Uranus - The 7th Planet from our Sun
Orbit
and rotation
Axial tilt
Internal
structure
Internal heat
Atmosphere
Planetary
rings
Moons
Uranus 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 different bulk chemical
composition 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, although similar to Jupiter's and Saturn's in
its primary composition of hydrogen and helium, 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 (−224.2 °C), 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.
Uranus
is the only planet whose name is derived from a figure from Greek mythology, from the Latinized version of
the Greek god of the sky, Ouranos. 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 revolution about the Sun. 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).
Orbit
and rotation
Uranus
orbits the Sun once every 84 Earth years. Its average distance from the Sun is
roughly 3 billion km
(about 20 AU). 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.[46] 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 September 23, 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 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. In contrast to the other planets, whose motions around the Sun resemble
that of spinning tops, Uranus's motion can rather be visualized as that of,
respectively, a ball rolling on the ecliptic plane near solstices and a spinning rifle
bullet near equinoxes.
Uranus
reached its most recent equinox on December 7, 2007.
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 mantlein 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 densityis around 9 g/cm3, with
a pressure in the center 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 science to
determine 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 and 24 973 ± 20 km,
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 °C), 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. 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. The tenuous corona of the atmosphere 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 and pressures from 100 to 0.1 bar
(10 MPa to 10 kPa); the stratosphere, spanning
altitudes between 50 and 4000 km and pressures of between 0.1 and 10−10 bar (10 kPa to 10 µPa); and the thermosphere/corona
extending from 4,000 km to as high as 50,000 km from the surface. There
is no mesosphere.
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,[69] 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 vapor, carbon monoxide and carbon dioxide in the upper atmosphere,
which can only originate from an external source such as in falling dust and comets.
Planetary
rings
The
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 March 10, 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 2passed
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.
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 areMiranda, 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, 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 kilometres 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.
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