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| + | '''Uranus''' is the seventh [[planet]] from the sun. |
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| − | Please note the formatting and layout of this infobox has been matched with the other bodies of the Solar System. Please do not arbitrarily change it without discussion.
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| − | Additional parameters for this template are available at [[Template:Infobox Planet]].
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| − | {{Infobox Planet
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| − | | bgcolour = #c0ffff
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| − | | name = Uranus
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| − | | symbol = [[Image:Uranus symbol.svg|25px|Astronomical symbol of Uranus]]
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| − | | image = [[Image:Uranus Voyager 2.jpg|240px|Image taken by the ''[[Voyager 2]]'' spacecraft]]
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| − | | caption = Uranus, as seen by [[Voyager 2]]
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| − | | discovery = yes
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| − | | discoverer = [[William Herschel]]
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| − | | discovered = [[March 13]], [[1781]]
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| − | | orbit_ref =
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| − | <ref name=horizons>{{cite web
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| − | | last = Yeomans | first = Donald K. |date = July 13, 2006
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| − | | url = http://ssd.jpl.nasa.gov/?horizons
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| − | | title = HORIZONS System | publisher = NASA JPL
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| − | | accessdate = 2007-08-08 }} — At the site, go to the "web interface" then select "Ephemeris Type: ELEMENTS", "Target Body: Uranus Barycenter" and "Center: Sun".</ref>{{Ref_label|A|a|none}}
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| − | | epoch = [[J2000]]
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| − | | aphelion = 3,004,419,704 [[kilometer|km]]<br />20.08330526 [[Astronomical unit|AU]]
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| − | | perihelion = 2,748,938,461 km<br />18.37551863 AU
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| − | | semimajor = 2,876,679,082 km<br />19.22941195 AU
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| − | | eccentricity = 0.044405586
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| − | | period = 30,799.095 [[day]]s<br />84.323326 [[julian year (astronomy)|yr]]
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| − | | synodic_period = 369.66 days<ref name="fact">{{cite web|url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html|title = Uranus Fact Sheet|publisher = NASA|last = Williams|first = Dr. David R.|accessdate = 2007-08-10|date = January 31, 2005}}</ref>
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| − | | avg_speed = 6.81 km/s<ref name="fact"/>
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| − | | inclination = 0.772556°<br />6.48° to [[Sun]]'s equator
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| − | | asc_node = 73.989821°
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| − | | arg_peri = 96.541318°
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| − | | mean_anomaly = 142.955717°
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| − | | satellites = [[Uranus' natural satellites|27]]
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| − | | physical_characteristics = yes
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| − | | flattening = 0.0229 ± 0.0008{{Ref_label|B|b|none}}
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| − | | equatorial_radius = 25,559 ± 4 km<br />4.007 Earths<ref name=Seidelmann2007/>{{Ref_label|C|c|none}}
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| − | | polar_radius = 24,973 ± 20 km<br />3.929 Earths<ref name=Seidelmann2007/>{{Ref_label|C|c|none}}
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| − | | surface_area = 8.1156{{e|9}} km²<ref name="nasafact">{{cite web|url = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus&Display=Facts|title = NASA: Solar System Exploration: Planets: Uranus: Facts & Figures|publisher = NASA|last = Munsell|first = Kirk|accessdate = 2007-08-13|date = May 14, 2007}}</ref>{{Ref_label|C|c|none}}<br />15.91 Earths
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| − | | volume = 6.833{{e|13}} km³<ref name="fact">{{cite web|url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html|title = Uranus Fact Sheet|publisher = NASA|last = Williams|first = Dr. David R.|accessdate = 2007-08-13|date = January 31, 2005}}</ref>{{Ref_label|C|c|none}}<br />63.086 Earths
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| − | | mass = 8.6810 ± 13{{e|25}} [[kilogram|kg]]<br />14.536 Earths<ref name=Jacobson1992/><br/>
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| − | [[standard gravitational parameter|GM]]=5,793,939 ± 13 km³/s²
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| − | | density = 1.27 g/cm³<ref name=fact/>{{Ref_label|C|c|none}}
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| − | | surface_grav = 8.69 [[Acceleration|m/s²]]<ref name=fact/><{{Ref_label|C|c|none}}<br />0.886 [[g-force|g]]
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| − | | escape_velocity = 21.3 km/s<ref name=fact/>{{Ref_label|C|c|none}}
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| − | | sidereal_day = [[retrograde motion|−]]0.71833 day<br/>{{nowrap|17 [[hour|h]] 14 [[minute|min]] 24 [[second|s]]}}<ref name=Seidelmann2007/>
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| − | | rot_velocity = 2.59 km/s<br />9,320 km/h
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| − | | axial_tilt = 97.77°<ref name=Seidelmann2007/>
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| − | | right_asc_north_pole = {{nowrap|17 h 9 min 15 s}}<br />257.311°<ref name=Seidelmann2007/>
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| − | | declination = −15.175°<ref name=Seidelmann2007/>
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| − | | albedo = 0.300 ([[Bond albedo|bond]])<br/>
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| − | 0.51 ([[Geometric albedo|geom.]])<ref name="fact"/>
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| − | | magnitude = 5.9<ref name=ephemeris/> to 5.32<ref name="fact"/>
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| − | | angular_size = 3.3"–4.1"<ref name="fact"/>
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| − | | temperatures = yes
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| − | | temp_name1 = 1 [[bar (unit)|bar]] level<ref name=Podolak1995/>
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| − | | min_temp_1 =
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| − | | mean_temp_1 = 76 [[kelvin|K]]
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| − | | max_temp_1 =
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| − | | temp_name2 = 0.1 bar <br/>([[tropopause]])<ref name=Lunine1993/>
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| − | | min_temp_2 = 49 K
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| − | | mean_temp_2 = 53 K
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| − | | max_temp_2 = 57 K
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| − | | adjectives = Uranian
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| − | | atmosphere = yes
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| − | | atmosphere_ref =
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| − | <ref name=Lunine1993/><ref name=Lindal1987/><ref name=Conrath1987/>{{Ref_label|D|d|none}}
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| − | | scale_height = 27.7 km<ref name="fact"/>
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| − | | atmosphere_composition = ''(Below 1.3 bar)''
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| − | <table>
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| − | <tr><td>
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| − | 83 ± 3%</td><td>[[Hydrogen]] (H<sub>2</sub>)
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| − | </td></tr><tr><td>
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| − | 15 ± 3%</td><td>[[Helium]]
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| − | </td></tr><tr><td>
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| − | 2.3%</td><td>[[Methane]]</td></tr><tr><td>
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| − | 0.009%<br>(0.007–0.015%)</td><td>[[Hydrogen deuteride]] (HD)<ref name=Feuchtgruber1999>{{cite journal|last=Feuchtgruber|first=H.|coauthors=Lellouch, E.; B. Bezard; et.al.|title=Detection of HD in the atmospheres of Uranus and Neptune: a new determination of the D/H ratio|year=1999|journal=Astronomy and Astrophysics|volume=341|pages=L17–L21|url=http://adsabs.harvard.edu/abs/1999A%26A...341L..17F}}</ref>
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| − | </td></tr><tr><td>
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| − | '''Ices''':</td><td>
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| − | </td></tr><tr><td>
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| − | </td><td>[[Ammonia]]
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| − | </td></tr><tr><td>
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| − | </td><td>[[water]]
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| − | </td></tr><tr><td>
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| − | </td><td>[[ammonium hydrosulfide]] (NH<sub>4</sub>SH)
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| − | </td></tr><tr><td>
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| − | </td><td>[[methane]] (CH<sub>4</sub>)
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| − | </td></tr></table>
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| − | }}
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| − | '''Uranus''' ({{pronEng|ˈjÊŠÉ™rÉ™nÉ™s}} or {{IPA|/jʊˈreɪnÉ™s/}}<ref>{{cite encyclopedia|title=Uranus|encyclopedia=Oxford English Dictionary|
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| − | edition=Second edition|year=1989}}</ref>) is the seventh [[planet]] from the [[Sun]] and the third-largest and fourth-most massive planet in the [[solar system]]. It is named after the ancient Greek deity of the sky ([[Uranus (mythology)|Uranus]], ''[[wiktionary:οá½Ïανός|{{polytonic|Οá½Ïανός}}]]''), the father of [[Cronus|Kronos]] ([[Saturn (mythology)|Saturn]]) and grandfather of [[Zeus]] ([[Jupiter (mythology)|Jupiter]]). Uranus was the first planet discovered in [[modern times]]. Though it is visible to the naked eye like the five [[classical planet]]s, it was never recognized as a planet by ancient observers due to its dimness.<ref>{{cite web|title=MIRA's Field Trips to the Stars Internet Education Program|work=Monterey Institute for Research in Astronomy|url=http://www.mira.org/fts0/planets/101/text/txt001x.htm|accessdate=2007-08-27}}</ref> 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]].
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| − | Uranus and [[Neptune]] have internal and [[atmospheric chemistry|atmospheric composition]]s different from those of the larger [[gas giant]]s [[Jupiter (planet)|Jupiter]] and [[Saturn (planet)|Saturn]]. As such, astronomers sometimes place them in a separate category, the "[[ice giant]]s". Uranus' atmosphere, while similar to Jupiter and Saturn in being composed primarily of [[hydrogen]] and [[helium]], contains a higher proportion of "ices" such as [[water]], [[ammonia]] and [[methane]], along with the usual traces of [[hydrocarbon]]s. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 [[kelvin|K]] (−224 [[Celsius|°C]]). It has a complex, layered [[cloud]] structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.<ref name=Lunine1993/>
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| − | Like the other giant planets, Uranus has a [[ring system]], a [[magnetosphere]], and numerous [[natural satellite|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.<ref name=Smith1986/> Seen from Earth, Uranus' rings can appear to circle the planet like an [[Target archery|archery target]] and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appear edge-on. In 1986, images from ''[[Voyager 2]]'' showed Uranus as a virtually featureless planet in visible light without the cloud bands or [[storms]] associated with the other giants.<ref name=Smith1986/> However, terrestrial observers have seen signs of [[season]]al change and increased [[weather]] activity in recent years as Uranus approached its [[equinox]]. The [[wind]] speeds on Uranus can reach 250 meters per second.<ref name=Sromovsky2005/>
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| − | == Discovery==
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| − | 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 [[Taurus (constellation)|Tauri]] and observed it at least six times. The French astronomer, [[Pierre Lemonnier]], observed Uranus at least twelve times between 1750 and 1769,<ref>{{cite web | title=Uranus—About Saying, Finding, and Describing It | publisher=Astronomy Briefly |url=http://www.thespaceguy.com/Uranus.htm | last=Dunkerson |first=Duane |publisher=thespaceguy.com |accessdate=2007-04-17}}</ref> including on four consecutive nights.
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| − | Sir [[William Herschel]] observed the planet on [[13 March]] [[1781]] while in the garden of his house at 19 New King Street in the town of [[Bath, Somerset|Bath]], [[Somerset]] (now the [[Herschel Museum of Astronomy]]),<ref>{{cite web|title=Bath Preservation Trust|url=http://www.bath-preservation-trust.org.uk/|accessdate=2007-09-29}}</ref> but initially reported it (on [[26 April]] [[1781]]) as a "[[comet]]".<ref>{{cite journal|title=Account of a Comet, By Mr. Herschel, F. R. S.; Communicated by Dr. Watson, Jun. of Bath, F. R. S.|author=William Herschel|journal=Philosophical Transactions of the Royal Society of London|Volume=71|pages=492–501|url=http://adsabs.harvard.edu/abs/1781RSPT...71..492H}}</ref> Herschel "engaged in a series of observations on the parallax of the fixed stars",<ref>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</ref> using a telescope of his own design.
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| − | He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".<ref>Royal Astronomical Society MSS W.2/1.2, 23; quoted in Miner p. 8</ref> 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".<ref>RAS MSS Herschel W.2/1.2, 24, quoted in Miner p. 8</ref> 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:<ref> Journal of the Royal Society and Royal Astronomical Society 1, 30; quoted in Miner p. 8</ref>
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| − | {{cquote|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.}}
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| − | 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".<ref>RAS MSS Herschel W1/13.M, 14 quoted in Miner p. 8</ref>
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| − | 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.<ref name=georgeforbes>{{cite web|title=History of Astronomy|author=George Forbes|year=1909|url=http://www.vinnysa1store.com/historyofastronomy2.html#8|accessdate=2007-08-07}}</ref> 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".<ref>Johann Elert Bode, Berliner Astronomisches Jahrbuch, p. 210, 1781, quoted in Miner p. 11</ref> Bode concluded that its near-circular orbit was more like a planet than a comet.<ref>Miner p. 11</ref>
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| − | 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."<ref name=Dreyer>{{cite book|author=[[J. L. E. Dreyer]], |year=1912|title=The Scientific Papers of Sir William Herschel|publisher=Royal Society and Royal Astronomical Society|volume=1|pages=100}}</ref> In recognition of his achievement, [[George III of the United Kingdom|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.<ref name="Miner12">Miner p. 12</ref>
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| − | ===Naming===
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| − | 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."<ref>RAS MSS Herschel W.1/12.M, 20, quoted in Miner p. 12</ref> 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.<ref>{{cite journal | url=http://web.archive.org/web/20060210222142/http://vesuvius.jsc.nasa.gov/er/seh/hersc.html | title= Voyager at Uranus | year=1986 | journal=NASA JPL | pages=400–268 | volume=7 | issue=85}}</ref> He explained this decision in a letter to Joseph Banks:<ref name=Dreyer />
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| − | [[Image:William Herschel01.jpg|thumb|left|upright|William Herschel, discoverer of Uranus]]
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| − | {{cquote|''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''.}}
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| − | Astronomer [[Jérôme Lalande]] proposed the planet be named ''Herschel'' in honour of its discoverer.<ref name=Francisca>{{cite web|title=The meaning of the symbol H+o for the planet Uranus|author=Francisca Herschel|year=1917|work=The Observatory|url=http://adsabs.harvard.edu/abs/1917Obs....40..306H|accessdate=2007-08-05}}</ref> Bode, however, opted for ''Uranus'', the Latinized version of the [[Greek mythology|Greek god]] of the sky, [[Uranus (mythology)|Ouranos]]. Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.<ref name="Miner12"/><ref name="planetsbeyond">{{Cite book | title=Planets Beyond: Discovering the Outer Solar System | id=ISBN 0-486-43602-0 | last=Littmann| first=Mark | publisher=Courier Dover Publications | year=2004 | pages=pp. 10–11}}</ref><ref>{{cite web | title=Astronomy in Berlin | publisher=Brian Daugherty | url=http://bdaugherty.tripod.com/astronomy/berlin.html | accessdate=2007-05-24 |last=Daugherty | first=Brian}}</ref> The earliest citation of the name Uranus in an official publication is in 1823, a year after Herschel's death.<ref>{{cite web | title=Query Results from the ADS Database | publisher=Smithsonian/NASA Astrophysics Data System (ADS) | url=http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20db_key=AST&qform=AST&sim_query=YES&ned_query=YES&aut_logic=OR&obj_logic=OR&author=&object=&start_mon=00&start_year=1787&end_mon=12&end_year=1900&ttl_req=YES&ttl_logic=OR&title=uranus&txt_logic=OR&text=&nr_to_return=100&start_nr=1&jou_pick=ALL&ref_stems=&data_and=ALL&group_and=ALL&start_entry_day=&start_entry_mon=&start_entry_year=&end_entry_day=&end_entry_mon=&end_entry_year=&min_score=&sort=SCORE&data_type=SHORT&aut_syn=YES&ttl_syn=YES&txt_syn=YES&aut_wt=1.0&obj_wt=1.0&ttl_wt=0.3&txt_wt=3.0&aut_wgt=YES&obj_wgt=YES&ttl_wgt=YES&txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1&start_nr=101&start_cnt=101&start_nr=201&start_cnt=201 | accessdate=2007-05-24}}</ref><ref>{{cite journal| author=Friedrich Magnus Schwerd| title= Opposition des Uranus 1821| journal= Astronomische Nachrichten| volume=1| pages= 18–21}}</ref> The name ''Georgium Sidus'' or "the Georgian" was still used infrequently (by the British alone) for some time thereafter; the final holdout was [[HM Nautical Almanac Office]], which did not switch to ''Uranus'' until 1850.<ref name="planetsbeyond"/>
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| − | The preferred pronunciation of the name ''Uranus'' among astronomers is {{IPA|[ˈjÊŠÉ™rÉ™nÉ™s]}}, with the first [[syllable]] stressed and a short ''a''
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| − | {{Unicode|('''Å«r'''ănÅs)}};<ref>{{cite web|title=How to speak like a BBC newsreader|work=Daily Mail|year=2006|url=http://www.dailymail.co.uk/pages/live/articles/news/news.html?in_article_id=411233&in_page_id=1770|accessdate=2007-12-13}}</ref> this is more classically correct than the alternate
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| − | {{IPA|[jʊˈɹeɪ.nÉ™s]}}, with stress on the second syllable and a "long a" {{Unicode|(Å«r'''Ä'''nÅs)}},
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| − | which is often used in the English-speaking world.
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| − | Uranus is the only planet whose name is derived from a figure from [[Greek mythology]] rather than [[Roman mythology]]. (The Roman equivalent would have been [[Caelus]].) The adjective of Uranus is "Uranian". The element [[uranium]], discovered in 1789, was named in its honour by its discoverer, [[Martin Klaproth]].<ref>{{cite web|title=Science in Flux|author=Mark D Bowles|work=NASA History Series|url=http://history.nasa.gov/sp4317.pdf|format=PDF|year=2006|accessdate=2007-08-04}}</ref>
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| − | Its [[astronomical symbol]] is [[Image:Uranus symbol.svg|20px|Astronomical symbol for Uranus]]. 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.<ref>{{cite web|title=Planet symbols|work=NASA Solar System exploration|url=http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=167|accessdate=2007-08-04}}</ref> Its [[astrological symbol]] is [[Image:Uranus's astrological symbol.svg|20px|]], 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").<ref name=Francisca /> In the [[Chinese language|Chinese]], [[Japanese language|Japanese]], [[Korean language|Korean]], and [[Vietnamese language|Vietnamese]] languages, the planet's name is literally translated as the ''sky king star'' (天王星).<ref>{{cite web |url=http://www.eternalsailormoon.org/help.html#myth | title=Sailormoon Terms and Information | publisher=The Sailor Senshi Page
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| − | |accessdate=2006-03-05}}</ref><ref>{{cite journal | url=http://amateurastronomy.org/EH/Oct97.txt |title=Asian Astronomy 101
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| − | |journal=Hamilton Amateur Astronomers |month=October |year=1997 |volume=4 |issue=11 |accessdate=2007-08-05}}</ref>
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| − | ==Orbit and rotation==
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| − | [[Image:Uranusandrings.jpg|thumb|[[Hubble Space Telescope|HST]] image of Uranus showing cloud bands, rings, and moons]]
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| − | Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 [[1,000,000,000 (number)|billion]] km (about 20 [[Astronomical Units|AU]]). The intensity of sunlight on Uranus is about 1/400 that of Earth.<ref>{{cite web|title=Next Stop Uranus| url=http://www.astrosociety.org/education/publications/tnl/04/04.html|year=1986|accessdate=2007-06-09}}</ref> Its orbital elements were first calculated in 1783 by [[Pierre-Simon Laplace]].<ref name=georgeforbes /> 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.<ref>{{cite web|title=Mathematical discovery of planets|author=J J O'Connor and E F Robertson|url=http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Neptune_and_Pluto.html|X|year=1996|accessdate=2007-06-13}}</ref>
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| − | 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 two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.<ref>{{cite web|title=Uranus|work=NASA World Book|author=Peter J. Gierasch and Philip D. Nicholson|url=http://www.nasa.gov/worldbook/uranus_worldbook.html|year=2004|accessdate=2007-06-09}}</ref>
| + | |
| − | | + | |
| − | ===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 makes its exchange of seasons completely unlike those of the other major planets. Other planets can be visualized to rotate like tilted spinning [[top]]s relative to the plane of the solar system, while Uranus rotates more like a tilted rolling [[ball]]. Near the time of Uranian [[solstice]]s, 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 as 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.<ref>{{cite web|title=Hubble captures rare, fleeting shadow on Uranus|author=Lawrence Sromovsky|work=University of Wisconsin Madison|url=http://www.news.wisc.edu/releases/12826.html|year=2006|accessdate=2007-06-09}}</ref> Near the time of the [[equinox]]es, the Sun faces the equator of Uranus giving a period of day-night cycles similar to those seen on most of the other planets. Uranus reached its most recent equinox on [[7 December]] [[2007]].<ref>{{cite conference|last=Hammel|first=Heidi B.|title=Uranus nears Equinox. |booktitle=A report from the 2006 Pasadena Workshop|date=September 5, 2006|url=http://www.apl.ucl.ac.uk/iopw/uworkshop_060905.pdf}}</ref><ref name=weather>{{cite web |url=http://www.sciencedaily.com/releases/2006/10/061001211630.htm |title=Hubble Discovers Dark Cloud In The Atmosphere Of Uranus |publisher=Science Daily |accessdate=2007-04-16}}</ref>
| + | |
| − | {| class="wikitable"
| + | |
| − | |-
| + | |
| − | ! Northern hemisphere
| + | |
| − | ! Year
| + | |
| − | ! Southern hemisphere
| + | |
| − | |-
| + | |
| − | |align="center"|Winter solstice
| + | |
| − | |align="center"|1902, 1986
| + | |
| − | |align="center"|Summer solstice
| + | |
| − | |-
| + | |
| − | |align="center"|Vernal equinox
| + | |
| − | |align="center"|1923, 2007
| + | |
| − | |align="center"|Autumnal equinox
| + | |
| − | |-
| + | |
| − | |align="center"|Summer solstice
| + | |
| − | |align="center"|1944, 2028
| + | |
| − | |align="center"|Winter solstice
| + | |
| − | |-
| + | |
| − | |align="center"|Autumnal equinox
| + | |
| − | |align="center"|1965, 2049
| + | |
| − | |align="center"|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.<ref>{{cite book|author=Jay T.Bergstralh, Ellis Miner, Mildred Matthews|title=Uranus|year=1991|pages= 485–486}}</ref> Uranus' south pole was pointed almost directly at the Sun at the time of ''[[Voyager 2]]'''s flyby in 1986. 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.<ref>{{cite web |url=http://www.hnsky.org/iau-iag.htm |title=Report of the IAU/IAG working group on cartographic coordinates and rotational elements of the planets and satellites: 2000|work=IAU|year=2000|accessdate=2007-06-13}}</ref><ref>{{cite web |url=http://pds.jpl.nasa.gov/documents/sr/stdref_021015/Chapter02.pdf |format=PDF |title=Cartographic Standards |work=NASA|accessdate=2007-06-13}}</ref> 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.<ref>{{cite web |url=http://roger.ecn.purdue.edu/~masl/documents/masl/coords.html |title=Coordinate Frames Used in MASL|year=2003|accessdate=2007-06-13}}</ref> 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!"<ref>{{cite journal|journal=Sky at Night Magazine|first=Patrick|last=Moore|title=Observing the green giant|pages=47|date=September|year=2006| url=http://www.skyatnightmagazine.com/viewIssue.asp?id=625}}</ref>
| + | |
| − | | + | |
| − | === Visibility===
| + | |
| − | From 1995 to 2006, Uranus' [[apparent magnitude]] fluctuated between +5.6 and +5.9, placing it just within the limit of [[naked eye]] visibility at +6.5.<ref name=ephemeris>{{cite web|title=Twelve Year Planetary Ephemeris: 1995 - 2006|author=Fred Espenak|work=NASA|url=http://sunearth.gsfc.nasa.gov/eclipse/TYPE/TYPE.html|year=2005|accessdate=2007-06-14}}</ref> 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]].<ref name=ephemeris /> At opposition, Uranus is visible to the naked eye in dark, un-[[light pollution|light polluted]] skies, and becomes an easy target even in urban conditions with binoculars.<ref name=nasafact>{{cite web|url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html|title= NASA's Uranus fact sheet|accessdate=2007-06-13}}</ref> 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 (moon)|Titania]] and [[Oberon (moon)|Oberon]], may be visible.<ref>{{cite web|title=Uranus: the Threshold Planet of 2006|author=Gary T. Nowak|url=http://www.vtastro.org/Articles/uranus2006.html|year=2006|accessdate=2007-06-14}}</ref>
| + | |
| − | | + | |
| − | ==Physical characteristics==
| + | |
| − | ===Internal structure===
| + | |
| − | [[Image:Uranus, Earth size comparison.jpg|thumb|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 1.27 g/cm³ makes it the second least dense planet, after Saturn.<ref name=Jacobson1992>{{cite journal|last=Jacobson|first=R.A.|coauthors=Campbell, J.K.; Taylor, A.H.; Synnott, S.P.|title=The masses of Uranus and its major satellites from Voyager tracking data and Earth-based Uranian satellite data|journal=The Astronomical Journal|volume=103|issue=6|pages=2068–2078|year=1992|doi=10.1086/116211| url=http://adsabs.harvard.edu/abs/1992AJ....103.2068J}}</ref> Though having a diameter slightly larger than Neptune (roughly four times Earth's), it is less massive.<ref name=Seidelmann2007/> These values indicate that it is made primarily of various [[volatiles|ices]], such as [[water]], [[ammonia]], and [[methane]].<ref name=Podolak1995>{{cite journal|last=Podolak|first=M.|coauthors=Weizman, A.; Marley, M.|title=Comparative model of Uranus and Neptune|journal=Planet. Space Sci.|volume=43|issue=12|pages=1517–1522|year=1995| url=http://adsabs.harvard.edu/abs/1995P%26SS...43.1517P}}</ref> 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.<ref name=Podolak1995/><ref name=Podolak2000>{{cite journal|last= Podolak|first=M.|coauthors=Podolak, J.I.; Marley, M.S.|title=Further investigations of random models of Uranus and Neptune |journal=Planet. Space Sci.|volume=48|pages=143–151|year=2000| url=http://adsabs.harvard.edu/abs/2000P%26SS...48..143P}}</ref> [[Hydrogen]] and [[helium]] constitute only a small part of the total, with between 0.5 and 1.5 Earth masses.<ref name=Podolak1995/> The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by [[rock (geology)|rocky material]].<ref name=Podolak1995/>
| + | |
| − | | + | |
| − | The standard model of Uranus' structure is that it consists of three layers: a rocky [[core (geology)|core]] in the center, an icy [[mantle (geology)|mantle]] in the middle and an outer gaseous [[hydrogen]]/[[helium]] envelope.<ref name=Podolak1995/><ref name=Faure2007/> 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.<ref name=Podolak1995/><ref name=Faure2007/> Uranus' core [[density]] is around 9 g/cm³, with a [[pressure]] at the core/mantle boundary of 8 million [[bar (unit)|bars]] (800 [[gigapascal|GPa]]) and a temperature of about 5000 [[kelvin|K]].<ref name=Podolak2000/><ref name=Faure2007>{{cite encyclopedia|last= Faure|first=Gunter|coauthors=Mensing, Teresa|title=Uranus: What Happened Here?|encyclopedia=Introduction to Planetary Science|year=2007|publisher=Springer Netherlands|editor=Faure, Gunter; Mensing, Teresa M.|doi=10.1007/978-1-4020-5544-7_18|}}</ref> 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]].<ref name=Podolak1995/><ref name=Faure2007/> This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.<ref name=Atreya2006>{{cite journal|last=Atreya|first=S.|coauthors=Egeler, P.; Baines, K. |title=Water-ammonia ionic ocean on Uranus and Neptune?|journal=Geophysical Research Abstracts|volume=8|pages=05179|year=2006|format=pdf| url=http://www.cosis.net/abstracts/EGU06/05179/EGU06-J-05179-1.pdf}}</ref> The bulk compositions of Uranus and Neptune are very different from those of [[Jupiter]] and [[Saturn]], with ice dominating over gases, hence justifying their separate classification as [[ice giant]]s.
| + | |
| − | | + | |
| − | While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow us to determine which model is correct.<ref name=Podolak2000/> The [[fluid]] interior structure of Uranus means that it has no [[solid]] [[surface]]. The gaseous atmosphere gradually transitions into the internal liquid layers.<ref name=Podolak1995/> However for the sake of convenience an [[oblate spheroid]] of revolution, where pressure equals 1 [[bar (unit)|bar]] (100 kPa), is designated conditionally as a ‘surface’. It has [[equator]]ial and [[Geographical pole|polar]] radii of {{nowrap|25,559 ± 4}} and {{nowrap|24,973 ± 20 km}}, respectively.<ref name=Seidelmann2007>{{cite journal|last= Seidelmann|first= P. Kenneth|coauthors= Archinal, B. A.; A’hearn, M. F.; et.al.|title= Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006|journal= Celestial Mech. Dyn. Astr. |volume=90|pages=155–180|year=2007|doi=10.1007/s10569-007-9072-y|url=http://adsabs.harvard.edu/doi/10.1007/s10569-007-9072-y}}</ref> This surface will be used throughout this article as a zero point for [[altitude]]s.
| + | |
| − | | + | |
| − | ==== Internal heat ====
| + | |
| − | Uranus' [[internal heat]] appears markedly lower than that of the other giant planets; in astronomical terms, it has a low [[thermal flux]].<ref name=1986Hanel/><ref name=Sromovsky2005/> 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.<ref name=Sromovsky2005/> 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 {{nowrap|1.06 ± 0.08}} times the solar energy absorbed in its [[atmosphere]].<ref name=Pearl1990/><ref name=Lunine1993/> In fact, Uranus' heat flux is only {{nowrap|0.042 ± 0.047}} W/m², which is lower than the internal heat flux of Earth of about 0.075 [[flux|W/m²]].<ref name=Pearl1990/> The lowest temperature recorded in Uranus' tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.<ref name=Pearl1990/><ref name=Lunine1993/>
| + | |
| − | | + | |
| − | 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.<ref>{{cite journal|title=Ten Mysteries of the Solar System: Why is Uranus So Cold?|author=David Hawksett|journal=Astronomy Now|date=August|year=2005|pages=73}}</ref> Another hypothesis is that some form of barrier exists in Uranus' upper layers which prevents the core's heat from reaching the surface.<ref name=Podolak1995/> For example, [[convection]] may take place in a set of compositionally different layers, which may inhibit the upward [[Heat conduction|heat transport]].<ref name=Lunine1993/><ref name=Pearl1990/>
| + | |
| − | | + | |
| − | ===Atmosphere===
| + | |
| − | {{main|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]].<ref name=Lunine1993/> Remote sensing capability extends down to roughly 300 km below the 1 bar (100 kPa) level, with a corresponding pressure around 100 bar (10 MPa) and temperature of 320 [[kelvin (unit)|K]].<ref name=dePater1991>{{cite journal|last=dePater|first=Imke|coauthors=Romani, Paul N.; Atreya, Sushil K.|title=Possible Microwave Absorption in by {{nowrap|H<sub>2</sub>S}} gas Uranus’ and Neptune’s Atmospheres|journal=Icarus|volume=91|pages=220–233|year=1991|doi=10.1016/0019-1035(91)90020-T| url=http://www-personal.umich.edu/~atreya/Articles/1991_Microwave_Absorption.pdf|format=PDF}}</ref> The tenuous [[corona]] of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.<ref name=Herbert1987/> The Uranian atmosphere can be divided into three layers: the [[troposphere]], between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; (10 MPa to 10 kPa) the [[stratosphere]], spanning altitudes between 50 and 4000 km and pressures of between {{nowrap|0.1 and 10<sup>–10</sup> bar}} (10 kPa to 10 [[micropascal|µPa]]), and the [[thermosphere]]/[[corona]] extending from 4,000 km to as high as 50,000 km from the surface.<ref name=Lunine1993>{{cite journal|title=The Atmospheres of Uranus and Neptune|last=Lunine|first=Jonathan. I.|journal = Annual Review of Astronomy and Astrophysics|volume=31|pages=217–263|year=1993|doi=10.1146/annurev.aa.31.090193.001245| url=http://adsabs.harvard.edu/abs/1993ARA%26A..31..217L}}</ref> 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]].<ref name=Lunine1993/> The helium molar fraction, i.e. the number of helium [[atom]]s per [[molecule]] of gas, is {{nowrap|0.15 ± 0.03}}<ref name=Conrath1987>{{cite journal|author=B. Conrath ''et al.''|title=The helium abundance of Uranus from Voyager measurements|journal=Journal of Geophysical Research|volume=92|pages=15003-15010|yar=1987|url=http://adsabs.harvard.edu/abs/1987JGR....9215003C}}</ref> in the upper troposphere, which corresponds to a mass fraction {{nowrap|0.26 ± 0.05}}.<ref name=Lunine1993/><ref name=Pearl1990/> This value is very close to the protosolar helium mass fraction of {{nowrap|0.275 ± 0.01}},<ref name=Lodders2003>{{cite journal|last=Lodders|first= Katharin|title= Solar System Abundances and Condensation Temperatures of the Elements|journal=The Astrophysical Journal|volume=591|pages=1220–1247 |year=2003|doi=10.1086/375492|url=http://adsabs.harvard.edu/abs/2003ApJ...591.1220L}}</ref> indicating that helium has not settled in the center of the planet as it has in the gas giants.<ref name=Lunine1993/> The third most abundant constituent of the Uranian [[atmosphere]] is [[methane]] {{nowrap|(CH<sub>4</sub>)}}.<ref name=Lunine1993/> Methane possesses prominent [[absorption band]]s in the [[visible]] and [[near-infrared]] (IR) making Uranus [[aquamarine (color)|aquamarine]] or [[cyan]] in color.<ref name=Lunine1993/> 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 (unit)|bar]] (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.<ref name=Lunine1993/><ref name=Lindal1987/><ref name=1986Tyler/> The mixing ratio{{Ref_label|E|e|none}} 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.<ref name=Bishop1990>{{cite journal|last=Bishop|first=J.|coauthors=Atreya, S.K.; Herbert, F.; and Romani, P.|title=Reanalysis of Voyager 2 UVS Occultations at Uranus: Hydrocarbon Mixing Ratios in the Equatorial Stratosphere|journal=Icarus|volume=88|pages=448–463|year=1990| doi=10.1016/0019-1035(90)90094-P| url=http://www-personal.umich.edu/~atreya/Articles/1990_Reanalysis.pdf|format=PDF}}</ref> 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.<ref name=Lunine1993/><ref name=dePater1989>{{cite journal|last= dePater|first=Imke|coauthors=Romani, Paul N.; Atreya, Sushil K.|title=Uranius Deep Atmosphere Revealed|journal=Icarus|volume=82|issue=12|pages=288–313|year=1989|doi=10.1016/0019-1035(89)90040-7| url=http://www-personal.umich.edu/~atreya/Articles/1989_Uranus_Deep_Atm.pdf|format=PDF}}</ref> In addition to methane, trace amounts of various [[hydrocarbon]]s are found in the upper atmosphere of Uranus, which are thought to be produced from methane by [[photolysis]] induced by the solar [[ultraviolet]] (UV) radiation.<ref name=Summers1989/> They include [[ethane]] {{nowrap|(C<sub>2</sub>H<sub>6</sub>)}}, [[acetylene]] {{nowrap|(C<sub>2</sub>H<sub>2</sub>)}}, [[methylacetylene]] {{nowrap|(CH<sub>3</sub>C<sub>2</sub>H)}}, [[diacetylene]] {{nowrap|(C<sub>2</sub>HC<sub>2</sub>H)}}.<ref name=Bishop1990/><ref name=Burdorf2006>{{cite journal|last=Burgorf|first=Martin|coauthors=Orton, Glenn; van Cleve, Jeffrey; et.al.|title=Detection of new hydrocarbons in Uranus' atmosphere by infrared spectroscopy|journal=Icarus|volume=184|year=2006|pages=634–637| doi=10.1016/j.icarus.2006.06.006| url=http://adsabs.harvard.edu/abs/2006Icar..184..634B}}</ref><ref name=Encrenaz2003/> Spectroscopy has also uncovered traces of water vapor, [[carbon monoxide]] and [[carbon dioxide]] in the upper atmosphere, which can only originate from an external source such as infalling dust and [[comet]]s.<ref name=Encrenaz2003>{{cite journal|last=Encrenaz |first=Therese|title=ISO observations of the giant planets and Titan: what have we learnt?|journal=Planet. Space Sci.|volume=51| pages=89–103|year=2003|doi=10.1016/S0032-0633(02)00145-9| url=http://adsabs.harvard.edu/abs/2003P%26SS...51...89E}}</ref><ref name=Burdorf2006/><ref name=Encrenaz2004>{{cite journal|last=Encrenaz|first=Th.|coauthors=Lellouch, E.; Drossart, P.|title=First detection of CO in Uranus|journal=Astronomy&Astrophysics|year=2004|volume=413|pages=L5–L9|doi=10.1051/0004-6361:20034637| url=http://www-personal.umich.edu/~atreya/Articles/2004_First_Detection.pdf|format=PDF|accessdate=2007-08-05}}</ref>
| + | |
| − | | + | |
| − | ====Troposphere====
| + | |
| − | [[Image:Tropospheric profile Uranus.png|thumb|400px|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.<ref name=Lunine1993/> The temperature falls from about 320 K at the base of the nominal troposphere at −300 km to 53 K at 50 km.<ref name=dePater1991/><ref name=1986Tyler>{{cite journal|last=Tyler|first=J.L.|coauthors=Sweetnam, D.N.; Anderson, J.D.; et.al. |title=Voyger 2 Radio Science Observations of the Uranian System: Atmosphere, Rings, and Satellites|journal=Science|volume=233|pages=79–84| year=1986| url=http://adsabs.harvard.edu/abs/1986Sci...233...79T}}</ref> 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.<ref name=Lunine1993/><ref name=1986Hanel>{{cite journal|last=Hanel|first=R.|coauthors=Conrath, B.; Flasar, F.M.; et.al. |title=Infrared Observations of the Uranian System|journal=Science|volume=233|pages=70–74|year=1986| url=http://adsabs.harvard.edu/abs/1986Sci...233...70H}}</ref> The tropopause region is responsible for the vast majority of the planet’s thermal [[far infrared]] emissions, thus determining its [[effective temperature]] of {{nowrap|59.1 ± 0.3 K}}.<ref name=1986Hanel/><ref name=Pearl1990>{{cite journal|last=Pearl|first=J.C.|coauthors=Conrath, B.J.; Hanel, R.A.; and Pirraglia, J.A.|title=The Albedo, Effective Temperature, and Energy Balance of Uranus as Determined from Voyager IRIS Data|journal=Icarus|volume=84|pages=12–28|year=1990| doi=10.1016/0019-1035(90)90155-3|url=http://adsabs.harvard.edu/abs/1990Icar...84...12P}}</ref>
| + | |
| − | | + | |
| − | The troposphere is believed to possess a highly complex cloud structure; [[cloud|water clouds]] are hypothesised to lie in the pressure range of {{nowrap|50 to 100 bar}} (5 to 10 MPa), [[ammonium hydrosulfide]] clouds in the range of {{nowrap|20 to 40 bar}} (2 to 4 MPa), [[ammonia]] or [[hydrogen sulfide]] clouds at between 3 and 10 bar (0.3 to 1 MPa) and finally directly detected thin [[methane]] clouds at {{nowrap|1 to 2 bar}} (0.1 to 0.2 MPa).<ref name=Lunine1993/><ref name=dePater1991/><ref name=Atreya2005>{{cite journal|last=Atreya|first=Sushil K.|coauthors=Wong, Ah-San |title=Coupled Clouds and Chemistry of the Giant Planets – a Case for Multiprobes |journal= Space Sci. Rev.|volume=116|pages=121–136|year=2005|doi=10.1007/s11214-005-1951-5| url=http://adsabs.harvard.edu/abs/2005SSRv..116..121A}}</ref><ref name=Lindal1987>{{cite journal|last=Lindal|first=G.F.|coauthors=Lyons, J.R.; Sweetnam, D.N.; et.al.|title=The Atmosphere of Uranus: Results of Radio Occultation Measurements with Voyager 2 |journal=J. of Geophys. Res.|volume=92|pages=14,987–15,001|year=1987|url=http://adsabs.harvard.edu/abs/1987JGR....9214987L}}</ref> The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes, which will be discussed below.<ref name=Sromovsky2005>{{cite journal|last=Sromovsky|first=L.A.|coauthors=Fry, P.M.|title=Dynamics of cloud features on Uranus|journal=Icarus|volume=179|pages=459–483|year=2005| doi=10.1016/j.icarus.2005.07.022|url=http://adsabs.harvard.edu/abs/2005Icar..179..459S}}</ref>
| + | |
| − | | + | |
| − | ====Upper atmosphere====
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| − | 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]].<ref name=Herbert1987>{{cite journal|last=Herbert|first=Floyd|coauthors=Sandel, B.R.; Yelle, R.V.; et.al.|title=The Upper Atmosphere of Uranus: EUV Occultations Observed by Voyager 2|journal=J. of Geophys. Res.|volume=92| pages=15,093–15,109|year=1987| url=http://www-personal.umich.edu/~atreya/Articles/1987_Upper_Atm_Uranus.pdf|format=PDF}}</ref> The heating of the stratosphere is caused by absorption of solar [[UV]] and [[Infrared|IR]] radiation by [[methane]] and other [[hydrocarbon]]s that form in this part of the atmosphere as a result of methane [[photolysis]].<ref name=Bishop1990/><ref name=Summers1989>{{cite journal|last=Summers|first=Michael E.|coauthors=Strobel, Darrell F.|title=Photochemistry of the Atmosphere of Uranus|journal=The Astrophysical Journal|volume=346|pages=495–508|year=1989|doi=10.1086/168031| url=http://adsabs.harvard.edu/abs/1989ApJ...346..495S}}</ref> Heating from the hot thermosphere may also be significant.<ref name=Herbert1999/><ref name=Young2001>{{cite journal|last=Young|first=Leslie A.|coauthors= Bosh, Amanda S.; Buie, Marc; et.al.|title= Uranus after Solstice: Results from the 1998 November 6 Occultation |journal=Icarus|volume=153|pages=236–247|year=2001|doi=10.1006/icar.2001.6698| url=http://www.boulder.swri.edu/~layoung/eprint/ur149/Young2001Uranus.pdf| format=PDF}}</ref> 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 m[[bar (unit)|bar]] (1000 to 10 kPa) and temperatures of between 75 and 170 K.<ref name=Bishop1990/> The most abundant hydrocarbons are [[acetylene]] and [[ethane]] with [[mixing ratio]]s of around 10<sup>−7</sup> relative to [[hydrogen]], which is similar to the mixing ratios of methane and [[carbon monoxide]] at these altitudes.<ref name=Bishop1990/><ref name=Burdorf2006/><ref name=Encrenaz2004/> Heavier hydrocarbons and [[carbon dioxide]] have mixing ratios three orders of magnitude lower.<ref name=Burdorf2006/> The abundance ratio of water is around 7{{e|−9}}.<ref name=Encrenaz2003/> Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause forming [[haze]] layers,<ref name=Summers1989/> 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 planet]]s.<ref name=Bishop1990/><ref name=Herbert1999>{{cite journal|last=Herbert|first=Floyd|coauthors=Sandel, Bill R. |title=Ultraviolet Observations of Uranus and Neptune|journal=Planet. Space Sci. |volume=47|pages=1119–1139|year=1999| url=http://adsabs.harvard.edu/abs/1999P%26SS...47.1119H}}</ref>
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| − | The outermost layer of the Uranian atmosphere is the thermosphere and [[corona]], which has a uniform temperature around 800 to 850 K.<ref name=Lunine1993/><ref name=Herbert1999/> The heat sources necessary to sustain such a high value are not understood, since neither solar [[UV|far UV]] and [[UV|extreme UV]] 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.<ref name=Herbert1987/><ref name=Herbert1999/> In addition to [[molecular hydrogen]], the thermosphere-corona contains a large proportion of free [[hydrogen atom]]s. 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.<ref name=Herbert1987/><ref name=Herbert1999/> This extended corona is a unique feature of Uranus.<ref name=Herbert1999/> Its effects include a [[drag (physics)|drag]] on small particles orbiting Uranus, causing a general depletion of [[dust]] in the Uranian rings.<ref name=Herbert1987/> The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the [[ionosphere]] of Uranus.<ref name=1986Tyler/> Observations show that the ionosphere occupies altitudes from 2,000 to 10,000 km.<ref name=1986Tyler/> The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.<ref name=Herbert1999/><ref name=Trafton1999>{{cite journal|last=Trafton|first=L.M.|coauthors=Miller, S.; Geballe, T.R.; et.al. |title= H2 Quadrupole and H3+ Emission from Uranus: the Uranian Thermosphere, Ionosphere, and Aurora|journal=The Astrophysical Journal|volume=524|pages=1059–1023|year=1999| doi=10.1086/307838|url=http://adsabs.harvard.edu/abs/1999ApJ...524.1059T}}</ref> The ionosphere is mainly sustained by solar UV radiation and its density depends on the [[solar activity]].<ref name=Encrenaz2003b>{{cite journal|last=Encrenaz|first=Th.|coauthors=Drossart, P.; Orton, G.; et.al|title=The rotational temperature and column density of H<sup>+</sup><sub>3</sub> in Uranus|year=2003|journal=Planetary and Space Sciences|volume=51|pages=1013–1016| url=http://www-personal.umich.edu/~atreya/Articles/2003_Rotational_Temperature.pdf| doi=10.1016/S0032-0633(03)00132-6|format=PDF}}</ref> [[Auroral]] activity is not as significant as at Jupiter and Saturn.<ref name=Herbert1999/><ref name=Lam1997>{{cite journal|last=Lam|first=Hoanh An|coauthors=Miller, Steven; Joseph, Robert D.; et.al|title=Variation in the {{nowrap|H<sup>+</sup><sub>3</sub>}} emission from Uranus|year=1997|journal=The Astrophysical Journal|volume=474|pages=L73–L76| url=http://adsabs.harvard.edu/abs/1997ApJ...474L..73L|doi=10.1086/310424}}</ref>
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| − | ===Planetary rings===
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| − | {{main|Rings of Uranus}}
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| − | [[Image:Uranian rings PIA01977 modest.jpg|thumb|Uranus' inner rings. The bright outer ring is the epsilon ring, eight other rings are present]]
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| − | [[Image:uranusrings.jpg|upright|thumb|An enhanced colour schematic of the inner rings]]Uranus has a faint [[planetary ring]] system, composed of dark particulate matter up to ten meters in diameter.<ref name=Smith1986/> It was the second ring system to be discovered in the Solar System after [[Rings of Saturn|Saturn's]].<ref name=Esposito2002/> Thirteen distinct rings are presently known, the brightest being the epsilon ring. 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.<ref name=Esposito2002>{{cite journal |last=Esposito|first=L.W.|authorlink=Larry W. Esposito|title=Planetary rings|journal=Reports On Progress In Physics |year=2002|volume=65|pages=1741–1783 |url=http://www.iop.org/EJ/article/0034-4885/65/12/201/r21201.pdf| format=pdf}}</ref><ref name=summary />
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| − | [[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 [[Celestial body atmosphere|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.<ref>{{cite web|title=The rings of Uranus|author=J. L. Elliot, E. Dunham & D. Mink|work= Cornell University|url=http://www.nature.com/nature/journal/v267/n5609/abs/267328a0.html|year=1977|accessdate=2007-06-09}}</ref> The rings were directly imaged when ''[[Voyager 2]]'' passed Uranus in 1986.<ref name=Smith1986/> ''[[Voyager 2]]'' also discovered two additional faint rings bringing the total number to eleven.<ref name=Smith1986/>
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| − | 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 of 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 (moon)|Mab]], shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13.<ref>{{cite web|title=NASA's Hubble Discovers New Rings and Moons Around Uranus|work=Hubblesite|url=http://hubblesite.org/newscenter/archive/releases/2005/33/| year=2005|accessdate=2007-06-09}}</ref> In April 2006, images of the new rings with the [[Keck Observatory]] yielded the colours of the outer rings: the outermost is blue and the other red.<ref name=dePater2006>{{cite journal|last=dePater|first=Imke|coauthors=Hammel, Heidi B.; Gibbard, Seran G.; Showalter Mark R. |title=New Dust Belts of Uranus: Two Ring, red Ring, Blue Ring|journal=Science|volume=312|pages=92-94| year=2006|doi=10.1126/science.1125110|url=http://adsabs.harvard.edu/abs/2006Sci...312...92D}}</ref><ref>{{Cite web| title=Blue ring discovered around Uranus|publisher=UC Berkeley News|last=Sanders|first=Robert| url=http://www.berkeley.edu/news/media/releases/2006/04/06_bluering.shtml|date=[[2006-04-06]]|accessdate=2006-10-03}}</ref>
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| − | One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light.<ref name=dePater2006/><ref>{{cite web|title=Blue ring of Uranus linked to sparkling ice|author=Stephen Battersby|work=NewScientistSpace| url=http://space.newscientist.com/article/dn8960|year=2006|accessdate=2007-06-09}}</ref> The planet's inner rings appear grey.<ref name=dePater2006/>
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| − | 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".<ref>{{Cite news |title=Uranus rings 'were seen in 1700s' |publisher=BBC News |url=http://news.bbc.co.uk/1/hi/sci/tech/6569849.stm|date=[[April 19]][[2007]]| accessdate=2007-04-19}}</ref>
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| − | 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.<ref name=dePater2006/> 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.<ref>{{cite web|title=Did William Herschel Discover The Rings Of Uranus In The 18th Century?|work=Physorg.com|url=http://www.physorg.com/news95949762.html|year=2007|accessdate=2007-06-20}}</ref>
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| − | ===Magnetic field===
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| − | [[Image:Uranian Magnetic field.gif|thumb|left|300px|The magnetic field of Uranus as seen by Voyager 2 in 1986. S and N are magnetic south and north poles.]]
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| − | 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]].<ref name=1986Ness/>
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| − | ''Voyager'''s observations revealed that the [[magnetic field]] is peculiar, both because it does not originate from the planet's geometric center, and because it is tilted at 59° from the axis of rotation.<ref name=1986Ness>{{cite journal|last=Ness|first=Norman F.|coauthors=Acuna, Mario H.; Behannon, Kenneth W.; et.al. |title=Magnetic Fields at Uranus|journal=Science|volume=233|pages=85–89|year=1986|url=http://adsabs.harvard.edu/abs/1986Sci...233...85N}}</ref><ref name=Russell993/> In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by as much as one third of the planetary radius.<ref name=1986Ness/> 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 (unit)|gauss]] (10 [[microtesla|µT]]), whereas in the northern hemisphere it can be as high 1.1 gauss (110 µT).<ref name=1986Ness/> The average field at the surface is 0.23 gauss (23 µT).<ref name=1986Ness/> 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.<ref name=Russell993/> The dipole moment of Uranus is 50 times that of Earth.<ref name=1986Ness/><ref name=Russell993>{{cite journal|last=Russell|first=C.T.|title= Planetary Magnetospheres |journal=Rep. Prog. Phys.|volume=56|pages=687–732|year=1993 |url=http://www.iop.org/EJ/article/0034-4885/56/6/001/rp930601.pdf|format=pdf}}</ref> Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.<ref name=Russell993/> 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.<ref name=Atreya2006/><ref>{{cite journal|last=Stanley|first=Sabine|coauthors=Bloxham, Jeremy|title=Convective-region geometry as the cause of Uranus’ and Neptune’s unusual magnetic fields|journal=Letters to Nature|volume=428|pages=151–153| url=http://mahi.ucsd.edu/johnson/ES130/stanley2004-nature.pdf|format=PDF|accessdate=2007-08-05|year=2004|doi=10.1038/nature02376}}</ref>
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| − | 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 [[magnetosphere#Magnetic Tails|magnetotail]] and [[radiation belt]]s.<ref name=1986Ness/><ref name=Russell993/><ref name=Krimigis1986>{{cite journal|last=Krimigis|first=S.M.|coauthors=Armstrong, T.P.; Axford, W.I.; et.al.|title=The Magnetosphere of Uranus: Hot Plasma and radiation Environment|journal=Science|volume=233|pages=97–102|year=1986| url=http://adsabs.harvard.edu/abs/1986Sci...233...97K}}</ref> Overall, the structure of the magnetosphere of Uranus is different from that of [[Jupiter]]'s and more similar to that of Saturn's.<ref name=1986Ness/><ref name=Russell993/> Uranus' [[magnetosphere#Magnetic Tails|magnetotail]] trails behind the planet into space for millions of kilometers and is twisted by the planet's sideways rotation into a long corkscrew.<ref name=1986Ness/><ref>{{cite web|title=Voyager: Uranus: Magnetosphere|url=http://voyager.jpl.nasa.gov/science/uranus_magnetosphere.html|work=NASA|year=2003|accessdate=2007-06-13}}</ref>
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| − | Uranus' magnetosphere contains [[charged particle]]s: [[proton]]s and [[electron]]s with small amount of {{nowrap|H<sub>2</sub><sup>+</sup>}} [[ion]]s.<ref name=Russell993/><ref name=Krimigis1986/> No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona.<ref name=Krimigis1986/> The ion and electron energies can be as high as 4 and 1.2 [[megaelectronvolt]]s, respectively.<ref name=Krimigis1986/> The density of low energy (below 100 [[electronvolt]]s) ions in the inner magnetosphere is about 2 cm<sup>−3</sup>.<ref name=Bridge1986>{{cite journal|last= Bridge|first=H.S.|coauthors=Belcher, J.W.; Coppi, B.; et.al. |title=Plasma Observations Near Uranus: Initial Results from Voyager 2 |journal=Science|volume=233|pages=89–93|year=1986|url=http://adsabs.harvard.edu/abs/1986Sci...233...89B}}</ref> The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.<ref name=Krimigis1986/> 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.<ref name=Krimigis1986/> This may be the cause of the uniformly dark colouration of the moons and rings.<ref name=summary>{{cite web|title=Voyager Uranus Science Summary|work=NASA/JPL|url=http://www.solarviews.com/eng/vgrur.htm|year=1988|accessdate=2007-06-09}}</ref> Uranus has relatively well developed [[aurora (astronomy)|aurora]]e, which are seen as bright arcs around both magnetic poles.<ref name=Herbert1999/> However, unlike Jupiter's, Uranus' aurorae seem to be insignificant for the energy balance of the planetary [[thermosphere]].<ref name=Lam1997/>
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| − | ==Climate==
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| − | {{main|Climate of Uranus}}
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| − | [[Image:Uranuscolour.png|thumb|Uranus' southern hemisphere in approximate natural colour (left) and in higher wavelengths (right), showing its faint cloud bands and atmospheric "hood" as seen by Voyager 2]]
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| − | Uranus' atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles.<ref name=Sromovsky2005/> When ''[[Voyager 2]]'' flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.<ref name=Smith1986/><ref name=planetary>{{cite web|title=No Longer Boring: 'Fireworks' and Other Surprises at Uranus Spotted Through Adaptive Optics|author=Emily Lakdawalla |work=The Planetary Society|url=http://www.planetary.org/news/2004/1111_No_Longer_Boring_Fireworks_and_Other.html|year=2004|accessdate=2007-06-13}}</ref> One proposed explanation for this dearth of features is that Uranus' [[internal heat]] appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus' tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than [[Neptune]].<ref name=Pearl1990/><ref name=Lunine1993/>
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| − | ===Banded structure, winds and clouds===
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| − | [[Image:Uranian wind speeds.png|thumb|left|upright|Zonal wind speeds on Uranus. Shaded areas show the southern collar and its future northern counterpart. The red curve is a symmetrical fit to the data.]]
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| − | 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).<ref name=Smith1986/> 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.<ref name=Smith1986/><ref name=Hammel2005>{{cite journal|last=Hammel|first=H.B.|coauthors=de Pater, I.; Gibbard, S.; et.al.|title=Uranus in 2003: Zonal winds, banded structure, and discrete features|journal=Icarus|volume=175|pages=534–545|year=2005| doi=10.1016/j.icarus.2004.11.012|url=http://www.llnl.gov/tid/lof/documents/pdf/316112.pdf|format=pdf}}</ref> 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 (unit)|bar]] (see above).<ref name=Rages2004/> 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 telescopes|Keck]] telescope observed neither a collar nor a polar cap in the northern hemisphere.<ref name=Hammel2005/> So Uranus appears to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.<ref name=Hammel2005/>
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| − | In addition to large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.<ref name=Smith1986/> In all other respects Uranus looked like a dynamically dead planet in 1986. However in the 1990s the number of the observed bright cloud features grew considerably.<ref name=Sromovsky2005/> The majority of them were found in the northern hemisphere as it started to become visible.<ref name=Sromovsky2005/> 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.<ref name=Karkoschka2001/> Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.<ref name=Hammel2005b/> They appear to lie at a higher [[altitude]].<ref name=Hammel2005b/> 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.<ref name=Sromovsky2005/><ref name=planetary/> 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.<ref name=Sromovsky2005/> The dark spots common on [[Neptune]] had never been observed on Uranus before 2006, when the first such feature was imaged.<ref name=DarkSpot/>
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| − | [[Image:Uranus Dark spot.jpg|thumb|The first dark spot observed on Uranus. Image obtained by [[Advanced Camera for Surveys|ACS]] on [[Hubble Space Telescope|HST]] in 2006.]]
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| − | The tracking of numerous cloud features allowed determination of [[Zonal and meridional|zonal]] winds blowing in the upper [[troposphere]] of Uranus.<ref name=Sromovsky2005/> 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.<ref name=Sromovsky2005/><ref name=Hammel2005/> Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.<ref name=1986Hanel/><ref name=Sromovsky2005/> 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.<ref name=Sromovsky2005/> Windspeeds at −40° latitude 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.<ref name=Sromovsky2005/> In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.<ref name=Sromovsky2005/><ref name=Hammel2005/><ref name=Hammel2001>{{cite journal|last=Hammel|first=H.B.|coauthors=Rages, K.; Lockwood, G.W.; et.al.|title=New Measurements of the Winds of Uranus|journal=Icarus| volume=153|pages=229–235|year=2001| doi=10.1006/icar.2001.6689| url=http://adsabs.harvard.edu/abs/2001Icar..153..229H}}</ref>
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| − | ===Seasonal variation===
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| − | [[Image:Uranus clouds.jpg|thumb|left|upright|Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible.]]
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| − | For a short period from March to May 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a [[Neptune]]-like appearance.<ref name=Hammel2005b>{{cite journal|last=Hammel|first=H.B.|coauthors=de Pater, I.; Gibbard, S.G.; et.al.|title=New cloud activity on Uranus in 2004: First detection of a southern feature at 2.2 µm|volume=175|year=2005|pages=284–288|doi=10.1016/j.icarus.2004.11.016| url=http://www.llnl.gov/tid/lof/documents/pdf/316113.pdf|format=pdf | journal = Icarus}}</ref><ref>{{cite web|last=Devitt|first=Terry|url=http://www.news.wisc.edu/10402.html|title=Keck zooms in on the weird weather of Uranus|publisher=University of Wisconsin-Madison |year=2004|accessdate=2006-12-24}}</ref> Observations included record-breaking wind speeds of 229 m/s (824 km/h) and a persistent thunderstorm referred to as "Fourth of July fireworks".<ref name=planetary/> 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.<ref name=DarkSpot>{{cite web| url=http://www.physorg.com/pdf78676690.pdf|title=Hubble Discovers a Dark Cloud in the Atmosphere of Uranus|last=Sromovsky|first=L. |coauthors=Fry, P.;Hammel, H.;Rages, K|publisher=physorg.com|accessdate=2007-08-22|format=pdf}}</ref> 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.<ref name=weather /><ref name=Hammel2007/> 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 (astronomy)|Photometry]] over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two [[spectral band]]s, with maxima occurring at the [[solstice]]s and minima occurring at the [[equinox]]es.<ref name=Lockwood2006>{{cite journal|last=Lockwood|first=G.W.|coauthors=Jerzykiewicz, MikoÅ‚aj|title=Photometric variability of Uranus and Neptune, 1950–2004|journal=Icarus| volume=180|pages=442–452|year=2006|doi=10.1016/j.icarus.2005.09.009 |url=http://adsabs.harvard.edu/abs/2006Icar..180..442L}}</ref> A similar periodic variation, with maxima at the solstices, has been noted in [[microwave]] measurements of the deep troposphere begun in the 1960s.<ref name=Klein2006>{{cite journal|last=Klein|first=M.J.|coauthors=Hofstadter, M.D.|title=Long-term variations in the microwave brightness temperature of the Uranus atmosphere|journal=Icarus| volume=184|pages=170–180|year=2006| doi=10.1016/j.icarus.2006.04.012 |url=http://adsabs.harvard.edu/abs/2006Icar..184..170K}}</ref> [[Stratosphere|Stratospheric]] temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.<ref name=Young2001/> The majority of this variability is believed to occur due to changes in the viewing [[geometry]].<ref name=Karkoschka2001>{{cite journal|last=Karkoschka|first=Erich|title=Uranus’ Apparent Seasonal Variability in 25 HST Filters|journal=Icarus| volume=151|pages=84–92|year=2001|doi=10.1006/icar.2001.6599 |url=http://adsabs.harvard.edu/abs/2001Icar..151...84K}}</ref>
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| − | 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.<ref name=Hammel2007>{{cite journal|last=Hammel|first=H.B.|coauthors=Lockwood, G.W.|title=Long-term atmospheric variability on Uranus and Neptune|journal=Icarus|year=2007|volume=186|pages=291–301|doi=10.1016/j.icarus.2006.08.027| url=http://adsabs.harvard.edu/abs/2007Icar..186..291H}}</ref> During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.<ref name=Lockwood2006/> This information implies that the visible pole brightens some time before the solstice and darkens after the [[equinox]].<ref name=Hammel2007/> 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.<ref name=Hammel2007/> Finally in the 1990s, as Uranus moved away from its [[solstice]], [[Hubble Space Telescope|Hubble]] and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),<ref name=Rages2004>{{cite journal|last=Rages|first=K.A.|coauthors=Hammel, H.B.; Friedson, A.J.|title=Evidence for temporal change at Uranus’ south pole|journal=Icarus|volume=172| pages=548–554|year=2004 |doi=10.1016/j.icarus.2004.07.009 |url=http://adsabs.harvard.edu/abs/2004Icar..172..548R}}</ref> while the northern hemisphere demonstrates increasing activity,<ref name=planetary/> such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.<ref name=Hammel2005b/>
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| − | The mechanism of physical changes is still not clear.<ref name=Hammel2007/> Near the summer and winter [[solstice]]s, 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]] [[cloud]]s and [[haze]] layers located in the [[troposphere]].<ref name=Rages2004/> The bright collar at −45° latitude is also connected with methane clouds.<ref name=Rages2004/> Other changes in the southern polar region can be explained by changes in the lower cloud layers.<ref name=Rages2004/> The variation of the [[microwave]] [[Emission (electromagnetic radiation)|emission]] from the planet is probably caused by a changes in the deep tropospheric [[Circulation (fluid dynamics)|circulation]], because thick polar clouds and haze may inhibit convection.<ref name=Hofstadter2003/> Now that the spring and autumn [[equinox]]es are arriving on Uranus, the dynamics are changing and convection can occur again.<ref name=planetary /><ref name=Hofstadter2003>{{cite journal|last=Hofstadter|first=Mark D.|coauthors=and Butler, Bryan J.|title=Seasonal change in the deep atmosphere of Uranus|journal=Icarus|volume=165|pages=168–180| year=2003|doi=10.1016/S0019-1035(03)00174-X |url=http://adsabs.harvard.edu/abs/2003Icar..165..168H}}</ref>
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| − | ==Formation==
| + | |
| − | {{seealso|Nebular hypothesis}}
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| − | Many argue that the differences between the ice giants and the gas giants extend to their formation.<ref name=Thommes1999/><ref name=Brunini1999>{{cite journal|last=Brunini|first=Adrian|coauthors=Fernandez, Julio A.|title=Numerical simulations of the accretion of Uranus and Neptune |journal=Plan. Space Sci.|volume=47|pages=591–605|year=1999|doi=10.1016/S0032-0633(98)00140-8| url=http://adsabs.harvard.edu/abs/1999P%26SS...47..591B}}</ref> 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.<ref name=Thommes1999/><ref name=Brunini1999/> 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.<ref name=Thommes1999/><ref name=Brunini1999/> 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.<ref name=Brunini1999/><ref name=Jewitt2006>{{cite journal|last=Sheppard|first=Scott S.|coauthors=Jewitt, David; Kleyna, Jan|title=An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness|journal=The Astronomical Journal| volume=129|pages=518–525| url=http://arxiv.org/PS_cache/astro-ph/pdf/0410/0410059v1.pdf|format=PDF|year=2006| doi=10.1086/426329}}</ref><ref name=Thommes1999/> 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.<ref name=Thommes1999>{{cite journal|last=Thommes|first=Edward W.|coauthors=Duncan, Martin J.; Levison, Harold F.|title=The formation of Uranus and Neptune in the Jupiter-Saturn region of the Solar System|journal=Nature|volume=402|pages=635–638| url=http://www.boulder.swri.edu/~hal/PDF/un-scat_nature.pdf|year=1999|doi=10.1038/45185|format=pdf}}</ref> However, more recent simulations, which take into account [[planetary migration]], seem to be able to form Uranus and Neptune near their present locations.<ref name=Brunini1999/>
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| − | | + | |
| − | ==Moons==
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| − | {{main|Moons of Uranus}}
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| − | {{seealso|Timeline of discovery of Solar System planets and their natural satellites}}
| + | |
| − | [[Image:Uranian moon montage.jpg|thumb|400px|Major moons of Uranus compared, at their proper relative sizes (montage of [[Voyager 2]] photographs)]]
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| − | | + | |
| − | Uranus has 27 known [[natural satellites]].<ref name=Jewitt2006/> The names for these satellites are chosen from characters from the works of [[Shakespeare]] and [[Alexander Pope]].<ref name=Faure2007/><ref name=Nineplanets>{{cite web |title=Uranus |url=http://www.nineplanets.org/uranus.html|publisher=nineplanets.org |accessdate=2007-07-03}}</ref> The five main satellites are [[Miranda (moon)|Miranda]], [[Ariel (moon)|Ariel]], [[Umbriel (moon)|Umbriel]], [[Titania (moon)|Titania]] and [[Oberon (moon)|Oberon]].<ref name=Faure2007/> 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 (moon)|Triton]] alone.<ref name=Jacobson1992/> 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 (planet)|Saturn]], making Titania the [[List of natural satellites by diameter|eighth largest moon]] in the [[Solar System]]. The moons have relatively low [[albedo]]s; ranging from 0.20 for [[Umbriel (moon)|Umbriel]] to 0.35 for [[Ariel (moon)|Ariel]] (in green light).<ref name=Smith1986>{{cite journal|last= Smith|first=B.A.|coauthors=Soderblom, L.A.; Beebe, A.; et.al. |title=Voyager 2 in the Uranian System: Imaging Science Results|journal=Science|volume=233|pages=97–102| year=1986|url=http://adsabs.harvard.edu/abs/1986Sci...233...43S}}</ref> The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include [[ammonia]] and [[carbon dioxide]].<ref name=Hussmann2006>{{cite journal|last=Hussmann|first=Hauke|coauthors=Sohl, Frank; Spohn, Tilman|title=Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects|journal=Icarus|volume=185|pages=258–273|year=2006|doi=10.1016/j.icarus.2006.06.005| url=http://adsabs.harvard.edu/abs/2006Icar..185..258H}}</ref><ref name=summary/>
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| − | | + | |
| − | Among the satellites, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.<ref name=Smith1986/><ref name=summary /> [[Miranda (moon)|Miranda]] possesses fault canyons 20 kilometers deep, terraced layers, and a chaotic variation in surface ages and features.<ref name=Smith1986/> Miranda's past geologic activity is believed to have been driven by [[tidal heating]] at a time when its orbit was more eccentric than currently, probably as a result of a formerly present 3:1 [[orbital resonance]] with Umbriel.<ref>{{cite journal|last=Tittemore|first=W. C. |coauthors=Wisdom, J.|title=Tidal evolution of the Uranian satellites III. Evolution through the Miranda-Umbriel 3:1, Miranda-Ariel 5:3, and Ariel-Umbriel 2:1 mean-motion commensurabilities|journal=Icarus|volume=85|issue=2|pages=394–443
| + | |
| − | |publisher=Elsevier Science|date=June 1990
| + | |
| − | |url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-473182X-22Y&_coverDate=06%2F30%2F1990&_alid=431841654&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000052082&_version=1&_urlVersion=0&_userid=1234512&md5=d7959dcca75860d54783b9dda43cacba
| + | |
| − | |doi=10.1016/0019-1035(90)90125-S }}</ref> [[Rift|Extensional]] processes associated with upwelling [[diapir]]s are likely the origin of the moon's 'racetrack'-like [[Corona (planetary geology)|coronae]].<ref>{{cite journal|author=[http://science.jpl.nasa.gov/people/Pappalardo/ Pappalardo, R. T.]|coauthors=Reynolds, S. J., Greeley, R.
| + | |
| − | |title=Extensional tilt blocks on Miranda: Evidence for an upwelling origin of Arden Corona|journal=Journal of Geophysical Research|volume=102|issue=E6|pages=13,369–13,380|publisher=Elsevier Science|date= 1997-06-25
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| − | |url=http://www.agu.org/pubs/crossref/1997/97JE00802.shtml }}</ref><ref>{{cite web|last = Chaikin|first = Andrew| authorlink = Andrew Chaikin| title = Birth of Uranus' Provocative Moon Still Puzzles Scientists|work=Space.Com|publisher=ImaginovaCorp.|date = 2001-10-16
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| − | |url = http://www.space.com/scienceastronomy/solarsystem/miranda_creation_011016-1.html|accessdate = 2007-12-07 }}</ref> Similarly, Ariel is believed to have once been held in a 4:1 resonance with Titania.<ref>{{cite journal|title=Tidal Heating of Ariel|last=Tittemore|first=W.C.|journal=Icarus |volume=87 |pages=110–139|year=1990| url=http://adsabs.harvard.edu/abs/1990Icar...87..110T|doi= 10.1016/0019-1035(90)90024-4
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| − | }}</ref>
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| − | | + | |
| − | ==Exploration==
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| − | {{main|Exploration of Uranus}}
| + | |
| − | [[Image:Uranus Final Image.jpg|right|thumb|left|upright|A picture of Uranus taken by [[Voyager 2]] as it headed to Neptune]]
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| − | 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,<ref name=1986Tyler/> discovered 10 new [[Uranus' natural satellites|moons]] and studied the planet's unique weather, caused by its [[axial tilt]] of 97.77°; and examined its [[Rings of Uranus|ring system]].<ref>{{cite web|title=Voyager: The Interstellar Mission: Uranus|work=JPL|url=http://voyager.jpl.nasa.gov/science/uranus.html|year=2004|accessdate=2007-06-09}}</ref><ref name=Smith1986/> It also studied the [[magnetic field]], its irregular structure, its tilt and its unique corkscrew [[magnetosphere|magnetotail]] brought on by Uranus' sideways orientation.<ref name=1986Ness/> 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.<ref name=summary /><ref name=Smith1986/>
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| − | ==See also==
| + | |
| − | {{portal|Solar System|Solar system.jpg}}
| + | |
| − | *[[Uranus in fiction]]
| + | |
| − | *[[Planets in astrology#Uranus|Uranus in astrology]]
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| − | *[[Colonization of the outer Solar System]]
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| − | {{-}}
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| − | | + | |
| − | ==Notes==
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| − | <div class="references-small">
| + | |
| − | <ol type="a">
| + | |
| − | <li>{{Note_label|A|a|none}} Orbital elements refer to the barycenter of the Uranus system, and are the instantaneous [[osculating orbit|osculating]] values at the precise [[J2000]] epoch. Barycenter quantities are given because, in contrast to the planetary center, they do not experience appreciable changes on a day-to-day basis from to the motion of the moons.
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| − | <li>{{Note_label|B|b|none}} Calculated using data from Seidelmann, 2007.<ref name=Seidelmann2007/>
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| − | <li>{{Note_label|C|c|none}} Refers to the level of 1 bar atmospheric pressure.
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| − | <li>{{Note_label|D|d|none}} Calculation of He, H<sub>2</sub> and CH<sub>4</sub> molar fractions is based on a 2.3% mixing ratio of methane to hydrogen and the 15/85 He/H<sub>2</sub> proportions measured at the tropopause.
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| − | <li>{{Note_label|E|e|none}} Mixing ratio is defined as the number of molecules of a compound per a molecule of hydrogen.</li>
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| − | </ol>
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| − | </div>
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| − | | + | |
| − | ==References==
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| − | {{Reflist|2}}
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| − | | + | |
| − | ==External links==
| + | |
| − | {{sisterlinks|Uranus}}
| + | |
| − | * [http://www.eso.org/public/outreach/press-rel/pr-2007/phot-37-07.html Edge On! ESO Press Release]
| + | |
| − | * [http://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html NASA's Uranus fact sheet]
| + | |
| − | * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus Uranus Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
| + | |
| − | * [http://www2.keck.hawaii.edu/news/science/uranus/ 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
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| − | ** ''[http://space.com/scienceastronomy/051222_uranus.html New Moons and Rings found at Uranus]'', [[SPACE.com]]
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| − | ** ''[http://www.msnbc.msn.com/id/10574903/ Two more rings discovered around Uranus]'', [[MSNBC]]
| + | |
| − | * [http://www.projectshum.org/Planets/uranus.html Planets—Uranus] A kid's guide to Uranus.
| + | |
| − | * [http://photojournal.jpl.nasa.gov/targetFamily/Uranus Uranus] at [[Jet Propulsion Laboratory]]'s planetary photojournal.
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| − | * [http://skytonight.com/news/4435217.html Spring Has Sprung on Uranus]
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| − | * [http://www.astronomycast.com/astronomy/episode-62-uranus/ Astronomy Cast: Uranus]
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