Optical properties of Venusian clouds

The cause of the inhomogeneous absorption observed in the Venusian clouds at near-ultraviolet wavelengths has been a significant question in the field of Venusian research for decades. Many possible chemical species have been proposed as the cause of the so-called “unknown UV absorption”, but none have been conclusively proven to be able to explain the observations. In this work, two candidates are considered: OSSO and ferric chloride (FeCl3).
Cis-OSSO and trans-OSSO, two isomers of S2O2, are expected to be produced in the Venusian mesosphere by the third body recombination reaction of two SO molecules. Their combined predicted absorption cross-sections have been reported to provide a good match to the spectrum of the unknown absorber, but 1D models suggest only low concentrations would be produced. 3D chemical and dynamical modelling was carried out to investigate the concentrations of OSSO likely in the Venusian atmosphere. The injection of additional SO from ablation of meteoric material was also modelled. The resulting concentrations were used in a radiative transfer model (SOCRATES), and the OSSO found to be able to explain less than 1% of the observed near-UV absorption.
FeCl3 has been proposed to explain the absorption if present in the micron-radius cloud droplets. However, representative absorption spectra of FeCl3 are not available in the literature. Laboratory experiments to measure the absorbance of ferric chloride in sulphuric acid were performed, and the absorption spectrum measured over time. The resulting spectra were used in the SOCRATES model, and the observations satisfactorily reproduced by 2 – 3 wt% FeCl3 in the sub-micron (“mode 1”) cloud droplets. By comparison with the predicted chemical formation and accumulation of FeCl3 in the atmosphere from the reaction of Fe ablated from cosmic dust with gas-phase HCl and Cl, ~40 % of the observed absorption can be explained by FeCl3.