Iron-sulfur chemistry can explain the ultraviolet absorber in the clouds of Venus

The clouds of Venus are believed to be composed of sulfuric acid (H₂SO₄) and minor constituents including iron-bearing compounds, and their respective concentrations vary with height in the thick Venusian atmosphere. This study experimentally investigates possible iron-bearing mineral phases that are stable under the unique conditions within Venusian clouds. Our results demonstrate that ferric iron can react with sulfuric acid to form two mineral phases: rhomboclase [(H₅O₂)Fe(SO₄)₂·3H₂O] and acid ferric sulfate [(H₃O)Fe(SO₄)₂]. A combination of these two mineral phases and dissolved Fe³⁺ in varying concentrations of sulfuric acid are shown to be good candidates for explaining the 200- to 300-nm and 300- to 500-nm features of the reported unknown UV absorber. We, therefore, hypothesize a rich and largely unexplored heterogeneous chemistry in the cloud droplets of Venus that has a large effect on the optical properties of the clouds and the behavior of trace gas species throughout Venus's atmosphere.