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 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. Key Words: Venus— Clouds—Life—Habitability—Sulfuric Acid—Life Cycle—Aerial Biosphere. Astrobiology 21, xxx–xxx. 1. Introduction and Overview 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). 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. How the clouds could become inhabited is not known. 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). Life may even have been directly seeded by impacts from Earth ejecta (Melosh, 1988; Reyes-Ruiz et al., 2012; Beech et al., 2018). A more commonly agreed on, and perhaps more conceivable scenario, is that life originated on the surface, as it most likely did on Earth, and migrated into the clouds. Recent modeling by Way et al. (2016) suggests the existence of habitable surface and oceans as late as *700 Mya. Consideration of life on Venus is extensively summarized in a recent paper (Limaye et al., 2018). Almost all previous work on life in Venusian clouds does not specify what exactly ‘‘life in the clouds’’ means (for one exception see Schulze-Makuch et al., 2004). Does microscopic life reside inside cloud liquid droplets? Or is microscopic life free floating in the air between cloud droplets? Earth has an aerial biosphere created by microbial life that regularly migrates to clouds from the ground (Vaı¨tilingom et al., 2012; Amato et al., 2017). Studies of Earth’s aerial biosphere shows that microbes mostly reside inside cloud droplets but some are free-floating in the atmosphere (Section 4.1). Some microbes (both inside and outside droplets) are found to be metabolically active, even though there is no evidence, as of yet, of cell division (Amato et al., 2019). Microbial life cannot reside in Earth’s atmosphere indefinitely, mostly because of lack of continuous cloud cover, and after days to weeks microbes are deposited back down on Earth’s surface (Burrows et al., 2009; Bryan et al., 2019). Earth’s aerial biosphere is intimately connected to the habitable surface of the planet. We argue that life, if it exists in Venus’ atmosphere, must reside inside cloud liquid droplets for the majority of its life cycle (Section 2.1). Life engulfed by cloud droplets will be protected from a fatal net loss of liquid to the atmosphere, an unavoidable problem for any free-floating microbial lifeforms. But a droplet habitat implies a lifetime limitation. As liquid droplets coalesce and grow, they eventually reach a size that, due to gravity, settles out of the temperate layers of the atmosphere at an appreciable rate. Over time, the population of inhabited droplets should therefore decline to zero. (Droplet fragmentation—which would reduce particle size—does not occur in Venusian atmosphere conditions [Section 3.4].) The conundrum is that there is no way for liquid droplets—and hence life inside of them—to persist indefinitely in Venus’ temperate atmosphere layers. In this article, we describe this problem, and a solution to it. We hypothesize how life escapes being ‘‘rained out’’ down to inhospitably hot atmosphere layers by cycling between small, desiccated spores and larger, metabolically active, droplet-inhabiting cells. Venusian life escapes settling to the surface by forming a resistant, spore-like form that survives the evaporation of the inevitable downward droplet flow to atmosphere layers of high temperature. The desiccated spores become suspended in the Venus atmosphere lower haze layer, which we thus call a ‘‘depot’’ for desiccated microbial life. Because the dynamics of the relatively stagnant lower haze layer are not well known, the main uncertainty in our life cycle hypothesis is how the spores are transported back up into the clouds again. The spores most likely travel upward by vertical mixing induced by gravity waves, and once in the cloud layer they form the nucleus of a new droplet. The depot is ‘‘leaky,’’ that is spores will also vertically mix downward to atmosphere layers with fatally high temperatures. Our proposed life cycle includes cell division that occurs in the larger droplets, and sporulation for individual cells, enabling cell numbers to be replenished against loss. In this work, we use the terms ‘‘microbial life’’ or ‘‘microbes’’ for microscopic life, without intending to imply that hypothetical Venusian microbes might in any way be taxonomically related to microbial life on Earth. We use the term ‘‘spore’’ to denote a cell in a dormant state of longterm metabolic inactivity, which is further resistant to (and protected from) environmental stresses. We begin with a highlighted review on the very harsh and inhospitable conditions in the Venusian atmosphere and related, required assumptions for life to exist (Section 2). We next present our Venusian life cycle hypothesis (Section 3), which optimistically assumes that the challenges described in Section 2 can be met. We put the hypothesis in the context with Earth’s aerial biosphere and other characteristics of the Venusian hypothesized aerial biosphere in Section 4. We conclude with a summary in Section 5. This paper is available on arxiv under CC BY 4.0 DEED license. 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. 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 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. Key Words: Venus— Clouds—Life—Habitability—Sulfuric Acid—Life Cycle—Aerial Biosphere. Astrobiology 21, xxx–xxx. 1. Introduction and Overview 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). 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. How the clouds could become inhabited is not known. 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). Life may even have been directly seeded by impacts from Earth ejecta (Melosh, 1988; Reyes-Ruiz et al., 2012; Beech et al., 2018). A more commonly agreed on, and perhaps more conceivable scenario, is that life originated on the surface, as it most likely did on Earth, and migrated into the clouds. Recent modeling by Way et al. (2016) suggests the existence of habitable surface and oceans as late as *700 Mya. Consideration of life on Venus is extensively summarized in a recent paper (Limaye et al., 2018). Almost all previous work on life in Venusian clouds does not specify what exactly ‘‘life in the clouds’’ means (for one exception see Schulze-Makuch et al., 2004). Does microscopic life reside inside cloud liquid droplets? Or is microscopic life free floating in the air between cloud droplets? Earth has an aerial biosphere created by microbial life that regularly migrates to clouds from the ground (Vaı¨tilingom et al., 2012; Amato et al., 2017). Studies of Earth’s aerial biosphere shows that microbes mostly reside inside cloud droplets but some are free-floating in the atmosphere (Section 4.1). Some microbes (both inside and outside droplets) are found to be metabolically active, even though there is no evidence, as of yet, of cell division (Amato et al., 2019). Microbial life cannot reside in Earth’s atmosphere indefinitely, mostly because of lack of continuous cloud cover, and after days to weeks microbes are deposited back down on Earth’s surface (Burrows et al., 2009; Bryan et al., 2019). Earth’s aerial biosphere is intimately connected to the habitable surface of the planet. We argue that life, if it exists in Venus’ atmosphere, must reside inside cloud liquid droplets for the majority of its life cycle (Section 2.1). Life engulfed by cloud droplets will be protected from a fatal net loss of liquid to the atmosphere, an unavoidable problem for any free-floating microbial lifeforms. But a droplet habitat implies a lifetime limitation. As liquid droplets coalesce and grow, they eventually reach a size that, due to gravity, settles out of the temperate layers of the atmosphere at an appreciable rate. Over time, the population of inhabited droplets should therefore decline to zero. (Droplet fragmentation—which would reduce particle size—does not occur in Venusian atmosphere conditions [Section 3.4].) The conundrum is that there is no way for liquid droplets—and hence life inside of them—to persist indefinitely in Venus’ temperate atmosphere layers. In this article, we describe this problem, and a solution to it. We hypothesize how life escapes being ‘‘rained out’’ down to inhospitably hot atmosphere layers by cycling between small, desiccated spores and larger, metabolically active, droplet-inhabiting cells. Venusian life escapes settling to the surface by forming a resistant, spore-like form that survives the evaporation of the inevitable downward droplet flow to atmosphere layers of high temperature. The desiccated spores become suspended in the Venus atmosphere lower haze layer, which we thus call a ‘‘depot’’ for desiccated microbial life. Because the dynamics of the relatively stagnant lower haze layer are not well known, the main uncertainty in our life cycle hypothesis is how the spores are transported back up into the clouds again. The spores most likely travel upward by vertical mixing induced by gravity waves, and once in the cloud layer they form the nucleus of a new droplet. The depot is ‘‘leaky,’’ that is spores will also vertically mix downward to atmosphere layers with fatally high temperatures. Our proposed life cycle includes cell division that occurs in the larger droplets, and sporulation for individual cells, enabling cell numbers to be replenished against loss. In this work, we use the terms ‘‘microbial life’’ or ‘‘microbes’’ for microscopic life, without intending to imply that hypothetical Venusian microbes might in any way be taxonomically related to microbial life on Earth. We use the term ‘‘spore’’ to denote a cell in a dormant state of longterm metabolic inactivity, which is further resistant to (and protected from) environmental stresses. We begin with a highlighted review on the very harsh and inhospitable conditions in the Venusian atmosphere and related, required assumptions for life to exist (Section 2). We next present our Venusian life cycle hypothesis (Section 3), which optimistically assumes that the challenges described in Section 2 can be met. We put the hypothesis in the context with Earth’s aerial biosphere and other characteristics of the Venusian hypothesized aerial biosphere in Section 4. We conclude with a summary in Section 5. This paper is available on arxiv under CC BY 4.0 DEED license. This paper is available on arxiv under CC BY 4.0 DEED license. available on arxiv

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Life in Venusian Clouds: A Hypothesis for Microbial Survival and Life Cycle

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