Atmosphere of Uranus - Composition

Composition

The composition of the Uranian atmosphere is different from that of Uranus as a whole, consisting mainly of molecular hydrogen and helium. The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium, was determined from the analysis of Voyager 2 far infrared and radio occultation observations. The currently accepted value is 0.152 ± 0.033 in the upper troposphere, which corresponds to a mass fraction 0.262 ± 0.048. This value is very close to the protosolar helium mass fraction of 0.2741 ± 0.0120, indicating that helium has not settled towards the centre of the planet as it has in the gas giants.

The fourth most abundant constituent of the Uranian atmosphere is methane (CH₄), the presence of which has been known for some time as a result of the ground-based spectroscopic observations. Methane possesses prominent absorption bands in the visible and near-infrared, making Uranus aquamarine or cyan in colour. Below the methane cloud deck at 1.3 bar methane molecules account for about 2.3% of the atmosphere by molar fraction; about 10 to 30 times that found in the Sun. The mixing ratio is much lower in the upper atmosphere due to the extremely low temperature at the tropopause, which lowers the saturation level and causes excess methane to freeze out. Methane appears to be undersaturated in the upper troposphere above the clouds having a partial pressure of only 30% of the saturated vapor pressure there. The concentration of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere is poorly known. However, as with methane, their abundances are probably greater than solar values by a factor of at least 20 to 30, and possibly by a factor of a few hundred.

Knowledge of the isotopic composition of Uranus's atmosphere is very limited. To date the only known isotope abundance ratio is that of deuterium to light hydrogen: 5.5+3.5
−1.5 × 10−5, which was measured by the Infrared Space Observatory (ISO) in the 1990s. It appears to be higher than the protosolar value of 2.25 ± 0.35×10−5 measured in Jupiter. The deuterium is found almost exclusively in hydrogen deuteride molecules which it forms with normal hydrogen atoms.

Infrared spectroscopy, including measurements with Spitzer Space Telescope (SST), and UV occultation observations, found trace amounts of complex hydrocarbons in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by solar UV radiation. They include ethane (C₂H₆), acetylene (C₂H₂), methylacetylene (CH₃C₂H), diacetylene (C₂HC₂H). Infrared spectroscopy also uncovered traces of water vapour, carbon monoxide and carbon dioxide in the stratosphere, which are likely to originate from an external source such as infalling dust and comets.