Water activity in Venus’s uninhabitable clouds and other planetary atmospheres

John E. Hallsworth* (Corresponding Author), Thomas Koop, Tiffany D. Dallas, María-Paz Zorzano, Juergen Burkhardt, Olga V. Golyshina, Javier Martín-Torres, Marcus K. Dymond, Philip Ball, Christopher P. McKay

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

23 Citations (Scopus)


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.
Original languageEnglish
Pages (from-to)665–675
Number of pages10
JournalNature Astronomy
Issue number7
Early online date28 Jun 2021
Publication statusPublished - 31 Jul 2021


  • air microbiology
  • Astrobiology
  • atmospheric chemistry


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