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.

最近金星大气中磷化氢的暗示重新激发了人们对云中生命这一想法的兴趣。然而，此类分析通常忽略了水分活度在宜居性中的作用，水分活度是衡量水相对可用性的指标。在这里，我们通过对温度和水汽丰度的观测来计算金星和其他太阳系行星云中的水分活度。我们发现构成金星云大部分的硫酸液滴的水分活度值≤0.004，比已知极端微生物的 0.585 限值低两个数量级。考虑到其他行星，火星上结冰的水活度≤0.537，略低于可居住范围，而条件在生物学上是允许的（>0.00）。585）在木星的云层中（尽管其他因素，例如它们的成分可能会限制它们的可居住性）。相比之下，地球的对流层条件通常在生物学上是允许的，而在平流层中部之上，大气变得过于干燥，无法活跃生命。当前研究中使用的方法也可以应用于太阳系外行星。