|
Uranus is a frigid world tipped on its side; most other large
planets spin more or less in the same direction that the Sun does, but some event early in Uranus history tipped its spin axis all the way over, past the horizontal.
It has long been thought that the tilt resulted from a great collision, but recently it has been suggested that the tilt was
caused by complex gravitational interactions between the giant planets as they formed and migrated to their current positions
Uranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It
is named after the ancient Greek deity of the sky Uranus (Ancient Greek) the father of Cronus (Saturn) and grandfather of
Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet
by ancient observers because of its dimness and slow orbit. Sir William Herschel announced its discovery on March 13, 1781,
expanding the known boundaries of the Solar System for the first time in modern history. Uranus was also the first planet
discovered with a telescope.
Uranus is similar in composition to Neptune, and both are composed of different elements
from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category,
the "ice giants". Uranus's atmosphere, while similar to Jupiter and Saturn's in its primary composition of hydrogen
and helium, contains more "ices" such as water, ammonia and methane, along with traces of hydrocarbons. It is the
coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (–224 °C). It has a complex,
layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer
of clouds. In contrast 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 the planets
because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, its north
and south poles lie where most other planets have their equators. Seen from Earth, Uranus's rings can sometimes appear to
circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008
the rings appeared edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light
without the cloud bands or storms associated with the other giants. However, terrestrial observers have seen signs of seasonal
change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach
250 meters per second (900 km/h, 560 mph).
Uranus had been observed on many occasions before its discovery as a
planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed
the planet at least six times, cataloging it as 34 Tauri. The French astronomer, Pierre Lemonnier, observed Uranus at least
twelve times between 1750 and 1769, including on four consecutive nights. Sir William Herschel observed the planet on 13 March
1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy),
but 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.The pronunciation of the name Uranus preferred
among astronomers is with stress on the first syllable as in Latin Uranus; in contrast to the colloquial with stress on the
second syllable and a long a, though both are considered acceptable. Because, in the English-speaking world, u·ra'·n?s
sounds like "your anus", the former pronunciation also saves embarrassment: as Dr. Pamela Gay, an astronomer at
Southern Illinois University, noted on her podcast, to avoid "being made fun of by any small schoolchildren ... when
in doubt, don't emphasise anything and just say ur'·?·n?s. And then run, quickly."
Uranus is the only planet whose name is derived from a figure from Greek
mythology rather than Roman mythology: the Greek arrived in English by way of the Latin "Uranus". The adjective
of Uranus is "Uranian". Its astronomical symbol is . It 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. Its astrological
symbol is , 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 name"). In the
Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star.
Hubble
Space Telescope image of Uranus showing cloud bands, rings, and moonsUranus revolves around the Sun once every 84 Earth years.
Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400
that 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 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. However, as on all giant planets,
its upper atmosphere experiences very strong winds in the direction of rotation. At some latitudes, such as about two-thirds
of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation
in as little as 14 hours.
Uranus has an axial tilt of 97.77 degrees, so its axis of rotation is approximately parallel
with the plane of the Solar System. This gives it seasonal changes completely unlike those of the other major planets. Other
planets can be visualized to rotate like tilted spinning tops on the plane of the Solar System, while Uranus rotates more
like a tilted rolling ball. Near the time of Uranian solstices, one pole faces the Sun continuously while the other pole faces
away. Only a narrow strip around the equator experiences a rapid day-night cycle, but with the Sun very low over the horizon
as in the Earth's polar regions. 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.
From 1995 to 2006, Uranus's apparent magnitude fluctuated between
+5.6 and +5.9, placing it just within the limit of naked eye visibility at +6.5.[10] 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, the planet 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.
Size comparison of Earth and UranusUranus's mass is roughly 14.5 times that of the Earth, making
it the least massive of the giant planets, while its density of 1.27 g/cm³ makes it the second least dense planet, after
Saturn. Though having a diameter slightly larger than Neptune's (roughly four times Earth's), it is less massive. These values
indicate 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, as different figures emerge depending on the model chosen; however, 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 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 core in the center, 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 percent of Uranus's; the mantle comprises
the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about
0.5 Earth masses and extending for the last 20 percent of Uranus's radius. Uranus's core density is 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 bulk compositions of Uranus
and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate
classification as ice giants.
The Uranus System. Credit ESOUranus has 27 known natural satellites. The names for
these satellites are chosen from characters from 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 the gas giants; indeed,
the combined mass of the five major satellites would be less than half that of Triton alone. The largest of the 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
moon of Saturn, making Titania the eighth largest moon in the Solar System. The moons have relatively low albedos; ranging
from 0.20 for Umbriel to 0.35 for Ariel (in green light). The moons are ice-rock conglomerates composed of roughly fifty percent
ice and fifty percent rock. The ice may include ammonia and carbon dioxide.
Among the satellites, Ariel appears
to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest. Miranda possesses fault canyons
20 kilometers deep, terraced layers, and a chaotic variation in surface ages and features. Miranda's past geologic activity
is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a
result of a formerly present 3:1 orbital resonance with Umbriel. Extensional processes associated with upwelling diapirs are
the likely origin of the moon's 'racetrack'-like coronae. Similarly, Ariel is believed to have once been held in a 4:1 resonance
with Titania.
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. Uranus, by contrast,
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. In fact, 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.
Hypotheses for this discrepancy include
that when Uranus was hit by a supermassive impactor, which caused it to expel most of its primordial heat, leaving it with
a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus's upper layers which 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.
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 remarkably over two planetary radii from the nominal
surface at 1 bar pressure. 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 the Uranian atmosphere
is different from the composition of whole planet, consisting as it does 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 very close to the protosolar helium mass fraction of 0.275
± 0.01, indicating that helium has not settled in the center of the planet as it has in the gas giants. The third most
abundant constituent of the Uranian atmosphere is methane (CH4). Methane possesses prominent absorption bands in the visible
and near-infrared (IR) making Uranus aquamarine or cyan in color. 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[e] is much lower in the upper atmosphere owing 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. However 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), 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
infalling dust and comets.
Size: 3rd largest planet - 51,118 kilometers -- 4.007 Earths
Calendar: 1 Uranus year
= 83.749 Earth years; 1 Uranus day = 0.72 Earth days
Orbit: 2,872,460,000 km - 19.201 Earth orbits
Axial tilt:
97.77 degrees (retrograde, and tilted almost horizontal)
|
