The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere

Abstract

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. Most discussions of an aerial biosphere in the Venus 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 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. 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. (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 Venus atmosphere stagnant lower haze layer (33-48 km altitude). We, thus, call the Venusian lower haze layer a ''depot'' for desiccated microbial life. 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 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 Astrobiology of Alien Worlds: Known and Unknown Forms of Life

“…A related model was suggested by Seager et al [154], who envisioned hydrophilic filaments in addition to the elemental sulfur that could accumulate the critical liquids the microbes would need. They suggested that the life cycle in the Venusian atmosphere would involve drying out of the microorganisms as liquid droplets containing them evaporated during settling.…”

“…And microbes have been shown to be physiologically active and metabolizing in cloud droplets [153], but so far reproduction has not been demonstrated in the aerial habitat, so Earth's atmosphere cannot yet be considered as a permanent habitat for life. However, if it is not, that should not be surprising, because natural selection has likely focused on temporary survival rather than adopting a life cycle permanently sustained in Earth's clouds given that the environmental conditions on Earth's surface are so well-suited for life [154]. Another challenge for clouds on Earth as a permanent habitat is that they are not continuous, so any microorganism will eventually be deposited back on the surface by precipitation within a few days or weeks at most [155].…”

Phosphine gas in the cloud decks of Venus

“…Initial modelling based on terrestrial biochemistry suggests that biochemical reduction of phosphate to PH 3 is thermodynamically feasible under Venus cloud conditions (W. Bains et al, manuscript in preparation). We have also described a possible life cycle for a Venusian aerial biosphere 61 .…”

Life on Venus? Scientists hunt for the truth

“…That led the discovery team to conclude that there must be some mechanism replenishing the gas, hinting at either biological production or an unknown chemical process that scientists cannot yet explain. Researchers have tentatively suggested 3 that in the region of the atmosphere where phosphine was found -away from the crushing pressures and scorching Venus's atmosphere is highly acidic and contains very little water.…”