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.
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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.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Abel Méndez, Dirk Schulze-Makuch and Nicholas Tosca for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary Figs. 1 and 2, text on biophysical limits of terrestrial microbes, text relating to the equilibration of droplets, text for Fig. 3, text relating to validation of water activity for H2SO4–H2O mixtures, text for Fig. 6b, and references.
Supplementary Table 1a: water-vapour-pressure data (in torr) as a function of temperature and sulfuric acid concentrations in liquid H2SO4–H2O mixtures. Supplementary Table 1b: water-activity data as a function of temperature and sulfuric acid concentrations in liquid H2SO4–H2O mixtures. Supplementary Table 2a: water-activity data as a function of temperature and sulfuric acid concentrations in liquid H2SO4–H2O mixtures. Supplementary Table 2b: water-activity data as a function of temperature and sulfuric acid concentrations in liquid H2SO4–H2O mixtures from Wilson22. Supplementary Table 3: properties and typical conditions for major cloud types in Earth’s atmosphere.
Source Data for Fig. 1 (water activity of liquid H2SO4–H2O mixtures as a function of temperature and sulfuric acid concentration).
Source Data for Fig. 2 (water activity of the Venusian atmosphere).
Source Data for Fig. 4 (water activity in the Jovian atmosphere).
Source Data for Fig. 5 (average water-vapour mixing ratio profiles in Earth’s atmosphere).
Source Data for Fig. 6a (relative humidity and water-activity ranges at different altitudes in Earth’s atmosphere).
Source Data for Fig. 7 (generalized exoplanet water-activity analysis).
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Hallsworth, J.E., Koop, T., Dallas, T.D. et al. Water activity in Venus’s uninhabitable clouds and other planetary atmospheres. Nat Astron 5, 665–675 (2021). https://doi.org/10.1038/s41550-021-01391-3
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