Composition of Uranian Atmosphere
The planets of our solar system from innermost (the closest to the Sun) to outermost are respectively Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Based on their positions, Mercury, Venus, Earth and Mars are considered as inner planets while Jupiter, Saturn, Uranus and Neptune are regarded as outer planets. Based on their compositions, inner planets are terrestrial or rocky planets meaning they are mostly composed of silicate rocks or metals. With different chemical compositions in comparison to inner planets, outer planets are either gas giants or ice giants. Jupiter and Saturn are gas giants meaning they are giant planets mostly composed of hydrogen and helium. Uranus and Neptune are ice giants meaning they are giant planets mostly composed of ices where here ices refer to compounds such as water, ammonia and methane which have elements heavier than hydrogen and helium and can be considered as volatiles when compared to silicate rocks or metals.
Uranus, the seventh planet from the Sun, among other planets of the Solar System has the third rank in radius after Jupiter and Saturn and has the fourth rank in mass after Jupiter, Saturn and Neptune. Its radius and mass are respectively about 4 and 14.5 times as large as those of Earth. Uranus is believed to be composed of a rocky (silicate/iron–nickel) core in the centre, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The outer gaseous envelope of Uranus or Uranian atmosphere is mainly composed of molecular hydrogen (about 82.5% by volume) and helium (about 15.2% by volume) gases. The next abundant constituent of Uranian atmosphere is methane gas (2.3% by volume). Uranian atmosphere is also considered to have less volatile compounds such as ammonia, water and hydrogen sulfide in its deep layers with poorly known amounts, traces of water vapour, carbon monoxide and carbon dioxide in the upper atmosphere possibly from external sources such as infalling dust and comets, and trace amounts of various hydrocarbons such as ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H) as a result of photolysis of methane by ultraviolet (UV) radiation from the Sun. Uranian atmosphere also contains layers of clouds where uppermost clouds of Uranian atmosphere are made of methane. Aquamarine or cyan color of Uranus is attributed to absorption bands of methane (CH4) in the visible and near-infrared (IR) regions of electromagnetic spectrum.