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Uranus  

Uranus, as seen by Voyager 2
Discovery
Discovered by	William Herschel
Discovery date	March 13, 1781
Orbital characteristics[1][2]
Epoch J2000
Aphelion	3,004,419,704 km 20.08330526 AU
Perihelion	2,748,938,461 km 18.37551863 AU
Semi-major axis	2,876,679,082 km 19.22941195 AU
Eccentricity	0.044405586
Synodic period	369.66 days[3]
Average orbital speed	6.81 km/s[3]
Mean anomaly	142.955717°
Inclination	0.772556° 6.48° to Sun's equator
Longitude of ascending node	73.989821°
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]
Surface area	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=5,793,939 ± 13 km³/s² 14.536 Earths[7]
Mean density	1.290 g/cm³[7][5]
Equatorial surface gravity	8.69 m/s²[3][5] 0.886 g
Escape velocity	21.3 km/s[3][5]
Sidereal rotation period	−0.71833 day 17 h 14 min 24 s[4]
Equatorial rotation velocity	2.59 km/s 9,320 km/h
Axial tilt	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]
Surface temp.    1 bar level    0.1 bar (tropopause)	min mean max 76 K[8] 49 K[9] 53 K[9] 57 K[9]
Adjectives	Uranian
Atmosphere[9][10][11][12]
Scale height	27.7 km [3]
Composition	(Below 1.3 bar) 83±3% Hydrogen (H2) 15±3% Helium 2.3% Methane 0.009% (0.007-0.015%) Hydrogen deuteride (HD)[13] Ices: Ammonia water ammonium hydrosulfide (NH4SH) methane (CH4)

Uranus (IPA: /jʊˈreɪnəs, ˈjʊərənəs/)^[14] is the seventh planet from the Sun, the third largest and fourth most massive planet in the solar system. It is named after the ancient Greek deity of the sky (Uranus, Οὐρανός), the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Uranus was the first planet discovered in modern times. Though it is visible to the naked eye like the five classical planets, it was never recognised as a planet by ancient observers due to its dimness.^[15] 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. This was also the first discovery of a planet made using a telescope.

Uranus and Neptune have different internal and atmospheric compositions from those of the larger gas giants Jupiter and Saturn. As such, 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 is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K, and has a complex layered cloud structure, in which water is thought to make up the lowest clouds, while methane makes up the uppermost layer of clouds.^[9]

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 most other planets have their equators.^[16] 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.^[16] However, ground-based observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approaches its equinox. The wind speeds on Uranus can reach 250 m/s.^[17]

Contents 1 Discovery 1.1 Naming 2 Orbit and rotation 2.1 Axial tilt 2.2 Visibility 3 Physical characteristics 3.1 Structure and composition 3.2 Atmosphere 3.3 Clouds 3.4 Planetary rings 3.5 Magnetic field 4 Formation 5 Moons 6 Exploration 7 See also 8 References

[edit] 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 and observed it at least six times. The French astronomer, Pierre Lemonnier, observed Uranus at least twelve times between 1750 and 1769,^[18] 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".^[19] Herschel "engaged in a series of observations on the parallax of the fixed stars",^[20] 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".^[21] 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".^[22] 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:^[23]

“

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".^[24]

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.^[25] 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".^[26] Bode concluded that its near-circular orbit was more like a planet than a comet.^[27]

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."^[28] 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.^[29]

[edit] 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".^[30]

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.^[31] He explained this decision in a letter to Joseph Banks:^[28]

“

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.^[32] 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.^[29][33][34] The earliest citation of the name Uranus in an official publication is in 1823, a year after Herschel's death.^[35][36] 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.^[33]

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.^[37] 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.^[38] 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.^[39] 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").^[32]

In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).^[40][41]

[edit] Orbit and rotation

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.^[42] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.^[25] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus' orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.^[43]

The rotational period of the interior of Uranus is 17 hours, 14 minutes. However, as on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. 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.^[44]

[edit] 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 visualized 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.^[45] 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.^[46]

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.^[47]

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).^[48][49] 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.^[50] 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!"^[51]

[edit] 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.^[52] 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.^[52] 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 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.^[53]

[edit] Physical characteristics

[edit] Structure and composition

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]

These values indicate 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 the model chosen; however, it must be between 9.3 and 13.5 Earth masses.^[8][54] Hydrogen and helium constitute only 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 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.^[8][55] 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][55] 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.^[54][55] 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][55] This fluid, which has a high electical conductivity, is sometimes called water–ammonia ocean.^[56] 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.^[54]

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.

[edit] Atmosphere

Main article: 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.^[9] 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.^[57] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.^[58] 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.^[9] There is no mesosphere.

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 is 0.15 ± 0.03^[11] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.^[9][59] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,^[60] indicating that helium has not settled in the centre of the planet as it has in the gas giants.^[9]

The third most abundant constituent of the Uranian atmosphere is methane (CH₄).^[9] Methane possesses prominent absorption bands in the visible and near-infrared making Uranus aquamarine or cyan in color.^[9] 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.^[9][10][61] 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.^[62] 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][63]

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.^[64] They include ethane (C₂H₆), acetylene (C₂H₂), methylacetylene (CH₃C₂H), diacetylene (C₂HC₂H).^[62][65][66] 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.^[66][65][67]

[edit] Clouds

Uranus' atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles.^[17] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.^[16][68]

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.^[69][17] 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.^[17] 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.^[59][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.^[59] The lowest temperature recorded in Uranus' tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.^[59][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][59]

For a short period in Autumn 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.^[70][71] Observations included record-breaking wind speeds of 824 km/h and a persistent thunderstorm referred to as "Fourth of July fireworks".^[68] 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.^[72] 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.^[46][73]

The mechanism of physical changes is still not clear.^[73] 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.^[74] The bright collar at −45° latitude is also connected with methane clouds.^[74] Other changes in the southern polar region can be explained by changes in the lower cloud layers.^[74] 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.^[75] Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.^[68][75]

[edit] Planetary rings

Main article: Rings of Uranus

Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter.^[16] It was the next ring system to be discovered in the Solar System after Saturn's.^[76] 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.^[77] The rings were directly imaged when Voyager 2 passed Uranus in 1986.^[16] Voyager 2 also discovered two addtional faint rings bringing the total number to eleven.^[16]

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.^[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.^[79][80] 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.^[79][81] The planet's inner rings appear grey.^[79]

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.^[76][82]

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".^[83] 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.^[79] 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.^[84]

[edit] Magnetic field

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.^[85]

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.^[85][86] 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.^[85] 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.^[85] The average field at the surface is 0.23 Gauss.^[85] 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.^[86] The dipole moment of Uranus is 50 times that of Earth.^[85][86] Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.^[86] 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.^[56][87]

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.^[85][86][88] Overall, the structure of the magnetosphere of Uranus is different from that of Jupiter's and more similar to that of Saturn's.^[85][86] 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.^[85][89]

Uranus' magnetosphere contains charged particles: protons and electrons with small amount of H₂⁺ ions.^[86][88] No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona.^[88] The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.^[88] The density of low energy (< 100 electronvolts) ions in the inner magnetosphere is about 2 cm^-3.^[90] The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.^[88] 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.^[88] This may be the cause of the uniformly dark colouration of the moons and rings.^[82] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.^[91] However, unlike Jupiter's, Uranus' aurore seem to be insignificant for the energy balance of the planetary thermosphere.^[92]

[edit] Formation

See also: Nebular hypothesis

Many argue that the differences between the ice giants and the gas giants extend to their formation.^[93][94] 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.^[93][94] 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.^[93][94] 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.^[94][95][93]

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.^[93] However more recent simulations, which take into account planetary migration, seem to be able to form Uranus and Neptune near their present locations.^[94]

[edit] Moons

Main article: Uranus' natural satellites

See also: Timeline of discovery of Solar System planets and their natural satellites

Uranus has 27 known natural satellites.^[95] The names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope.^[55][96] The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.^[55]

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).^[16] The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include ammonia and carbon dioxide.^[97][82]

Among the satellites, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.^[16][82] Miranda possesses fault canyons 20 kilometers deep, terraced layers, and a chaotic variation in surface ages and features.^[16] One hypothesis holds that Miranda may have been blasted completely apart by a massive impact some time in its past, and then reformed haphazardly.^[98][82]

[edit] Exploration

Main article: 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,^[61] discovered 10 new moons and studied the planet's unique weather, caused by its axial tilt of 97.77°; and examined its ring system.^[99][16]

Voyager 2 also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail brought on by Uranus' sideways orientation.^[85] 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.^[82][16]

[edit] See also

• Uranus in fiction
• Uranus in astrology
• Colonization of the outer Solar System

[edit] References