![]() In principle, life could arise in the clouds independent from the ground (Woese, 1979 Dobson et al., 2000) with material from meteoritic input (Sleep, 2018a, 2018b) from the asteroid belt (including Ceres, and even from Mars). How the clouds could become inhabited is not known. Today, only Venus' atmospheric cloud layers (a large region spanning from 48 to 60 km altitude) have seemingly habitable conditions-the surface (at 735 K) is too hot for any plausible solvent and for most organic covalent chemistry. Life on Venus has been a topic of speculation for more than half a century, with published papers ranging from science-fiction-like to invalid conjecture to legitimate hypothesis (Morowitz and Sagan, 1967 Grinspoon, 1997 Cockell, 1999 Schulze-Makuch and Irwin, 2002, 2006 Schulze-Makuch et al., 2004 Grinspoon and Bullock, 2007 Limaye et al., 2018). ![]() We also review the challenges for life in the extremely harsh conditions of the venusian atmosphere, refuting the notion that the “habitable” cloud layer has an analogy in any terrestrial environment. The spores eventually return to the cloud layer by upward diffusion caused by mixing induced by gravity waves, act as cloud condensation nuclei, and rehydrate for a continued life cycle. We, thus, call the venusian lower haze layer a “depot” for desiccated microbial life. (Droplet fragmentation-which would reduce particle size-does not occur in venusian atmosphere conditions.) We propose for the first time that the only way life can survive indefinitely is with a life cycle that involves microbial life drying out as liquid droplets evaporate during settling, with the small desiccated “spores” halting at, and partially populating, the venusian atmosphere stagnant lower haze layer (33–48 km altitude). However, the droplet habitat poses a lifetime limitation: Droplets inexorably grow (over a few months) to large enough sizes that are forced by gravity to settle downward to hotter, uninhabitable layers of the venusian atmosphere. We argue that life must reside inside liquid droplets such that it will be protected from a fatal net loss of liquid to the atmosphere, an unavoidable problem for any free-floating microbial life forms. Most discussions of an aerial biosphere in the venusian atmosphere temperate layers never address whether the life-small microbial-type particles-is free floating or confined to the liquid environment inside cloud droplets. ![]() We revisit the hypothesis that there is life in the venusian clouds to propose a life cycle that resolves the conundrum of how life can persist aloft for hundreds of millions to billions of years.
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