Talk:Uranus/atmosphere

Atmosphere
The outermost part of Uranus' gaseous envelope that is accessible to remote sensing, is called its atmosphere. Remote sensing capability extends down to roughly 300 km below the 1 bar level, with a corresponding pressure around 100 bar and temperature of 320 K. The atmosphere's upper boundary is also not well defined, as coronal gases denser than the background solar system gas are thought to extend tenuously to more than two planetary radii from the surface.

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. The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium, is approximately 0.15 ± 0.03, which corresponds to a mass fraction 0.26 ± 0.05. This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01, indicating that helium has not settled in the centre of the planet as it has in the gas giants.

The third 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 color. There are about 0.02 methane molecules per hydrogen molecule (the so called mixing ratio) below the pressure level of 1.3 bar; about 20 to 30 times that found in the Sun. The mixing ratio 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. 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.

Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause forming haze layers, which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere is significantly lower than in the stratospheres of the other giant planets. This, in addition to weak vertical mixing makes it less opaque and, as a result, colder than on the other giant planets.