Biologically Available Chemical Energy in the Temperate but Uninhabitable Venusian Cloud Layer: What Do We Want to Know?

The cloud layer has been hypothesized to be the most habitable region of Venus. In the lower clouds, both temperature and pressure fall within bounds that support reproduction of microbial life on Earth, although the water activity of the sulfuric acid cloud droplets makes the clouds uninhabitable to known life. In this study, we carried out an analysis of CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur) elements and potential redox couples in the cloud layer, and we used a microbial energetic growth model to investigate quantitatively the chemical energy available for microbial growth from methanogenesis, sulfate reduction, and hydrogen oxidation at temperatures between 278 and 350 K. The purpose was to improve knowledge of how far the venusian cloud layer comes from being habitable. Hydrogen oxidation was favorable at all temperatures; however, negative Gibbs free energies for sulfate reduction and methanogenesis depended critically on the assumed concentrations of electron donors, acceptors, and products. Improved measurements and the investigation of new molecules will allow us to better assess quantitatively how far Venus comes from possessing a habitable cloud layer and what would need to be different to make it habitable. We identify specific required measurements. These data will advance our understanding of the habitability of planetary atmospheres on extrasolar greenhouse worlds and the habitability of Earth when the planet eventually enters a greenhouse state.