Editing Uranus (section)

=== 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" />