From Wikipedia, the free encyclopedia
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'
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 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
Replica of the telescope used by Herschel to discover Uranus
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 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
Simulated
Earth view of Uranus from 1986 to 2030, from southern summer solstice
in 1986 to equinox in 2007 and northern summer solstice in 2028.
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.
Northern hemisphere
|
Year
|
Southern hemisphere
|
Winter solstice
|
1902, 1986
|
Summer solstice
|
Vernal equinox
|
1923, 2007
|
Autumnal equinox
|
Summer solstice
|
1944, 2028
|
Winter solstice
|
Autumnal equinox
|
1965, 2049
|
Vernal equinox
|
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
Size comparison of Earth and Uranus
Diagram of the interior of Uranus
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/cm
3 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/cm
3, 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 (CH
4). 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 (C
2H
6),
acetylene (C
2H
2),
methylacetylene (CH
3C
2H), and
diacetylene (C
2HC
2H). 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
Aurorae on Uranus taken by the Space Telescope Imaging Spectrograph (STIS) installed on Hubble.
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
Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated.
Zonal wind speeds on Uranus. Shaded areas show the southern collar
and its future northern counterpart. The red curve is a symmetrical fit
to the data.
Magnetosphere
The magnetic field of Uranus as observed by Voyager 2 in 1986. S and N are magnetic south and north poles.
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
Uranus's
southern hemisphere in approximate natural colour (left) and in shorter
wavelengths (right), showing its faint cloud bands and atmospheric
"hood" as seen by Voyager 2
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.
The first dark spot observed on Uranus. Image obtained by the HST ACS in 2006.
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
Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible (HST ACS image).
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.
Moons
Major moons of Uranus in order of increasing distance (left to right), at their proper relative sizes and albedos (collage of Voyager 2 photographs)
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
Animation about the discovering occultation in 1977. (Click on it to start)
Uranus has a complicated planetary ring system, which was the second such system to be discovered in the Solar System after Saturn's.
Uranus's aurorae against its equatorial rings, imaged by the Hubble
telescope. Unlike the aurorae of Earth and Jupiter, those of Uranus are
not in line with its poles, due to its lopsided magnetic field.
Exploration
Crescent Uranus as imaged by Voyager 2 while en route to Neptune
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.
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.
Many references to Uranus in popular culture and news involve
humor about one pronunciation of its name resembling that of the phrase
"your
anus".