Ferric chloride: the Venusian unknown UV absorber?

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 (FeCl₃) 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 FeCl₃ 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 FeCl₃.