Water activity in Venus's uninhabitable clouds and other planetary atmospheres
The recent suggestion of phosphine in Venus's atmosphere has regenerated interest in the idea of life in clouds. However, such analyses usually neglect the role of water activity, which is a measure of the relative availability of water, in habitability. Here we compute the water activity within the clouds of Venus and other Solar System planets from observations of temperature and water-vapour abundance. We find water-activity values of sulfuric acid droplets, which constitute the bulk of Venus's clouds, of ≤0.004, two orders of magnitude below the 0.585 limit for known extremophiles. Considering other planets, ice formation on Mars imposes a water activity of ≤0.537, slightly below the habitable range, whereas conditions are biologically permissive (>0.585) at Jupiter's clouds (although other factors such as their composition may play a role in limiting their habitability). By way of comparison, Earth's troposphere conditions are, in general, biologically permissive, whereas the atmosphere becomes too dry for active life above the middle stratosphere. The approach used in the current study can also be applied to extrasolar planets. ; We are grateful to S. L. Clegg (University of East Anglia, England, UK) for helpful discussions on the use of the E-AIM at low water activity and the provision of some code; C. S. Cockell (University of Edinburgh, Scotland, UK), D. Y. Sorokin (Winogradsky Institute of Microbiology, Russia) and A. Ventosa (University of Seville, Spain) for providing information about thermotolerance of halophiles; M. S. Marley (NASA Ames Research Center, CA, USA) for information on Jupiter and exoplanets; A. Méndez (University of Puerto Rico, Puerto Rico) for inputs relating to analysis of Earth's atmosphere; J. R. Lobry (University of Lyons, France) who helped with use of the cardinal pH model; N. J. Tosca (University of Cambridge, England, UK) for discussions about thermodynamic properties of aqueous sulfuric acid solutions; and E. L. J. Watkin (Curtin University, Australia) who provided information about stress tolerance of Acidihalobacter. J.E.H. was funded by the Biotechnology and Biological Sciences Research Council (BBSRC, United Kingdom) project BBF003471; M.-P.Z. was supported by projects PID2019-104205GB-C21 of Ministry of Science and Innovation and MDM-2017-0737 Unidad de Excelencia 'María de Maeztu'- Centro de Astrobiología (CSIC-INTA) (Spain); and O.V.G. was supported by the Centre of Environmental Biotechnology Project (grant 810280) funded by the European Regional Development Fund (ERDF) through the Welsh Government.