The ultraviolet dark clouds are an ephemeral phenomenon in the Venus atmosphere, apparently just near the limit of stability. The UV dark material is moderately abundant, perhaps 10%, since the contrasts between light and dark material are some 20%. The material should absorb light between 3000–4000 Å, and ideally should also have visible absorptions in the blue, as the overall spectral albedo of Venus indicates. Such a material is bromine dissolved in hydrobromic acid. Solar radiation near 2500 Å is sufficient to partially photolyze HBr into Br2. HBr in the Venus atmosphere is inferred to have a mixing ratio of 10−4 versus CO2. With a water vapor mixing ratio of 10−3, droplets of hydrobromic acid are possible. These droplets would eventually evaporate in the drier upper atmosphere. The refractive index of these hydrobromic acid droplets of 52% (by weight) composition is 1.46, within the limits set by polarization studies of Venus.

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