(Redirected from Uranus (planet))
'Uranus' () is the seventh
planet from the
Sun and third largest planet in the
solar system. It is named after the ancient Greek deity of the sky (
Uranus, ''), the father of
Kronos (
Saturn) and grandfather of
Zeus (
Jupiter). Uranus was the first planet discovered in modern times. It is visible to the naked eye like the five
classical planets, but it was never recognised as a planet by ancient observers due to its dimness.
[4] 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, and making the first discovery of a planet using a
telescope.
Uranus and
Neptune have different internal and atmospheric compositions from those of the larger
gas giants —
Jupiter and
Saturn, and astronomers sometimes place them in a separate category, the "
ice giants". Uranus' atmosphere, while still composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water,
ammonia and
methane, along with the usual traces of hydrocarbons. It also the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K. It has a complex cloud structure with water clouds presumably being the deepest and methane clouds at the top.
[ ]
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; its north and south poles lie where the other planets have their equators.[ Seen from Earth, Uranus' rings appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock. 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, ground-based observers have seen signs of the seasonal change and increased weather activity in recent years as Uranus approaches its equinox. The wind speeds on Uranus can reach 250 m/s.]
Discovery
Uranus had been observed on many occasions prior to its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed catalogued Uranus as 34 Tauri. Flamsteed observed Uranus at least six more times. The French astronomer, Pierre Lemonnier, observed Uranus at least 12 times between 1750 and 1769,[5] including on four consecutive nights.
Sir William Herschel obvserved the planet on March 13, 1781, but initially reported it (on April 26, 1781) as a "comet".[6] Herschel "engaged in a series of observations on the parallax of the fixed stars",[7] using his telescope at his new home in the town of Bath. He recorded in his journal, "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".[8] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".[9] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:[10]
Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "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".[11]
While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell estimated its distance as 18 times the distance of the Sun from the Earth, and no comet had yet been observed with a perihelion of even four times the Earth–Sun distance.[ History of Astronomy George Forbes ] 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".[12] Bode concluded that its near-circular orbit was more like a planet than a comet.[13]
The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact 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."[ The Scientific Papers of Sir William Herschel, J. L. E. Dreyer,, , , Royal Society and Royal Astronomical Society, 1912, ] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so the Royal Family could have a chance to look through his telescopes.[14]
Naming
Maskelyne asked Herschel to "do the astronomical world the faver ''(sic)'' to give a name to your planet, which is entirely your own, & which we are so much obliged to you for the discovery of".[15]
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.[16] He explained this decision in a letter to Joseph Banks:
William Herschel, discoverer of Uranus
Astronomer Jérôme Lalande proposed the planet be named ''Herschel'' in honour of its discoverer.[ The meaning of the symbol H+o for the planet Uranus Francesca Herschel ] Bode, however, opted for ''Uranus'', the Latinized version of the Greek god of the sky, Ouranos. Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.
Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. The adjective of Uranus is "Uranian". The element uranium, discovered in 1789, was named in its honour by its discoverer, Martin Klaproth.[21] The stressed syllable in the name ''Uranus'' is properly the first, because the penultimate vowel ''a'' is short and in an open syllable. Such syllables are never stressed in Latin.[22] The historically correct pronunciation of the name by English speakers is therefore . The historically incorrect pronunciation, , with stress on the second syllable and a "long a" has become very common.
Uranus' astronomical symbol is 
Astronomical symbol for Uranus
In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the ''sky king star'' (天王星).[24][25]
Orbit and rotation
HST image of Uranus showing cloud bands, rings, and moons
Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km. The intensity of sunlight on Uranus is about 1/400 that of Earth.[26] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.
The rotational period of the interior of Uranus is 17 hours, 14 minutes. However, like on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. In effect, at some latitudes, such as about 2/3 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.[28]
Axial tilt
Uranus axis of rotation lies on its side with respect to the plane of the solar system, with an axial tilt of 98 degrees. This gives it a completely different exchange of seasons to the other major planets. Other planets can be visulalized to rotate like tilted spinning tops relative to the plane of the solar system, while Uranus rotates more like a tilted rolling bowling ball. Near the time of Uranian solstices, one pole faces the Sun continually 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 like in the Earth's polar regions. At the other side of Uranus' 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.[29] 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 will reach its next equinox around December 2007.[ Hubble Discovers Dark Cloud In The Atmosphere Of Uranus ]
| 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, on average during the 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 which causes this is unknown. The reason for Uranus' 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.[30]
At the time of ''Voyager 2's flyby in 1986, Uranus' south pole was pointed almost directly at the Sun. The labeling 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 shall be the pole which points above the invariable plane of the solar system (regardless of the direction the planet is spinning).[31][32] However, a different convention is sometimes used, where a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation.[33] In terms of this latter coordinate system it was Uranus' ''north'' pole which was in sunlight in 1986. Astronomer Patrick Moore, commenting on the issue, summed it up by saying "Take your pick!"[34]
Visibility
From 1995 to 2006, Uranus' apparent magnitude fluctuated between +5.6 and +5.9, placing it just above the limit of naked eye visibility at +6.0.[ Twelve Year Planetary Ephemeris: 1995 - 2006 Fred Espenak ] 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.[ NASA's Uranus fact sheet ] 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.[35]
Physical characteristics
Structure and composition
Size comparison of Earth and Uranus
Uranus' mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets, while its density of 1.29g/cm³ makes it the second least dense planet after Saturn.[ The masses of Uranus and its major satellites from Voyager tracking data and Earth-based Uranian satellite data, , R.A., Jacobson, The Astronomical Journal, 1992 ] Though of a similar diameter to Neptune (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.[ Comparative model of Uranus and Neptune, , M., Podolak, Planet. Space Sci., 1995 ] The total mass of ice in Uranus' 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.[ Further investigations of random models of Uranus and Neptune, , M., Podolak, Planet. Space Sci., 2000 ] 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' structure is that it consists of three layers: a rocky 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 percent Uranus'; 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' radius.][ Uranus' core density is around 9 g/cm³, with a pressure at the core/mantle boundary of 8 million bar and a temperature of about 5000 K.][ This fluid, which has a high electical conductivity, is sometimes called water–ammonia ocean.][ Water-ammonia ionic ocean on Uranus and Neptune?, , S., Atreya, Geophysical Research Abstracts, 2006 ] 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, ice giants.
While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substational 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 us 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.[ However for the sake of convenience an oblate spheroid of revolution, where pressure equals 1 bar, is designated conditionally as a ‘surface’. It has equatorial and polar radii of 25,559 ± 4 and 24,973 ± 20 km, respectively.][ Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006, , P. Kenneth, Seidelmann, Celestial Mech. Dyn. Astr., 2007 ] This surface will be used throughout this article as a zero point for altitudes.
Atmosphere
Main articles: Atmosphere of Uranus
Although there is no well-defined solid surface within Uranus' interior, the outermost part of Uranus' 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 level, with a corresponding pressure around 100 bar and temperature of 320 K.][ Possible Microwave Absorbtion in by H2S gas Uranus’ and Neptune’s Atmospheres, , Imke, dePater, Icarus, 1991 ] 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; the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10–10 bar; and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface.][ The Atmospheres of Uranus and Neptune, , Jonathan. I., Lunine, Annual Review of Astronomy and Astrophysics, 1993 ] There is no mesosphere.
Composition
The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of molecular hydrogen and helium.[ The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium is 0.15 ± 0.03][ The helium abundance of Uranus from Voyager measurements, B. Conrath ''et al.'', , , Journal of Geophysical Research, ] 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,][ Solar System Abundances and Condensation Temperatures of the Elements, , Katharin, Lodders, The Astrophysical Journal, 2003 ] indicating that helium has not settled in the centre 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 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; about 20 to 30 times that 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.][ Reanalysis of Voyager 2 UVS Occultations at Uranus: Hydrocarbon Mixing Ratios in the Equatorial Stratosphere, , J., Bishop, Icarus, 1990 ] 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.[ Uranius Deep Atmosphere Revealed, , Imke, dePater, Icarus, 1989 ]
In addition to methane trace amounts of various hydrocarbons was found in the upper atmosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar UV radiation.[ They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), diacetylene (C2HC2H).][ Detection of new hydrocarbons in Uranus' atmosphere by infrared spectroscopy, , Martin, Burgorf, Icarus, 2006 ][ Spectroscopy 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.][ ISO observations of the giant planets and Titan: what have we learnt?, , Therese, Encrenaz, Planet. Space Sci., 2003 ][ First detection of CO in Uranus, , Th., Encrenaz, Astronomy&Astrophysics, 2004 ]
Troposphere
Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated.
The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.[ The temperature falls from about 320 K at the base of the nominal troposphere at −300 km to 53 K at 50 km.][ Voyger 2 Radio Science Observations of the Uranian System: Atmosphere, Rings, and Satellites, , J.L., Tyler, Science, 1986 ] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on planetary latitude.[ Infrared Observations of the Uranian System, , R., Hanel, Science, 1986 ] The tropopause region is responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.[ The Albedo, Effective Temperature, and Energy Balance of Uranus as Determined from Voyager IRIS Data, , J.C., Pearl, Icarus, 1990 ]
The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar, ammonium hydrosulfide clouds in the range of 20 and 40 bar, ammonia or hydrogen sulfide clouds at between 3 and 10 bar and finally directly detected thin methane clouds at 1 to 2 bar[ Coupled Clouds and Chemistry of the Giant Planets — a Case for Multiprobes, , Sushil K., Atreya, Space Sci. Rev., 2005 ][ The Atmosphere of Uranus: Results of Radio Occultation Measurements with Voyager 2, , G.F., Lindal, J. of Geophys. Res., 1987 ] The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes, which will be discussed below.[ Dynamics of cloud features on Uranus, , L.A., Sromovsky, Icarus, 2005 ]
Upper atmosphere
The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base of the thermosphere.[ The Upper Atmosphere of Uranus: EUV Occultations Observed by Voyager 2, , Floyd, Herbert, J. of Geophys. Res., 1987 ] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons, that form in this part of the atmosphere as a result of methane photolysis.[ Photochemistry of the Atmosphere of Uranus, , Michael E., Summers, The Astrophysical Journal, 1989 ] Heating from the hot thermosphere may also be significant.[ Uranus after Solstice: Results from the 1998 November 6 Occultation, , Leslie A., Young, Icarus, 2001 ] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 280 km corresponding to a pressure range of 10 to 0.1 mbar and temperatures of between 75 and 170 K.[ The most abundant hydrocarbons are acetylene and ethane with mixing ratios of around relative to hydrogen, which is similar to the mixing ratios of methane and carbon monoxide at these altitudes.][ Heavier hydrocarbons, carbon dioxide and water vapour have mixing ratios three orders of magnitude lower.][ Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause forming haze layers,][ which may be partly responsible for the bland appearance of Uranus. However, the concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.][ Ultraviolet Observations of Uranus and Neptune, , Floyd, Herbert, Planet. Space Sci., 1999 ]
The outmost layer of the Uranian atmosphere is thermosphere/corona, which has a uniform temperature around 800 to 850 K.[ The heat sources necessary to sustain such a high value are not understood, since neither solar FUV/EUV radiation nor auroral activity can provide the necessary energy, although weak cooling efficiency due to the lack of hydrocarbons in the upper part of the stratosphere may also contribute.][ In addition to molecular hydrogen, the thermosphere-corona contains a large proportion of free hydrogen atoms. Their small molecular mass together with the high temperatures may help to explain why the corona extends as far as 50,000 km or two Uranian radii from the planet.][ 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.][. The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.][ H2 Quadrupole and H3+ Emission from Uranus: the Uranian Thermosphere, Ionosphere, and Aurora, , L.M., Trafton, The Astrophysical Journal, 1999 ] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.[ The rotational temperature and column density of H+3 in Uranus, , Th., Encrenaz, Planetary and Space Sciences, 2003 ] Auroral activity is not as significant as at Jupiter and Saturn.[ Variation in the H+3 emission from Uranus, , Hoanh An, Lam, The Astrophysical Journal, 1997 ]
Planetary rings
Main articles: Rings of Uranus
An enhanced colour schematic of the inner rings
[ It was the next ring system to be discovered in the Solar System after Saturn's.][ 13 distinct rings are presently known, the brightest being the epsilon ring. ]
William Herschel claimed to have seen rings at Uranus in 1789 (see below), however this is doubtful as in the two following centuries no rings were noted by other observers. The ring system was definitively discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. However, when their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet.[36] The rings were directly imaged when ''Voyager 2'' passed Uranus in 1986.[ ''Voyager 2'' also discovered two addtional faint rings bringing the total number to eleven.]
In December 2005, the Hubble Space Telescope detected a pair of previously unknown bluish rings. The largest is located at twice the distance from the planet than the previously known rings. These new rings are so far from the planet that they are being called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. These two rings bring the total number of Uranus rings to 13.[37] In April 2006, images of the new rings with the Keck Observatory yielded the colours of the outer rings: one was blue and the other red.[ New Dust Belts of Uranus: Two Ring, red Ring, Blue Ring, , Imke, dePater, Science, 2006 ][38]
One hypothesis concerning the outer rings' blue colour is that it is composed of minute particles of water ice taken from the surface of Mab that are small enough to scatter blue light.[[39] The planet's inner rings appear grey.]
Uranus’ rings are probably quite young; gaps in their circumference as well as differences in their opacity suggest that they did not form with Uranus. The matter in the rings may once have been part of a moon which was shattered by a high-speed impact or tidal forces.[ Planetary rings, , L. W., Esposito, Reports On Progress In Physics, 2002 ]
Regarding William Herschel's observations in the 18th century, the first mention of a Uranian ring system comes from his notes detailing his observations of Uranus, which include the following passage: "February 22, 1789: A ring was suspected".[40]
Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case.[ Herschel's notes were published in a Royal Society journal in 1797. However, in the two centuries between 1797 and 1977 the rings are rarely mentioned, if at all. This casts serious doubt whether Herschel could have seen anything of the sort while hundreds of other astronomers saw nothing. Still, it has been claimed by some that Herschel actually gave accurate descriptions of the ring's size relative to Uranus, its changes as Uranus travelled around the Sun, and its colour.[41]]
Magnetic field
The magnetic field of Uranus as seen by Voyager 2 in 1986. S and N are magnetic south and north poles.
Prior to the arrival of ''Voyager 2'', no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, since it would then align with the planet's poles that lie in the ecliptic.[ ]
''Voyager's observations revealed that the magnetic field is peculiar, both because it does not originate from the planet's geometric centre, and because it is tilted at 59° from the axis of rotation.[ Magnetic Fields at Uranus, , Norman F., Ness, Science, 1986 ][ In fact the magnetic dipole is shifted from the centre of the planet 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, whereas in the northern hemisphere it can be as high 1.1 Gauss.][ The average field at the surface is 0.23 Gauss.][ In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its physical equator.][ The dipole moment of Uranus is 50 times that of Earth.][ Planetary Magnetospheres, , C.T., Russell, Rep. Prog. Phys., 1993 ] 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 giant planets, 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.][[42]]
Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock located at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail and radiation belts.[ The Magnetosphere of Uranus: Hot Plasma and radiation Environment, , S.M., Krimigis, Science, 1986 ] Overall, the structure of the magnetosphere of Uranus is different from that of Jupiter's and more similar to that of Saturn's.[ Uranus' magnetotail trails behind the planet into space for millions of kilometers and is twisted by the planet's sideways rotation into a long corkscrew.][[43] ]
Uranus' magnetosphere contains charged particles: protons and electrons with small amount of H2+ ions.[ No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona.][ The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.][ The density of low energy (< 100 electronvolts) ions in the inner magnetosphere is about 2 cm-3.][ Plasma Observations Near Uranus: Initial Results from Voyager 2, , H.S., Bridge, Science, 1986 ] The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.[ The particle flux is high enough to cause darkening or space weathering of the moon’s surfaces on an astronomically rapid timescale of 100,000 years.][ This may be the cause of the uniformly dark colouration of the moons and rings.][ Voyager Uranus Science Summary ] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.[ However, unlike Jupiter's, Uranus' aurore seem to be insignificant for the energy balance of the planetary thermosphere.]
Climate
Uranus' atmosphere is remarkably bland in comparison to the other gas giants, 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.][ No Longer Boring: 'Fireworks' and Other Surprises at Uranus Spotted Through Adaptive Optics Emily Lakdawalla ]
One proposed explanation for this dearth of cloud features is that Uranus' internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux.[ Why Uranus' internal temperature is so low is still not understood. Neptune, which is Uranus' 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' 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' tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.]
Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also 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' 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.][ ]
Banded structure, winds and clouds
The figure shows zonal winds on Uranus. The data of Smith et.al. 1986, Karkoschka et.al.1998 and Hammel et.al.2001 are from Hammel et.al.2001.[ The other data are from Hammel et.al. 2005][ and Sromovsky et.al 2005.][ Shaded areas show southern collar and its future northern counterpart. The red curve is a symmetrical fit to the data.]
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 (see figure on the right).[ Their boundary is located at about −45 degrees of latitude. A narrow band straddling the latitudinal range from −45 to −50 degrees is the brightest large feature on the visible surface of the planet.][ Uranus in 2003: Zonal winds, banded structure, and discrete features, , H.B., Hammel, Icarus, 2005 ] 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).[ Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. However, at the beginning of the twenty-first century, when the northern polar region came into view, Hubble Space Telescope (HST) and Keck telescope observed neither a collar nor a polar cap in the northern hemisphere.][ So Uranus appears to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.][ Details of possible seasonal variations in the planet's appearance will be discussed later. The visible latitudinal structure of Uranus is different from that of Jupiter and Saturn, which demonstrate multiple narrow and colorful bands.][ ]
In addition to 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. However in 1990s the number of the observed bright cloud features grew considerably.][ The majority of them was found in the northern hemisphere as it started to become visible.][ The common explanation of this fact is that bright clouds are easier to identify in the dark part of the planet, whereas in the southern hemisphere the bright collar masks them.][ 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, which is connected to fact that until 2004 (see below) no southern polar cloud had been observed at the wavelength 2.2 micrometres,][ which is sensitive to the methane absorption, while northern clouds have been regularly observed in this wavelength band. The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, while at least one southern cloud has persisted since Voyager flyby.][ Recent observation also discovered that cloud-features on Uranus have a lot in common with those on Neptune, although the weather on Uranus is much calmer.][ The dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature was imaged.]
The first dark spot on Uranus ever observed. The image is obtained by
ACS on
HST in 2006.
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 −100 to −50 m/s.][ 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 the planet's rotation. Windspeeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.][ Windspeeds at −40° lattitude range from 150 to 200 m/s. Since 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 240 m/s are observed near +50 degrees of latitude.][ These speeds sometimes lead to incorrect assertions that winds are faster in the northern hemisphere. In fact, latitude per latitude, winds are slightly slower in the northern part of Uranus, especially at the midlatitudes from ±20 to ±40 degrees.][ There is currently no agreement about whether any changes in windspeed have occurred since 1986,][ New Measurements of the Winds of Uranus, , H.B., Hammel, Icarus, 2001 ][ and nothing is known about much slower meridional winds.]
Seasonal variation
For a short period in Autumn 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.[ New cloud activity on Uranus in 2004: First detection of a southern feature at 2.2 µm, , H.B., Hammel, , 2005 ][44] Observations included record-breaking wind speeds of 824 km/h and a persistent thunderstorm referred to as "Fourth of July fireworks".[ On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus' surface, giving astronomers more insight into the planet's atmospheric activity.][ Hubble Discovers a Dark Cloud in the Atmosphere of Uranus ] Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus' 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' atmosphere has existed for less than 84 years, or one full Uranian year. A number of discoveries have however been made. 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 occuring at the solstices and minima occuring at the equinoxes.[ Photometric variability of Uranus and Neptune, 1950–2004, , G.W., Lockwood, Icarus, 2006 ] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[ Long-term variations in the microwave brightness temperature of the Uranus atmosphere, , M.J., Klein, Icarus, 2006 ] Stratospheric temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.[ ]
The visible magnitude of Uranus in two spectral bands (upper graph)[ adjusted for the distance and effective microwave temperature (lower graph).][ Blue band is centered at 470 nm, yellow at 550 nm.]
The majority of this variablilty is believed to occur due to changes in the viewing geometry. Uranus is an oblate spheroid, which causes its visible area to become larger when viewed from the poles. This explains in part its brighter appearance at solstices.[ Uranus is also known to exhibit strong meridional variations in albedo (see above).][ Uranus’ Apparent Seasonal Variability in 25 HST Filters, , Erich, Karkoschka, Icarus, 2001 ] For instance, the south polar region of Uranus is much brighter than the equatorial bands.[ In addition, both poles demonstrate elevated brightness in the microwave part of the spectrum,][ Seasonal change in the deep atmosphere of Uranus, , Mark D., Hofstadter, Icarus, 2003 ] while the polar stratosphere is known to be cooler than the equatorial one.[ So seasonal change seems to happen as follows: poles, which are bright both in visible and microwave spectral bands, come in to the view at solstices resulting in brighter planet, while the dark equator is visible mainly near equinoxes resulting in darker planet.][ In addition, occultations at solstices probe hotter equatorial stratosphere.][ ]
However there are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet 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 addition, some microwave data show increases in pole–equator contrast after the 1986 solstice.][ Finally 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),][ Evidence for temporal change at Uranus’ south pole, , K.A., Rages, Icarus, 2004 ] while the northern hemisphere demonstrates increasing activity,[ such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.][ In particular the bright polar collar present in the southern hemisphere at −45° is expected to appear in the northern part of the planet.][ Long-term atmospheric variability on Uranus and Neptune, , H.B., Hammel, Icarus, 2007 ]
The mechanism of physical changes is still not clear.[ Near the summer and winter solstices, Uranus' 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 the planet is probably caused by a 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.[ Numerical simulations of the accretion of Uranus and Neptune, , Adrian, Brunini, Plan. Space Sci., 1999 ] The Solar System is believed to have formed from a giant rotating ball of gas and dust known as the presolar nebula. As it condensed, it formed into a disc with a slowly collapsing Sun in the middle.[ Much of the nebula's gas, primarily hydrogen and helium, formed the Sun, while 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 onto 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.][ An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness, , Scott S., Sheppard, The Astronomical Journal, 2006 ]
Current theories of solar system formation have difficulty accounting for the presence of Uranus and Neptune so far out from Jupiter and Saturn. They are too large to have formed from the amount of material expected at that distance. Rather, some scientists expect that both formed closer to the Sun but were scattered outward by Jupiter.[ The formation of Uranus and Neptune in the Jupiter-Saturn region of the Solar System, , Edward W., Thommes, Nature, 1999 ] However more recent simulations, which take into account planetary migration, seem to be able to form Uranus and Neptune near their present locations.
Moons
Main articles: Uranus' natural satellites
Major moons of Uranus compared, at their proper relative sizes.
Uranus has 27 known natural satellites.[ The names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope.][ Uranus ] 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. The moons have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel (in green light).][ Voyager 2 in the Uranian System: Imaging Science Results, , B.A., Smith, Science, 1986 ] The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include ammonia and carbon dioxide.[ Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects, , Hauke, Hussmann, Icarus, 2006 ][ ]
Among the satellites, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.[ One hypothesis holds that Miranda may have been blasted completely apart by a massive impact some time in its past, and then reformed haphazardly.][ Modelling the disruption and reaccumulation of Miranda, , F., Marzari, Astron. Astrophys., 1998 ]
Exploration
A picture of Uranus taken by
Voyager 2 as it headed to Neptune
Main articles: Exploration of Uranus
In 1986 NASA's ''Voyager 2'' visited Uranus. This visit is the only attempt to investigate the planet from a short distance and no other visits are currently planned. Launched in 1977, ''Voyager 2'' made its closest approach to Uranus on January 24, 1986, coming within 81,500 kilometers of the planet's cloud tops, before continuing its journey to Neptune. ''Voyager 2'' studied structure and chemical composition of the atmosphere,[ discovered 10 new moons and studied the planet's unique weather, caused by its axial tilt of 97.77°; and examined its ring system.[45]]
Voyager 2 also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail brought on by Uranus' sideways orientation.[ It made the first detailed investigations of its five largest moons, and studied all nine of the system's known rings, discovering two new ones.]
See also
★ Uranus in fiction
★ Uranus in astrology
★ Colonization of the outer Solar System
References
1. Uranus Fact Sheet
2. NASA: Solar System Exploration: Planets: Uranus: Facts & Figures
3. Uranus Fact Sheet
4. MIRA's Field Trips to the Stars Internet Education Program
5. Uranus—About Saying, Finding, and Describing It
6. Account of a Comet, By Mr. Herschel, F. R. S.; Communicated by Dr. Watson, Jun. of Bath, F. R. S., William Herschel, , , Philosophical Transactions of the Royal Society of London,
7. Journal of the Royal Society and Royal Astronomical Society 1, 30, quoted in Ellis D. Miner, Uranus: The Planet, Rings and Satellites, New York, John Wiley and Sons, 1998 p. 8
8. Royal Astronomical Society MSS W.2/1.2, 23; quoted in Miner p. 8
9. RAS MSS Herschel W.2/1.2, 24, quoted in Miner p. 8
10. Journal of the Royal Society and Royal Astronomical Society 1, 30; quoted in Miner p. 8
11. RAS MSS Herschel W1/13.M, 14 quoted in Miner p. 8
12. Johann Elert Bode, Berliner Astronomisches Jahrbuch, p. 210, 1781, quoted in Miner p. 11
13. Miner p. 11
14. Miner p. 12
15. RAS MSS Herschel W.1/12.M, 20, quoted in Miner p. 12
16. Voyager at Uranus, , , , NASA JPL, 1986
17.
18. Astronomy in Berlin
19. Query Results from the ADS Database
20. Opposition des Uranus 1821, Friedrich Magnus Schwerd, , , Astronomische Nachrichten,
21. Science in Flux Mark D Bowles
22. Pocket Oxford Latin Dictionary, , , , Oxford University Press, 2005,
23. Planet symbols
24. Sailormoon Terms and Information
25. Asian Astronomy 101, , , , Hamilton Amateur Astronomers, 1997
26. Next Stop Uranus
27. Mathematical discovery of planets J J O'Connor and E F Robertson
28. Uranus Peter J. Gierasch and Philip D. Nicholson
29. Hubble captures rare, fleeting shadow on Uranus Lawrence Sromovsky
30. Uranus, Jay T.Bergstralh, Ellis Miner, Mildred Matthews, , , , 1991,
31. Report of the IAU/IAG working group on cartographic coordinates and rotational elements of the planets and satellites: 2000
32. Cartographic Standards
33. Coordinate Frames Used in MASL
34. Observing the green giant, , Patrick, Moore, Sky at Night Magazine, 2006
35. Uranus: the Threshold Planet of 2006 Gary T. Nowak
36. The rings of Uranus J. L. Elliot, E. Dunham & D. Mink
37. NASA's Hubble Discovers New Rings and Moons Around Uranus
38.
39. Blue ring of Uranus linked to sparkling ice Stephen Battersby
40.
41. Did William Herschel Discover The Rings Of Uranus In The 18th Century?
42. Convective-region geometry as the cause of Uranus’ and Neptune’s unusual magnetic fields, , Sabine, Stanley, Letters to Nature, 2004
43. Voyager: Uranus: Magnetosphere
44. Keck zooms in on the weird weather of Uranus
45. Voyager: The Interstellar Mission: Uranus
External links
★ Edge On! ESO Press Release
★ NASA's Uranus fact sheet
★ Uranus Profile by NASA's Solar System Exploration
★ Keck pictures of Uranus show best view from the ground—Press release with some photographs showing rings, satellites and clouds
★ News reports of December 22 2005 rings and moons discovery
★
★ ''New Moons and Rings found at Uranus'', SPACE.com
★
★ ''Two more rings discovered around Uranus'', MSNBC
★ Planets—Uranus A kid's guide to Uranus.
★ Uranus at Jet Propulsion Laboratory's planetary photojournal.
★ Spring Has Sprung on Uranus
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