Proposed energy-metabolisms cannot explain the atmospheric chemistry of Venus

TL;DR

This study tests proposed sulfur-based metabolisms for Venusian cloud life and finds they cannot fully explain observed atmospheric chemistry without conflicting with other data.

Contribution

It couples sulfur metabolism hypotheses with photochemical models to evaluate their viability in explaining Venus's atmospheric SO₂ depletion.

Findings

All three metabolisms can cause SO₂ depletion.

Metabolisms violate other observational constraints.

Maximum biomass density is estimated at 10^{-5} to 10^{-3} mg/m^3.

Abstract

Life in the clouds of Venus, if present in sufficiently high abundance, must be affecting the atmospheric chemistry. It has been proposed that abundant Venusian life could obtain energy from its environment using three possible sulfur energy-metabolisms. These metabolisms raise the possibility of Venus's enigmatic cloud-layer SO$_2$-depletion being caused by life. We here couple each proposed energy-metabolism to a photochemical-kinetics code and self-consistently predict the composition of Venus's atmosphere under the scenario that life produces the observed SO$_2$-depletion. Using this photo-bio-chemical kinetics code, we show that all three metabolisms can produce SO$_2$-depletions, but do so by violating other observational constraints on Venus's atmospheric chemistry. We calculate the maximum possible biomass density of sulfur-metabolising life in the clouds, before violating observational constraints, to be $\sim10^{-5}\,-\,10^{-3}\,{\rm mg\,m^{-3}}$. 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.