Microbial Life on Venus Requires a Unique Life Cycle to Overcome Extreme Conditions

Authors: (1) Sara Seager, Departments of Earth, Atmospheric and Planetary Sciences, Physics, Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; (2) Janusz J. Petkowski, Department of Earth; (3) Peter Gao, Department of Astronomy, University of California at Berkeley, California, USA; (4) William Bains, Department of Earth; (5) Noelle C. Bryan, Department of Earth; (6) Sukrit Ranjan, Department of Earth; (7) Jane Greaves, School of Physics and Astronomy, Cardiff University, Cardiff, United Kingdom and Institute of Astronomy, Cambridge University, Cambridge, United Kingdom. Table of Links Abstract and 1. Introduction and Overview Challenges and Assumptions for Life in the Venusian Clouds A Proposed Cycle for Venusian Aerial Microbial Life Discussion Summary, Acknowledgments, Author Disclosure Statement, Funding Information, and References 5. Summary Life in the Venusian clouds has long since been a popular if speculative topic. The clouds decks of Venus themselves are often described as conductive to life. We reassessed this notion and reviewed the severe and unique environmental challenges that life in the aerial biosphere of Venus would have to overcome. The challenges include: an extremely acidic environment, far more so than any known environment on Earth; very low water content; and nutrient scarcity. We also highlighted the assumption that life would have to reside inside protective cloud droplets (sulfuric acid mixed with water), and that any life would have to be photosynthetic to have enough energy for a variety of cellular processes. The main new point of this work is to present a life cycle concept. Assuming that life must reside inside cloud droplets, we resolve the subsequent conundrum of gravitationally settling droplets reaching hotter, uninhabitable regions by proposing a Venusian life cycle where a critical step is microbes drying out to become spores on reaching the relatively stagnant lower haze layer, which we call a leaky ‘‘depot.’’ The dried out spores would reside there until some of them can be transported back up to the temperate, habitable cloud layers, where they would act as CCN to promote cloud formation, becoming enveloped in cloud droplets to continue the life cycle. Acknowledgments The authors thank Joanna Petkowska-Hankel for the preparation of Figs. 1, 4, and 5. They also thank Daniel Koll for useful discussions. Author Disclosure Statement No competing financial interests exist. Funding Information The authors thank the Change Happens Foundation, the Heising-Simons Foundation, and the MIT Professor Amar G. Bose Research Grant Program for funding. 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Authors: Authors: (1) Sara Seager, Departments of Earth, Atmospheric and Planetary Sciences, Physics, Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; (2) Janusz J. Petkowski, Department of Earth; (3) Peter Gao, Department of Astronomy, University of California at Berkeley, California, USA; (4) William Bains, Department of Earth; (5) Noelle C. Bryan, Department of Earth; (6) Sukrit Ranjan, Department of Earth; (7) Jane Greaves, School of Physics and Astronomy, Cardiff University, Cardiff, United Kingdom and Institute of Astronomy, Cambridge University, Cambridge, United Kingdom. Table of Links Abstract and 1. Introduction and Overview Abstract and 1. Introduction and Overview Challenges and Assumptions for Life in the Venusian Clouds A Proposed Cycle for Venusian Aerial Microbial Life Discussion Summary, Acknowledgments, Author Disclosure Statement, Funding Information, and References Challenges and Assumptions for Life in the Venusian Clouds Challenges and Assumptions for Life in the Venusian Clouds A Proposed Cycle for Venusian Aerial Microbial Life A Proposed Cycle for Venusian Aerial Microbial Life Discussion Discussion Summary, Acknowledgments, Author Disclosure Statement, Funding Information, and References Summary, Acknowledgments, Author Disclosure Statement, Funding Information, and References 5. Summary Life in the Venusian clouds has long since been a popular if speculative topic. The clouds decks of Venus themselves are often described as conductive to life. We reassessed this notion and reviewed the severe and unique environmental challenges that life in the aerial biosphere of Venus would have to overcome. The challenges include: an extremely acidic environment, far more so than any known environment on Earth; very low water content; and nutrient scarcity. We also highlighted the assumption that life would have to reside inside protective cloud droplets (sulfuric acid mixed with water), and that any life would have to be photosynthetic to have enough energy for a variety of cellular processes. The main new point of this work is to present a life cycle concept. Assuming that life must reside inside cloud droplets, we resolve the subsequent conundrum of gravitationally settling droplets reaching hotter, uninhabitable regions by proposing a Venusian life cycle where a critical step is microbes drying out to become spores on reaching the relatively stagnant lower haze layer, which we call a leaky ‘‘depot.’’ The dried out spores would reside there until some of them can be transported back up to the temperate, habitable cloud layers, where they would act as CCN to promote cloud formation, becoming enveloped in cloud droplets to continue the life cycle. Acknowledgments The authors thank Joanna Petkowska-Hankel for the preparation of Figs. 1, 4, and 5. They also thank Daniel Koll for useful discussions. Author Disclosure Statement No competing financial interests exist. Funding Information The authors thank the Change Happens Foundation, the Heising-Simons Foundation, and the MIT Professor Amar G. Bose Research Grant Program for funding. 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