Uranus
Image taken by the Voyager 2 spacecraft Uranus, as seen by Voyager 2 | |||||||||||||||||||
Discovery | |||||||||||||||||||
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Discovered by | William Herschel | ||||||||||||||||||
Discovery date | March 13, 1781 | ||||||||||||||||||
Designations | |||||||||||||||||||
Adjectives | Uranian | ||||||||||||||||||
Symbol | ![]() | ||||||||||||||||||
Orbital characteristics[1][2] | |||||||||||||||||||
Epoch J2000 | |||||||||||||||||||
Aphelion | 3,004,419,704 km 20.083305 26 AU | ||||||||||||||||||
Perihelion | 2,748,938,461 km 18.37551863 AU | ||||||||||||||||||
2,876,679,082 km 19.22941195 AU | |||||||||||||||||||
Eccentricity | 0.044405586 | ||||||||||||||||||
369.66 days[3] | |||||||||||||||||||
Average orbital speed | 6.81 km/s[3] | ||||||||||||||||||
142.955717° | |||||||||||||||||||
Inclination | 0.772556° | ||||||||||||||||||
73.989821° | |||||||||||||||||||
Known satellites | 27 | ||||||||||||||||||
Physical characteristics | |||||||||||||||||||
Equatorial radius | 25,559 ± 4 km (4.007 Earths)[4][5] | ||||||||||||||||||
Polar radius | 24,973 ± 20 km (3.929 Earths)[4][5] | ||||||||||||||||||
8.1156×109 km²[6][5] (15.91 Earths) | |||||||||||||||||||
Volume | 6.833×1013 km³[3][5] (63.086 Earths) | ||||||||||||||||||
Mass | (8.6810 ± 13)×1025 kg (GM=5793939 ± 13 km³/s²) (14.536 Earths)[7] | ||||||||||||||||||
Mean density | 1.290 g/cm³[7][5] | ||||||||||||||||||
8.69 m/s²[3][5] (0.886 g) | |||||||||||||||||||
21.3 km/s[3][5] | |||||||||||||||||||
−0.718 33 day (17 h 14 min 24 s)[4] | |||||||||||||||||||
Equatorial rotation velocity | 2.59 km/s = 9320 km/h | ||||||||||||||||||
97.77°[4] | |||||||||||||||||||
North pole right ascension | 17 h 9 min 15 s (257.311°)[4] | ||||||||||||||||||
North pole declination | −15.175°[4] | ||||||||||||||||||
Albedo | 0.300 (bond) 0.51 (geom.)[3] | ||||||||||||||||||
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Atmosphere[3] | |||||||||||||||||||
27.7 km | |||||||||||||||||||
Composition by volume | [10]
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Uranus (/jʊˈreɪ.nəs, ˈjʊr.ə.nəs/) 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, [[wiktionary:οὐρανός|Οὐρανός]]), 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 recognized as a planet by ancient observers due to its dimness and its slow motion across the sky. 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 using a telescope. Uranus and Neptune have a different internal composition than gas giants — Jupiter and Saturn, and astronomers employ a new category, "ice giants", to describe them.
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system is unique in the solar system 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 without the cloud bands or storms associated with the other giants. However, ground-based observers have seen signs of weather activity in recent years as Uranus approaches its equinox.
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,[12] including on four consecutive nights.
Sir William Herschel discovered the planet on March 13, 1781, but initially reported it (on April 26, 1781) as a "comet".[13] Herschel "engaged in a series of observations on the parallax of the fixed stars",[14] 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".[15] 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".[16] 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:[17]
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 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".[18]
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.[19] 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".[20] Bode concluded that its near-circular orbit was more like a planet than a comet.[21]
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."[22] 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.[23]
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".[24]
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.[25] He explained this decision in a letter to Joseph Banks:[22]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/36/William_Herschel01.jpg/150px-William_Herschel01.jpg)
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.'
Astronomer Jérôme Lalande proposed the planet be named Herschel in honour of its discoverer.[26] 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.[23][27][28] The earliest citation of the name Uranus in an official publication is in 1823, a year after Herschel's death.[29][30] The name Georgium Sidus or "the Georgian" was still used infrequently (by the British alone) thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.[27]
The adjective of Uranus is "Uranian". The element uranium, discovered in 1789, was named in its honour by its discoverer, Martin Klaproth.[31] The stressed syllable in the name Uranus is properly the first, because the penultimate vowel a is short (ūrănŭs) and in an open syllable. Such syllables are never stressed in Latin.[32] The historically correct pronunciation of the name by English speakers is therefore [ˈjʊ.rə.nəs]. The historically incorrect pronunciation, [jʊˈɹeɪ.nəs], with stress on the second syllable and a "long a" (ūrānŭs) has become very common.
Uranus' 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.[33] 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").[26]
In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).[34][35]
Orbit and rotation
![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Uranusandrings.jpg/200px-Uranusandrings.jpg)
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.[36] Its orbital elements were calculated in 1783 by Pierre-Simon Laplace and its orbit was calculated in 1787 by Placidus Fixlmillner.[19][37] Discrepancies began to appear between the predicted and observed orbits. 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' orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.[38]
Uranus' rotational period is roughly 17 hours, 14 minutes. Like all giant planets, its core spins on average more slowly than its upper atmosphere. The fastest region of Uranus' atmosphere makes one rotation in only 14 hours.[39]
Axial tilt
Uranus lies on its side with an axial tilt of 98 degrees. Other planets rotate like tilted spinning tops relative to the plane of the solar system, but Uranus rotates like a tilted rolling bowling ball. Near the time of Uranian solstices, one pole faces the Sun continually while the other pole faces away. At the other side of Uranus' orbit the orientation of the poles towards the Sun is reversed. This gives each pole 42 years of continuous sunlight, followed by 42 years of darkness.[40] Near the time of the equinoxes, the Sun faces the equator of Uranus just as it does for the other planets. Uranus will reach its next equinox around December 2007.[41]
Season, Northern hemisphere | Year | Season, 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 |
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).[42][43] A different system is sometimes used, defining a body's north and south poles according to the right-hand rule in relation to the direction of rotation.[44] 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, sums it up by saying "Take your pick!"[45]
One result of this orientation is that the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles, although the underlying mechanism which causes this is unknown. The reason for Uranus' extreme axial tilt is also not known. It is speculated that during the formation of the Solar System, an Earth sized protoplanet collided with Uranus, causing the skewed orientation.[46]
Physical characteristics
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.[47] 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.[47] At opposition, Uranus is visible to the naked eye in dark, un-light polluted skies, and becomes an easy target even in urban conditions with binoculars.[6] In larger amateur telescopes with an objective diameter of between 15 and 22.5 cm (6 and 9 inches), the planet appears as a pale cyan disk with distinct limb darkening. With a large telescope of 25 cm (10 inches) or wider, cloud patterns, as well as some of the larger satellites, such as Titania and Oberon, may be visible.[48]
Structure and composition
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Uranus%2C_Earth_size_comparison.jpg/150px-Uranus%2C_Earth_size_comparison.jpg)
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.[7] Though of a similar diameter to Neptune (roughly four times Earth's), it is less massive.[4]
Uranus' density and mass suggest that it is made primarily of various ices, such as water, ammonia, and methane.[8] The total mass of ice in Uranus' interior is not precisely known, as different figures emerge depending on on the model chosen; however, it must be between 9.3 and 13.5 Earth masses.[8][49] Hydrogen and helium constitute a small part of the total, with between 0.5 and 1.5 Earth masses.[8] The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by rocky material.[8] The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, hence justifying their separate classification, ice giants.
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 outer gaseous hydrogen/helium envelope.[8][50] 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.[8][50] 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.[49][50] 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.[8][50]This fluid, which has a high electical conductivity, is sometimes called water–ammonia ocean.[51]
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 there is no way to determine which model is correct.[49]
The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.[8] 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.[4] This surface will be used throughout this article as a zero point for altitudes.
Atmosphere
The outermost part of Uranus' gaseous envelope, which is accessible to remote sensing, is called its atmosphere.[9] Its lower boundary is usually placed at roughly 300 km below the surface, with corresponding pressure around 100 bar and temperature of 320 K.[52] The atmosphere's upper boundary is not well defined, as gases are thought to extend tenuously to more than two planetary radii from the surface.[53]
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.[9] The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium, was determined from the analysis of Voyager 2 far infrared and radio occultation observations.[54] The currently accepted value is 0.15 ± 0.03, which corresponds to the mass fraction 0.26 ± 0.05.[9][55] This value is very close to the solar helium mass fraction of 0.275 ± 0.010, indicating that helium has not settled in the centre of the planet as it has in the gas giants.[9] The deuterium abundance ratio relative to light hydrogen 5.5+3.5−1.5×10−5 was measured in the 1990s by the Infrared Space Observatory (ISO), and appears to be higher than the protosolar value of 2.25 ± 0.35×10−5 measured in Jupiter.[11][56]
The third most abundant constituent of the Uranian atmosphere is methane (CH4), the presence of which has been known for some time as a result of the ground-based spectroscopic observations.[9] Methane possesses prominent absorption bands in the visible and near-infrared making Uranus aquamarine or cyan in color.[9] There are about 0.02 methane molecules per hydrogen molecule (the so called mixing ratio) below the pressure level of 1.3 bar; about 20 to 30 times that found in the Sun.[57][54][9] The mixing ratio is much lower in the upper atmosphere due to its extremely low temperature.[58] 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.[9][59]
Infrared spectroscopy, including measurements with Spitzer Space Telescope (SST),[60] and UV occultation observations,[58] found trace amounts of various hydrocarbons in the upper atmosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar UV radiation.[61] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), diacetylene (C2HC2H).[60][56] Infrared 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.[60][62][56]
Structure
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Tropospheric_profile_Uranus.png/300px-Tropospheric_profile_Uranus.png)
The Uranian atmosphere is divided into three layers: the troposphere, with altitudes rising from −300 to 50 km and pressures from 100 to 0.1 bar, the stratosphere, spanning altitudes of 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.[9]
The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.[9] The temperature falls from about 320 K at the base of troposphere at −300 km to 53 K at 50 km.[52][54] The temperatures in the upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on latitude. The lowest temperature, 49 K, is reached near 25° southern latitude.[9][64] The tropopause region is responsible for vast majority of the planet’s thermal far infrared emissions, thus defining its effective temperature of 59.1 ± 0.3 K.[64][55]
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 thin methane clouds situated at 1 to 2 bar.[9][52][63] Although Voyager 2 directly detected methane clouds via a radio occultation experiment,[57] all other cloud layers remain speculative. The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes, which will be discussed below.[65]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Uranian_stratosphere.png/200px-Uranian_stratosphere.png)
The middle layer of the Uranian atmosphere is 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.[53] The heating of the stratosphere is caused by absorption of the solar UV and IR radiation by methane and other hydrocarbons, which form in this part of the atmosphere as a result of the methane photolysis,[58][61] although additional heating from the hot thermosphere may be necessary.[66][67] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 280 km corresponding to the pressure range of 10 to 0.1 mbar and temperatures of between 75 and 170 K.[58] The most abundant hydrocarbons are acetylene and ethane with the mixing ratios of around ×10−7 relative to hydrogen, which is similar to the mixing ratios of methane and carbon monoxide at these altitudes.[62][58][60] Heavier hydrocarbons, carbon dioxide and water vapour have mixing ratios three orders of magnitude lower.[60] The stratosphere exhibits strong temporal and meridional variations of temperature and hydrocarbon mixing ratios; the poles are both poorer in hydrocarbons and cooler than the rest of the stratosphere.[58][61][67][66] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause forming haze layers,[61] which may be partly responsible for the bland appearance of Uranus. The amount of hydrocarbons in the Uranian stratosphere is significantly lower than in other giant planets, which in addition to weak vertical mixing makes it cleaner and less opaque and as a result colder than stratospheres of other giant planets.[58][66]
The outmost layer of the Uranian atmosphere is thermosphere/corona, which has a constant temperature around 800 to 850 K,[9][66] higher than 420 K in the thermosphere of Saturn.[68] The heat sources necessary to sustain such a high value are a problem, 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 contribute too.[53][66] Helium is thought to be absent here, because it diffusivelly separated at lower altitudes.[53] In addition to molecular hydrogen, the thermosphere-corona contains a large proportion of hydrogen atoms, which, together with high temperatures, may help to explain why the corona extends as far as 50,000 km or two Uranian radii from the planet.[53][66] Such an extended corona creates a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.[53] The extended corona is a unique feature of Uranus.[66] The hot thermosphere of Uranus produces hydrogen quadrupole emissions in the near-infrared.[69]
The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.[54] The main sources of information about it, except Voyager 2 measurements, are infrared emissions of H3+ ion detected from the ground.[69] The observations show that ionosphere occupies altitudes from 2,000 to 10,000 km.[54] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may be explained by the low mixing ratio of hydrocarbons in the stratosphere.[66][69] The ionosphere is mainly sustained by the solar UV radiation and its density depends on the solar activity.[70] The auroral activity is not as significant as in Jupiter and Saturn.[66][71] The ionosphere–thermosphere region is the source of UV emission from Uranus known as ‘dayglow’ or ‘electroglow’, which like H3+ IR radiation emanates exclusively from the sunlit part of the planet. This phenomenon, which occurs in thermospheres of all giant planets, was mysterious for some time, but now it is interpreted as a solar excited UV fluorescence of atomic and molecular hydrogen with a possible contribution from photoelectrons.[66]
Planetary rings
Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter.[72] It was the first ring system to be discovered in the Solar System after Saturn's.[73] The first mention of Uranus' ring system comes from William Herschel's notes detailing his observations of Uranus. These include the following passage: "February 22, 1789: A ring was suspected".[74] 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.[75] Herschel's notes were published in a Royal Society journal in 1797. Between 1797 and 1977 the rings are rarely mentioned, if at all; this has led some to conclude that Herschel erred in claiming to have found the ring. However, Dr Stuart Eves, of Surrey Satellite Technology Limited, claims 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. He theorises that the rings were missed for so long because over the course of the intervening two centuries they have darkened and expanded outward.[76] The ring system was rediscovered 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;[77] it was directly imaged when Voyager 2 passed Uranus in 1986.[72] Voyager 2 also discovered two addtional faint rings bringing their total number to eleven.[72]
In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located at twice the distance from the planet than the previously known rings. The new rings are so far from the planet that they are being called Uranus' "outer" ring system. Hubble also spotted two small satellites. One, Mab, shares its orbit with the outermost newly discovered ring. These two rings bring the total number of Uranus rings to 13.[78] 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.[75][79] One hypothesis concerning the outer ring's 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 visible blue light.[75][80] The planet's inner rings appear grey.[75]
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 rings may once have been a moon which was shattered by a high-speed impact or tidal forces.[73][81]
Magnetic field
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Uranian_Magnetic_field.gif/300px-Uranian_Magnetic_field.gif)
Prior to the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Voyager's observations revealed that magnetic field of Uranus 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.[82][83] 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.[82] This unusual geometry results in a highly asymmetric magnetosphere, where magnetic field in the southern hemisphere can be as low as 0.1 Gauss, where as magnetic field in the northern hemisphere can be as high 1.1 Gauss.[82] The average field at the surface is 0.23 Gauss.[82] 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.[83] The dipole moment of Uranus is 50 times that of Earth.[82][83] Neptune has a similarly displaced and tilted magnetic field, suggesting that such percularity may be a common feature of ice giants.[83] One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giant planets, which are generated deep within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.[51][84]
Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, since it would then line up with the planet's horizontal poles.[82] However, due to its lopsided nature, the field in fact lay in a position similar to those of the terrestrial planets at the time of Voyager flyby.[83] Despite its curious position, in all other respects Uranian magnetosphere is no different from 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.[82][83][85] The structure of the magnetosphere of Uranus is different from that of Jupiter's and more similar to that of Saturn's.[82][83] Uranus' magnetotail trails behind the planet into space for millions of miles and is twisted by the planet's sideways rotation into a long corkscrew.[82][86]
Uranus' magnetosphere is filled with charged particles: protons and electrons with small amount of H2+ ions.[83][85] No heavier ions have been detected. Many of these particles probably derive from the hot corona.[85] The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.[85] The density of low energy (< 100 electronvolts) ions in the inner magnitosphere is about 2 cm-3.[87] The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.[85] The particle flux is high enough to cause darkening of the moon’s surfaces at the timescale of 100,000 years.[85] This may have been responsible for the uniformly dark colouration of the moons and rings.[81] Uranus has relatively well developed aurorae observed as bright arcs around both magnetic poles.[66] However, unlike Jupiter's, Uranus' aurore seem to be insignificant in the energy balance of the planetary thermosphere.[71]
Climate
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Uranuscolour.png/300px-Uranuscolour.png)
Uranus' atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles.[65] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[72][88]
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.[64][65] 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.[65] 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.[55][9] 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.[55] The lowest temperature recorded in Uranus' upper atmosphere is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.[55][9]
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.[8] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.[9][55]
Banded structure, winds and clouds
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/16/Uranian_wind_speeds.png/200px-Uranian_wind_speeds.png)
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.[72] 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.[72][90] 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).[91] In the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south pole cap darkened noticeably, while the southern collar remained bright.[91] At the beginning of the twenty-first century, when the northern polar region came into view, neither a collar nor a polar cap were observered in that hemisphere.[90] So Uranus appears to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[90] Details of possible seasonal variations in the planet's appearance will be discussed later. The visible latitudinal structure of Uranus is very different from that of Jupiter and Saturn, which demonstrate multiple narrow and colorful bands.[65]
In addition to large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[72] 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.[65] The majority of them was found in the northern hemisphere as it started to become visible.[65] 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.[92] Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.[93] 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,[93] 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.[88][65] 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.[65] The dark spots common on Neptune had never been observed on Uranus before 2005, when the first such feature was imaged.[94]
The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.[65] 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.[65][90] Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.[65][64] 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.[65] 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.[65] In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.[65][90] 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.[65] This is especially true at the midlatitudes from ±20 to ±40 degrees. There is actually no agreement about whether any changes in windspeed have occurred since 1986.[89][90][65] Currently nothing is known about much slower meridional winds.[65]
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.[93][95] Observations included record-breaking wind speeds of 824 km/h (512 mph) and a persistent thunderstorm referred to as "Fourth of July fireworks".[88] 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.[94] 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.[41][96]
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 maximum occuring at the solstices and minimum occuring at the equinoxes.[97] A similar periodic variation, with maximum at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[98] Stratospheric temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.[67]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/fe/Uranus_Seasonal_variability.png/200px-Uranus_Seasonal_variability.png)
The majority of this variablilty is due 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.[97] Uranus is also known to exhibit strong meridional variations in albedo.[92] For instance, Voyager 2 found that, for wavelengths longer than 0.5 micrometres, the south polar region of Uranus is much brighter then the equatorial bands. Unfortunately Voyager arrived during during the height of the planet's southern summer and could not observe the northern hemisphere. In addition, the microwave mapping of Uranus showed that both south and north poles are brighter than equator,[99] while it is known that the polar stratosphere is cooler than the equatorial one.[67] Now it appears that brighter poles come in to the view at solstices resulting in the brighter planet,[92] while occultations in the same time 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, observations by Hubble Space Telescope (HST) and Keck telescopes since 2000 showed that the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.[96] During its previous northern solstice in 1945, Uranus displayed elevated levels of brightness, which suggests it was not always so dim.[97] This information suggests 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.[96] In addition some microwave data show increases in pole–equator contrast after the 1986 solstice.[99] Finally the same HST observations found that the south pole has started to darken,[91] while the northern hemisphere demonstrates increasing activity,[88] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon (see above).[93] In particular the bright polar collar present in the southern hemisphere at −45° is expected to appear in the northern part of the planet.[96]
The mechanism of physical changes is still not clear.[96] 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.[91] The bright collar at −45° latitude is also connected with methane clouds.[91] Other changes in the southern polar region can be explained by changes in the lower cloud layers.[91] 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.[99] Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.[88][99]
Formation
Many argue that the differences between the ice giants and the gas giants extend to their formation.[100][101] 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.[100][101] 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.[100][101] 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.[101][102][100]
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.[100] However more recent simulations, which take into account planetary migration, seem to be able to form Uranus and Neptune near their present locations.[101]
Moons
![](http://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Uranian_moon_montage.jpg/400px-Uranian_moon_montage.jpg)
Uranus has 27 known natural satellites.[102] The names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope.[50][103] The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.[50]
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.[7] 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).[72] The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include ammonia and carbon dioxide.[104][81]
Among the satellites, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.[72][81] Miranda possesses fault canyons 20 kilometres (12 miles) deep, terraced layers, and a chaotic variation in surface ages and features.[72] One hypothesis holds that Miranda may have been blasted completely apart by a massive impact some time in its past, and then reformed haphazardly.[105][81]
Exploration
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/af/Uranus_Final_Image.jpg/120px-Uranus_Final_Image.jpg)
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 kilometres (50,600 miles) of the planet's cloud tops, before continuing its journey to Neptune. Voyager 2 studied structure and chemical composition of the atmosphere,[54] discovered 10 new moons and studied the planet's unique weather, caused by its axial tilt of 97.77°; and examined its ring system.[106][72]
Voyager 2 also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail brought on by Uranus' sideways orientation.[82] 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.[81][72]
See also
References
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{{cite journal}}
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ignored (|author=
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- ^ Bridge, H.S. (1986). "Plasma Observations Near Uranus: Initial Results from Voyager 2". Science. 233: 89–93.
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{{cite journal}}
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suggested) (help) - ^ a b c d e f g Hammel, H.B. (2005). "Uranus in 2003: Zonal winds, banded structure, and discrete features" (pdf). Icarus. 175: 534–545. doi:10.1016/j.icarus.2004.11.012.
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{{cite journal}}
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(help); Unknown parameter|coauthors=
ignored (|author=
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ignored (help) - ^ a b Sromovsky, L. "Hubble Discovers a Dark Cloud in the Atmosphere of Uranus" (pdf). physorg.com. Retrieved 2007-08-22.
{{cite web}}
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ignored (|author=
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ignored (|author=
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ignored (|author=
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{{cite journal}}
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ignored (|author=
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ignored (|author=
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ignored (|author=
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ignored (|author=
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ignored (|author=
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{{cite journal}}
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ignored (|author=
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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
- Planets—Uranus A kid's guide to Uranus.
- Uranus at Jet Propulsion Laboratory's planetary photojournal.
- Spring Has Sprung on Uranus