Editing Uranus (section)

== Atmosphere ==
{{main|Atmosphere of Uranus}}
Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope that is accessible to remote sensing is called its [[atmosphere]].<ref name="Lunine 1993" /> Remote-sensing capability extends down to roughly 300&nbsp;km below the {{convert|1|bar|kPa|abbr=on}} level, with a corresponding pressure around {{convert|100|bar|MPa|abbr=on}} and temperature of {{convert|320|K|C F}}.<ref name="de Pater Romani et al. 1991" /> The tenuous [[thermosphere]] extends over two planetary radii from the nominal surface, which is defined to lie at a pressure of 1 bar.<ref name="Herbert Sandel et al. 1987" /> The Uranian atmosphere can be divided into three layers: the [[troposphere]], between altitudes of {{convert|-300|and|50|km|mi|abbr=on}} and pressures from 100 to 0.1&nbsp;bar (10&nbsp;MPa to 10&nbsp;kPa); the [[stratosphere]], spanning altitudes between {{convert|50|and|4000|km|mi|abbr=on}} and pressures of between {{nowrap|0.1 and 10<sup>−10</sup>&nbsp;bar}} (10&nbsp;kPa to 10&nbsp;[[micropascal|μPa]]); and the thermosphere extending from 4,000&nbsp;km to as high as 50,000&nbsp;km from the surface.<ref name="Lunine 1993" /> There is no [[mesosphere]].

=== Composition ===
[[File:Tropospheric profile Uranus new.svg|thumb|400x400px|Diagram of the Uranus atmosphere's composition and layers, along with the graph of its pressure]]
The composition of Uranus's atmosphere is different from its bulk, consisting mainly of [[molecular hydrogen]] and helium.<ref name="Lunine 1993" /> The helium [[Gas composition|molar fraction]], i.e. the number of helium [[atom]]s per molecule of gas, is {{val|0.15|0.03}}<ref name="Conrath Gautier et al. 1987" /> in the upper troposphere, which corresponds to a mass fraction {{val|0.26|0.05}}.<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" /> This value is close to the protosolar helium mass fraction of {{val|0.275|0.01}},<ref name="Lodders 2003" /> indicating that helium has not settled in its centre as it has in the gas giants.<ref name="Lunine 1993" /> The third-most-abundant component of Uranus's atmosphere is methane ({{chem2|CH4}}).<ref name="Lunine 1993" /> Methane has prominent [[absorption band]]s in the [[visible spectrum|visible]] and [[near-infrared]] (IR), making Uranus [[aquamarine (color)|aquamarine]] or [[cyan]] in colour.<ref name="Lunine 1993" /> Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of {{convert|1.3|bar|kPa|abbr=on}}; this represents about 20 to 30&nbsp;times the carbon abundance found in the Sun.<ref name="Lunine 1993" /><ref name="Lindal Lyons et al. 1987" /><ref name="Tyler 1986" />

The mixing ratio{{efn | Mixing ratio is defined as the number of molecules of a compound per a molecule of hydrogen. }} 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="Bishop Atreya et al. 1990" /> The abundances of less volatile compounds such as ammonia, water, and [[hydrogen sulfide]] in the deep atmosphere are poorly known. They are probably also higher than solar values.<ref name="Lunine 1993" /><ref name="de Pater Romani et al. 1989" /> Along with methane, trace amounts of various [[hydrocarbon]]s are found in the stratosphere of Uranus, which are thought to be produced from methane by [[photolysis]] induced by the solar [[ultraviolet]] (UV) radiation.<ref name="Summers & Strobel 1989" /> They include [[ethane]] ({{chem2|C2H6}}), [[acetylene]] ({{chem2|C2H2}}), [[methylacetylene]] ({{chem2|CH3C2H}}), and [[diacetylene]] ({{chem2|C2HC2H}}).<ref name="Bishop Atreya et al. 1990" /><ref name="Burgdorf Orton et al. 2006" /><ref name="Encrenaz 2003" /> Spectroscopy has also uncovered traces of water vapour, [[carbon monoxide]], and [[carbon dioxide]] in the upper atmosphere, which can only originate from an external source such as infalling dust and [[comet]]s.<ref name="Burgdorf Orton et al. 2006" /><ref name="Encrenaz 2003" /><ref name="Encrenaz Lellouch et al. 2004" />

=== Troposphere ===
The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude.<ref name="Lunine 1993" /> The temperature falls from about {{convert|320|K|C F|0}} at the base of the nominal troposphere at −300&nbsp;km to {{convert|53|K|C F|0}} at 50&nbsp;km.<ref name="de Pater Romani et al. 1991" /><ref name="Tyler 1986" /> The temperatures in the coldest upper region of the troposphere (the [[tropopause]]) actually vary in the range between {{convert|49|and|57|K|C F|0}} depending on planetary latitude.<ref name="Lunine 1993" /><ref name="Hanel Conrath et al. 1986" /> The tropopause region is responsible for the vast majority of Uranus's thermal [[far infrared]] emissions, thus determining its [[effective temperature]] of {{convert|59.1|±|0.3|K|C F|1}}.<ref name="Hanel Conrath et al. 1986" /><ref name="Pearl Conrath et al. 1990" />

The troposphere is thought to have a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of {{convert|50|to|100|bar|MPa|sigfig=1|abbr=on}}, [[ammonium hydrosulfide]] clouds in the range of {{convert|20|to|40|bar|MPa|sigfig=1|abbr=on}}, ammonia or [[hydrogen sulfide]] clouds at between {{convert|3|and|10|bar|MPa|sigfig=1|abbr=on}} and finally directly detected thin methane clouds at {{convert|1|to|2|bar|MPa|sigfig=1|abbr=on}}.<ref name="Lunine 1993" /><ref name="Lindal Lyons et al. 1987" /><ref name="de Pater Romani et al. 1991" /><ref name="Atreya Wong 2005" /> The troposphere is a dynamic part of the atmosphere, exhibiting strong winds, bright clouds, and seasonal changes.<ref name="Sromovsky & Fry 2005" />

=== Upper atmosphere ===
[[File:Adding to Uranus's legacy.tif|thumb|Uranus's upper atmosphere imaged by HST during the Outer Planet Atmosphere Legacy (OPAL) observing program.<ref>{{cite web |title=Adding to Uranus's legacy |url=https://www.spacetelescope.org/images/potw1906a/ |url-status=live |archive-url=https://web.archive.org/web/20190212011708/https://www.spacetelescope.org/images/potw1906a/ |archive-date=12 February 2019 |access-date=11 February 2019 |website=www.spacetelescope.org |language=en}}</ref>]]
The middle layer of the Uranian atmosphere is the [[stratosphere]], where temperature generally increases with altitude from {{convert|53|K|C F|0}} in the [[tropopause]] to between {{convert|800|and|850|K|C F|0}} at the base of the thermosphere.<ref name="Herbert Sandel et al. 1987" /> The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other [[hydrocarbon]]s,<ref name="Young et al. 2001" /> which form in this part of the atmosphere as a result of methane [[photolysis]].<ref name="Summers & Strobel 1989" /> Heat is also conducted from the hot thermosphere.<ref name="Young et al. 2001" /> The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 300&nbsp;km corresponding to a pressure range of 1,000 to 10&nbsp;Pa and temperatures of between {{convert|75|and|170|K|C F|0}}.<ref name="Bishop Atreya et al. 1990" /><ref name="Burgdorf Orton et al. 2006" />

The most abundant hydrocarbons are methane, [[acetylene]], and [[ethane]] with [[mixing ratio]]s of around {{10^|-7}} relative to hydrogen. The mixing ratio of [[carbon monoxide]] is similar at these altitudes.<ref name="Bishop Atreya et al. 1990" /><ref name="Burgdorf Orton et al. 2006" /><ref name="Encrenaz Lellouch et al. 2004" /> Heavier hydrocarbons and [[carbon dioxide]] have mixing ratios three orders of magnitude lower.<ref name="Burgdorf Orton et al. 2006" /> The abundance ratio of water is around 7{{e|-9}}.<ref name="Encrenaz 2003" /> Ethane and acetylene tend to condense in the colder lower part of the stratosphere and tropopause (below 10&nbsp;mBar level) forming haze layers,<ref name="Summers & Strobel 1989" /> which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.<ref name="Bishop Atreya et al. 1990" /><ref name="Herbert & Sandel 1999" />
[[File:PIA25951-Uranus-NorthPole-Cyclone-October2021.jpg|thumb|left|250px|Planet Uranus – North Pole – Cyclone ([[Very Large Array|VLA]]; October 2021)]]
The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature of around {{convert|800|K|C}} to {{convert|850|K|C}}.<ref name="Lunine 1993" /><ref name="Herbert & Sandel 1999" /> The heat sources necessary to sustain such a high level are not understood, as neither the solar UV nor the [[aurora (astronomy)|auroral]] activity can provide the necessary energy to maintain these temperatures. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1&nbsp;mBar pressure levels may contribute too.<ref name="Herbert Sandel et al. 1987" /><ref name="Herbert & Sandel 1999" /> In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass and high temperatures explain why the corona extends as far as {{convert|50000|km|mi|abbr=on}}, or two Uranian radii, from its surface.<ref name="Herbert Sandel et al. 1987" /><ref name="Herbert & Sandel 1999" />

This extended corona is a unique feature of Uranus.<ref name="Herbert & Sandel 1999" /> Its effects include a [[drag (physics)|drag]] on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.<ref name="Herbert Sandel et al. 1987" /> The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the [[ionosphere]] of Uranus.<ref name="Tyler 1986" /> Observations show that the ionosphere occupies altitudes from {{convert|2000|to|10000|km|mi|abbr=on}}.<ref name="Tyler 1986" /> 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="Herbert & Sandel 1999" /><ref name="Trafton Miller et al. 1999" /> The ionosphere is mainly sustained by solar UV radiation and its density depends on the [[Space weather|solar activity]].<ref name="Encrenaz Drossart et al. 2003" /> [[Auroral]] activity is insignificant as compared to Jupiter and Saturn.<ref name="Herbert & Sandel 1999" /><ref name="Lam Miller et al. 1997" />