Ferric Chloride: The Venusian Unknown UV Absorber?

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Abstract
Near‐ultraviolet imaging of the planet Venus reveals inhomogeneous absorption features in the otherwise bright clouds. This absorption has been studied for nearly 100 years, but identification of the near‐UV absorber remains one of the largest open questions in Venusian research. Based on a multiple scattering radiative transfer model, the observed absorption between 300 and 600 nm can be produced by 1.0–1.2 wt% ferric chloride (FeCl 3 ) in the mode 1 (∼0.2 μm radius) sulfuric acid cloud droplets—less than the in‐cloud iron mass loading measured during the Venera‐12 mission. We present a novel chemical network within a global chemistry‐climate model, which provides the required source of FeCl 3 in the upper cloud region from the reaction of Fe‐containing molecules, produced by the ablation of cosmic dust particles, with atmospheric HCl. A residence lifetime of ∼3,000 years is required to provide sufficient optical absorption from a purely meteoric source of FeCl 3 .

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