The composition of the Uranian atmosphere is different from the rest of the planet, consisting as it does mainly of molecular hydrogen and helium.[12] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03[15] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.[12][64] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,[68] indicating that helium has not settled in the center of the planet as it has in the gas giants.[12] The third most abundant constituent of the Uranian atmosphere is methane (CH4).[12] Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.[12] 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 (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.[12][14][69] The mixing ratio[h] is much lower in the upper atmosphere owing to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.[70] 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.[12][71] Along with methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[72] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).[70][73][74] 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 comets.[73][74][75]
Troposphere
The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.[12] The temperature falls from about 320 K at the base of the nominal troposphere at -300 km to 53 K at 50 km.[66][69] 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.[12][63] The tropopause region is responsible for the vast majority of Uranus's thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.[63][64]
The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar (5 to 10 MPa), ammonium hydrosulfide clouds in the range of 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 1 to 2 bar (0.1 to 0.2 MPa).[12][14][66][76] The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes.
Troposphere
The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.[12] The temperature falls from about 320 K at the base of the nominal troposphere at -300 km to 53 K at 50 km.[66][69] 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.[12][63] The tropopause region is responsible for the vast majority of Uranus's thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.[63][64]
The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar (5 to 10 MPa), ammonium hydrosulfide clouds in the range of 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 1 to 2 bar (0.1 to 0.2 MPa).[12][14][66][76] The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes.
