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 (CH 4 ), 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. 
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