Metabolic Signatures of an Aerial Biosphere in the Clouds of Venus: A Self-Consistent Photo-Bio-Chemical Model

Life in the clouds of Venus, if present, has been proposed to extract energy from its environment using sulfur-based metabolisms. These metabolisms link life to the chemistry of Venus's atmosphere and thus provide testable predictions of life's presence given current observations. In particular, these hypothetical metabolisms raise the possibility of Venus's enigmatic cloud-layer SO2-depletion being explained by life. We couple each proposed metabolic pathway to a photochemical-kinetics code and self-consistently predict the composition of Venus's atmosphere under the scenario that life produces the observed SO2-depletion. Using this photo-bio-chemical kinetics code, we show that all three metabolisms produce SO2-depletions which violate other observational constraints on Venus's atmospheric chemistry. For each metabolism, we estimate the maximum potential biomass density in the cloud layer before the observational constraints are violated. Our analysis shows that either the observed SO2-depletion is due to a currently unknown metabolism, or there is not a high-mass biosphere in Venus's clouds. The methods employed are equally applicable to aerial biospheres on Venus-like exoplanets, planets that are optimally poised for atmospheric characterisation in the near-future.