrestrial and gas giants,
which are generated within their cores, the
ice giants' magnetic fields are generated
by motion at relatively shallow depths, for
instance, in the water–ammonia ocean.
Another possible explanation for the magnetosphere's
alignment is that there are oceans of liquid
diamond in Uranus's interior that would deter
the magnetic field.Despite its curious alignment,
in other respects the Uranian magnetosphere
is like those of other planets: it has a bow
shock at about 23 Uranian radii ahead of it,
a magnetopause at 18 Uranian radii, a fully
developed magnetotail, and radiation belts.
Overall, the structure of Uranus's magnetosphere
is different from Jupiter's and more similar
to Saturn's.
Uranus's magnetotail trails behind it into
space for millions of kilometres and is twisted
by its sideways rotation into a long corkscrew.Uranus's
magnetosphere contains charged particles:
mainly protons and electrons, with a small
amount of H2 ions.
No heavier ions have been detected.
Many of these particles probably derive from
the thermosphere.
The ion and electron energies can be as high
as 4 and 1.2 megaelectronvolts, respectively.
The density of low-energy (below 1 kiloelectronvolt)
ions in the inner magnetosphere is about 2
cm−3.
The particle population is strongly affected
by the Uranian moons, which sweep through
the magnetosphere, leaving noticeable gaps.
The particle flux is high enough to cause
darkening or space weathering of their surfaces
on an astronomically rapid timescale of 100,000
years.
This may be the cause of the uniformly dark
colouration of the Uranian satellites and
rings.
Uranus has relatively well developed aurorae,
which are seen as bright arcs around both
magnetic poles.
Unlike Jupiter's, Uranus's aurorae seem to
be insignificant for the energy balance of
the planetary thermosphere.
== Climate ==
At ultraviolet and visible wavelengths, Uranus's
atmosphere is bland in comparison to the other
giant planets, even to Neptune, which it otherwise
closely resembles.
When Voyager 2 flew by Uranus in 1986, it
observed a total of ten cloud features across
the entire planet.
One proposed explanation for this dearth of
features is that Uranus's internal heat appears
markedly lower than that of the other giant
planets.
The lowest temperature recorded in Uranus's
tropopause is 49 K (−224 °C; −371 °F),
making Uranus the coldest planet in the Solar
System.
=== Banded structure, winds and clouds ===
In 1986, Voyager 2 found that the visible
southern hemisphere of Uranus can be subdivided
into two regions: a bright polar cap and dark
equatorial bands.
Their boundary is located at about −45°
of latitude.
A narrow band straddling the latitudinal range
from −45 to −50° is the brightest large
feature on its visible surface.
It is called a southern "collar".
The cap and collar are thought to be a dense
region of methane clouds located within the
pressure range of 1.3 to 2 bar (see above).
Besides the large-scale banded structure,
Voyager 2 observed ten small bright clouds,
most lying several degrees to the north from
the collar.
In all other respects Uranus looked like a
dynamically dead planet in 1986.
Voyager 2 arrived during the height of Uranus's
southern summer and could not observe the
northern hemisphere.
At the beginning of the 21st century, when
the northern polar region came into view,
the Hubble Space Telescope (HST) and Keck
telescope initially observed neither a collar
nor a polar cap in the northern hemisphere.
So Uranus appeared to be asymmetric: bright
near the south pole and uniformly dark in
the region north of the southern collar.
In 2007, when Uranus passed its equinox, the
southern collar almost disappeared, and a
faint northern collar emerged near 45° of
latitude.
In the 1990s, the number of the observed bright
cloud features grew considerably partly because
new high-resolution imaging techniques became
available.
Most were found in the northern hemisphere
as it started to become visible.
An early explanation—that bright clouds
are easier to identify in its dark part, whereas
in the southern hemisphere the bright collar
masks them – was shown to be incorrect.
Nevertheless there are differences between
the clouds of each hemisphere.
The northern clouds are smaller, sharper and
brighter.
They appear to lie at a higher altitude.
The lifetime of clouds spans several orders
of magnitude.
Some small clouds live for hours; at least
one southern cloud may have persisted since
the Voyager 2 flyby.
Recent observation also discovered that cloud
features on Uranus have a lot in common with
those on Neptune.
For example, the dark spots common on Neptune
had never been observed on Uranus before 2006,
when the first such feature dubbed Uranus
Dark Spot was imaged.
The speculation is that Uranus is becoming
more Neptune-like during its equinoctial season.The
tracking of numerous cloud features allowed
determination of zonal winds blowing in the
upper troposphere of Uranus.
At the equator winds are retrograde, which
means that they blow in the reverse direction
to the planetary rotation.
Their speeds are from −360 to −180 km/h
(−220 to −110 mph).
Wind speeds increase with the distance from
the equator, reaching zero values near ±20°
latitude, where the troposphere's temperature
minimum is located.
Closer to the poles, the winds shift to a
prograde direction, flowing with Uranus's
rotation.
Wind speeds continue to increase reaching
maxima at ±60° latitude before falling to
zero at the poles.
Wind speeds at −40° latitude range from
540 to 720 km/h (340 to 450 mph).
Because the collar obscures all clouds below
that parallel, speeds between it and the southern
pole are impossible to measure.
In contrast, in the northern hemisphere maximum
speeds as high as 860 km/h (540 mph) are observed
near 50° latitude.
=== Seasonal variation ===
For a short period from March to May 2004,
large clouds appeared in the Uranian atmosphere,
giving it a Neptune-like appearance.
Observations included record-breaking wind
speeds of 820 km/h (510 mph) and a persistent
thunderstorm referred to as "Fourth of July
fireworks".
On 23 August 2006, researchers at the Space
Science Institute (Boulder, Colorado) and
the University of Wisconsin observed a dark
spot on Uranus's surface, giving scientists
more insight into Uranus's atmospheric activity.
Why this sudden upsurge in activity occurred
is not fully known, but it appears that Uranus's
extreme axial tilt results in extreme seasonal
variations in its weather.
Determining the nature of this seasonal variation
is difficult because good data on Uranus's
atmosphere have existed for less than 84 years,
or one full Uranian year.
Photometry over the course of half a Uranian
year (beginning in the 1950s) has shown regular
variation in the brightness in two spectral
bands, with maxima occurring at the solstices
and minima occurring at the equinoxes.
A similar periodic variation, with maxima
at the solstices, has been noted in microwave
measurements of the deep troposphere begun
in the 1960s.
Stratospheric temperature measurements beginning
in the 1970s also showed maximum values near
the 1986 solstice.
The majority of this variability is thought
to occur owing to changes in the viewing geometry.There
are some indications that physical seasonal
changes are happening in Uranus.
Although Uranus is known to have a bright
south polar region, the north pole is fairly
dim, which is incompatible with the model
of the seasonal change outlined above.
During its previous northern solstice in 1944,
Uranus displayed elevated levels of brightness,
which suggests that the north pole was not
always so dim.
This information implies that the visible
pole brightens some time before the solstice
and darkens after the equinox.
Detailed analysis of the visible and microwave
data revealed that the periodical changes
of brightness are not completely symmetrical
around the solstices, which also indicates
a change in the meridional albedo patterns.
In the 1990s, as Uranus moved away from its
solstice, Hubble and ground-based telescopes
revealed that the south polar cap darkened
noticeably (except the southern collar, which
remained bright), whereas the northern hemisphere
demonstrated increasing activity, such as
cloud formations and stronger winds, bolstering
expectations that it should brighten soon.
This indeed happened in 2007 when it passed
an equinox: a faint northern polar collar
arose, and the southern collar became nearly
invisible, although the zonal wind profile
remained slightly asymmetric, with northern
winds being somewhat slower than southern.The
mechanism of these physical changes is still
not clear.
Near the summer and winter solstices, Uranus's
hemispheres lie alternately either in full
glare of the Sun's rays or facing deep space.
The brightening of the sunlit hemisphere is
thought to result from the local thickening
of the methane clouds and haze layers located
in the troposphere.
The bright collar at −45° latitude is also
connected with methane clouds.
Other changes in the southern polar region
can be explained by changes in the lower cloud
layers.
The variation of the microwave emission from
Uranus is probably caused by changes in the
deep tropospheric circulation, because thick
polar clouds and haze may inhibit convection.
Now that the spring and autumn equinoxes are
arriving on Uranus, the dynamics are changing
and convection can occur again.
== Formation ==
Many argue that the differences between the
ice giants and the gas giants extend to their
formation.
The Solar System is hypothesised to have formed
from a giant rotating ball of gas and dust
known as the presolar nebula.
Much of the nebula's gas, primarily hydrogen
and helium, formed the Sun, and the dust grains
collected together to form the first protoplanets.
As the planets grew, some of them eventually
accreted enough matter for their gravity to
hold on to the nebula's leftover gas.
The more gas they held onto, the larger they
became; the larger they became, the more gas
they held onto until a critical point was
reached, and their size began to increase
exponentially.
The ice giants, with only a few Earth masses
of nebular gas, never reached that critical
point.
Recent simulations of planetary migration
have suggested that both ice giants formed
closer to the Sun than their present positions,
and moved outwards after formation (the Nice
model).
== Moons ==
Uranus has 27 known natural satellites.
The names of these satellites are chosen from
characters in the works of Shakespeare and
Alexander Pope.
The five main satellites are Miranda, Ariel,
Umbriel, Titania, and Oberon.
The Uranian satellite system is the least
massive among those of the giant planets;
the combined mass of the five major satellites
would be less than half that of Triton (largest
moon of Neptune) alone.
The largest of Uranus's satellites, Titania,
has a radius of only 788.9 km (490.2 mi),
or less than half that of the Moon, but slightly
more than Rhea, the second-largest satellite
of Saturn, making Titania the eighth-largest
moon in the Solar System.
Uranus's satellites have relatively low albedos;
ranging from 0.20 for Umbriel to 0.35 for
Ariel (in green light).
They are ice–rock conglomerates composed
of roughly 50% ice and 50% rock.
The ice may include ammonia and carbon dioxide.Among
the Uranian satellites, Ariel appears to have
the youngest surface with the fewest impact
craters and Umbriel's the oldest.
Miranda has fault canyons 20 km (12 mi) deep,
terraced layers, and a chaotic variation in
surface ages and features.
Miranda's past geologic activity is thought
to have been driven by tidal heating at a
time when its orbit was more eccentric than
currently, probably as a result of a former
3:1 orbital resonance with Umbriel.
Extensional processes associated with upwelling
diapirs are the likely origin of Miranda's
'racetrack'-like coronae.
Ariel is thought to have once been held in
a 4:1 resonance with Titania.Uranus has at
least one horseshoe orbiter occupying the
Sun–Uranus L3 Lagrangian point—a gravitationally
unstable region at 180° in its orbit, 83982
Crantor.
Crantor moves inside Uranus's co-orbital region
on a complex, temporary horseshoe orbit.
2010 EU65 is also a promising Uranus horseshoe
librator candidate.
=== Planetary rings ===
The Uranian rings are composed of extremely
dark particles, which vary in size from micrometres
to a fraction of a metre.
Thirteen distinct rings are presently known,
the brightest being the ε ring.
All except two rings of Uranus are extremely
narrow – they are usually a few kilometres
wide.
The rings are probably quite young; the dynamics
considerations indicate that they did not
form with Uranus.
The matter in the rings may once have been
part of a moon (or moons) that was shattered
by high-speed impacts.
From numerous pieces of debris that formed
as a result of those impacts, only a few particles
survived, in stable zones corresponding to
the locations of the present rings.William
Herschel described a possible ring around
Uranus in 1789.
This sighting is generally considered doubtful,
because the rings are quite faint, and in
the two following centuries none were noted
by other observers.
Still, Herschel made an accurate description
of the epsilon ring's size, its angle relative
to Earth, its red colour, and its apparent
changes as Uranus travelled around the Sun.
The ring system was definitively discovered
on 10 March 1977 by James L. Elliot, Edward
W. Dunham, and Jessica Mink using the Kuiper
Airborne Observatory.
The discovery was serendipitous; they planned
to use the occultation of the star SAO 158687
(also known as HD 128598) by Uranus to study
its atmosphere.
When their observations were analysed, they
found that the star had disappeared briefly
from view five times both before and after
it disappeared behind Uranus.
They concluded that there must be a ring system
around Uranus.
Later they detected four additional rings.
The rings were directly imaged when Voyager
2 passed Uranus in 1986.
Voyager 2 also discovered two additional faint
rings, bringing the total number to eleven.In
December 2005, the Hubble Space Telescope
detected a pair of previously unknown rings.
The largest is located twice as far from Uranus
as the previously known rings.
These new rings are so far from Uranus that
they are called the "outer" ring system.
Hubble also spotted two small satellites,
one of which, Mab, shares its orbit with the
outermost newly discovered ring.
The new rings bring the total number of Uranian
rings to 13.
In April 2006, images of the new rings from
the Keck Observatory yielded the colours of
the outer rings: the outermost is blue and
the other one red.
One hypothesis concerning the outer ring's
blue colour is that it is composed of minute
particles of water ice from the surface of
Mab that are small enough to scatter blue
light.
In contrast, Uranus's inner rings appear grey.
Uranus's rings
== Exploration ==
In 1986, NASA's Voyager 2 interplanetary probe
encountered Uranus.
This flyby remains the only investigation
of Uranus carried out from a short distance
and no other visits are planned.
Launched in 1977, Voyager 2 made its closest
approach to Uranus on 24 January 1986, coming
within 81,500 km (50,600 mi) of the cloudtops,
before continuing its journey to Neptune.
The spacecraft studied the structure and chemical
composition of Uranus's atmosphere, including
its unique weather, caused by its axial tilt
of 97.77°.
It made the first detailed investigations
of its five largest moons and discovered 10
new ones.
It examined all nine of the system's known
rings and discovered two more.
It also studied the magnetic field, its irregular
structure, its tilt and its unique corkscrew
magnetotail caused by Uranus's sideways orientation.Voyager
1 was unable to visit Uranus because investigation
of Saturn's moon Titan was considered a priority.
This trajectory took Voyager 1 out of the
plane of the ecliptic, ending its planetary
science mission.The possibility of sending
the Cassini spacecraft from Saturn to Uranus
was evaluated during a mission extension planning
phase in 2009, but was ultimately rejected
in favour of destroying it in the Saturnian
atmosphere.
It would have taken about twenty years to
get to the Uranian system after departing
Saturn.
A Uranus orbiter and probe was recommended
by the 2013–2022 Planetary Science Decadal
Survey published in 2011; the proposal envisages
launch during 2020–2023 and a 13-year cruise
to Uranus.
A Uranus entry probe could use Pioneer Venus
Multiprobe heritage and descend to 1–5 atmospheres.
The ESA evaluated a "medium-class" mission
called Uranus Pathfinder.
A New Frontiers Uranus Orbiter has been evaluated
and recommended in the study, The Case for
a Uranus Orbiter.
Such a mission is aided by the ease with which
a relatively big mass can be sent to the system—over
1500 kg with an Atlas 521 and 12-year journey.
For more concepts see Proposed Uranus missions.
== In culture ==
In astrology, the planet Uranus () is the
ruling planet of Aquarius.
Because Uranus is cyan and Uranus is associated
with electricity, the colour electric blue,
which is close to cyan, is associated with
the sign Aquarius (see Uranus in astrology).
The chemical element uranium, discovered in
1789 by the German chemist Martin Heinrich
Klaproth, was named after the newly discovered
planet Uranus."Uranus, the Magician" is a
movement in Gustav Holst's orchestral suite
The Planets, written between 1914 and 1916.
Operation Uranus was the successful military
operation in World War II by the Soviet army
to take back Stalingrad and marked the turning
point in the land war against the Wehrmacht.
The lines "Then felt I like some watcher of
the skies/When a new planet swims into his
ken", from John Keats's "On First Looking
Into Chapman's Homer", are a reference to
Herschel's discovery of Uranus.Many references
to Uranus in popular culture and news involve
humor about one pronunciation of its name
resembling that of the phrase "your anus".
== See also ==
Outline of Uranus
2011 QF99 and 2014 YX49, the only two known
Uranus trojans
Colonisation of Uranus
Uranus in astrology
Uranus in fiction
Extraterrestrial diamonds (thought to be abundant
in Uranus)
== Notes ==
== References ==
== Further reading ==
Alexander, Arthur Francis O'Donel (1965).
The Planet Uranus – A History of Observation,
Theory and Discovery.
Miner, Ellis D. (1998).
Uranus: The Planet, Rings and Satellites.
New York: John Wiley and Sons.
ISBN 978-0-471-97398-0.
Bode, Johann Elert (1784).
Von dem neu entdeckten Planeten.
Verfasser.
Gore, Rick (August 1986).
"Uranus — Voyager Visits a Dark Planet".
National Geographic.
Vol. 170 no.
2.
pp. 178–194.
ISSN 0027-9358.
OCLC 643483454.
== External links ==
Uranus at Encyclopædia Britannica
Uranus at European Space Agency
NASA's Uranus fact sheet
Uranus Profile at NASA's Solar System Exploration
site
Planets – Uranus A kid's guide to Uranus.
Uranus at Jet Propulsion Laboratory's planetary
photojournal.
(photos)
Voyager at Uranus (photos)
Uranus (Astronomy Cast homepage) (blog)
Uranian system montage (photo)
Gray, Meghan; Merrifield, Michael (2010).
"Uranus".
Sixty Symbols.
Brady Haran for the University of Nottingham.
"How to Pronounce Uranus" by CGP Grey

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