Residence Time and Trajectory as Constraints on Cloud-Based Habitability

Rocky, geologically active worlds with water, including Venus, are often considered potential origins for Earthlike biochemistries. They are also some of the likeliest worlds to have clouds.Yet the requirements for cloud habitability – the potential for aerobiospheres – is rarely studied. Earth's clouds carry significant numbers of active microbes (Amato et al., 2017), which affect planetary weather(as condensation and ice nuclei), climate (alteration of cloud and surface albedo), and water and air chemistry (through metabolic processing) (Delort et al., 2010).However, we have not yet observed airborne microbial reproduction in the field, and one likely reason is that microbes suspended in clouds on Earth have residence times at best equivalent to typical generation times – hours to days.