Introducing the Venus Collection—Papers from the First Workshop on Habitability of the Cloud Layer

Abstract

We introduce the collection of papers from the first workshop on the habitability of the venusian cloud layer organized by the Roscosmos/IKI-NASA Joint Science Definition Team (JSDT) for Russia's Venera-D mission and hosted by the Space Research Institute in Moscow, Russia, during October 2–5, 2019. The collection also includes three papers that were developed independently of the workshop but are relevant to venusian cloud habitability.

1. Introduction

The Roscosmos/IKI-NASA Joint Science Definition Team (JSDT) for Russia's Venera-D mission held the first workshop in Moscow in October 2019 to discuss the potential habitability of the venusian cloud layer and possible landing site locations. The workshop was organized by the Venera-D JSDT sponsored by the Russian Space Agency (Roscosmos), Space Research Institute (Institut Kosmicheskii Issledovania [IKI], Russian Academy of Sciences), and NASA. The workshop was hosted by IKI as the second half of a 4-day meeting, with the first half devoted to Venera-D landing site options (October 2–5, 2019). This Astrobiology collection focuses on the potential for venusian habitability in the planet's middle atmosphere. The motivation for the workshop came from a suggestion to investigate the nature and identity of the ultraviolet unknown absorbers of incident solar radiation (Pérez-Hoyos et al., 2018; Titov et al., 2018). No satisfactory candidates have emerged for the ultraviolet absorption in the atmosphere (Limaye et al., 2021a); and the possibility of biological contribution to the incident solar radiation by as-yet-unknown absorbers––predominantly at ultraviolet wavelengths––has not actively been explored (Limaye et al., 2018). Similar ideas were stated earlier but without much discussion (Hapke and Nelson, 1975; Shimizu, 1977; Boyer 1986; Grinspoon, 1997; Schulze-Makuch et al., 2004). The possibility of life on Venus was proposed even before the dawn of the space age, when only minimal information was available about the planet's evolution to its current hot, arid surface state. Hypotheses at that time about the conditions and any hints of past water on Venus relied more on similarities to Earth's early evolution. Morowitz and Sagan (1967) were the first to propose the possibility of life in Venus' clouds and suggested that “If small amounts of minerals are stirred up to the clouds from the surface, it is by no means difficult to imagine an indigenous biology in the clouds of Venus.”

The survivability of microorganisms in the venusian clouds was investigated over two decades ago (Cockell, 1999). Recent interest in the possibility of past and present life on Venus arose from an investigation of the planet's climate over time (Way et al., 2016), which indicated that Venus was likely habitable with abundant liquid water on the surface for as long as 3 billion years, but the water was lost within the last 1.5 billion years. A different investigative approach based on geophysical modeling (Weller and Kiefer, 2020) yielded a similar assessment. Given that liquid water likely persisted on the planet's surface for most of its history, there was ample time for life to have emerged on Venus. If it did not emerge, the more difficult question, given our limited understanding of the origins of life on Earth, is why? This is a key question to answer if we want to understand the origin of life on Earth.

The search for life beyond Earth in our solar system began in earnest with Mars and has more recently expanded to include Europa, Enceladus, and Titan. Even the dwarf planet Ceres has been suggested as a target. Venus is generally excluded from such consideration, though new findings and investigations have raised interest in the past and present habitability of Venus. The potential for life to have arisen on Venus and migrating to the clouds once the surface environment became inhospitable is intriguing. Venus' current cloud layer, however, is an extreme environment that consists of acidic droplets with low water activity and unknown bioavailable nutrients. The possibility of extant life under these conditions was the focus of discussion during the Venera-D “Habitability of the Venus Cloud Layer” portion of the workshop.

2. First Venus Cloud Layer Habitability Workshop

The goal of the workshop was to seek broader scientific input for investigating the potential habitability of the venusian cloud layer including the potential biological nature of the ultraviolet solar radiation absorbers in the venusian atmosphere, one of the Venera-D science objectives. Key presentations and papers centered on this topic were held and are reflected in this issue of Astrobiology. This collection of papers provides a scientific foundation for advancing astrobiology research on Venus and guides assessment of the habitability of exoplanets via the study of Venus (Kane et al., 2019; Limaye et al., 2021b). A list of all the presentations given at the workshop can be found at http://venera-d.cosmos.ru/uploads/media/Habitability.zip.

3. Workshop Presentations

Approximately 50 scientists and students attended the combined Venera-D Landing Sites and Habitability workshop, which expanded over 4 days. Twenty-two presentations were given in the Venus Cloud Habitability workshop and included the following.

Michael Way (NASA/GISS, New York, New York, USA) discussed how climate modeling allows for greater understanding of Venus' water history (Way et al., 2016; Way and Del Genio 2020). For ancient Venus to have been habitable, it would have required surface liquid water and a slow rotational rate. In situ measurements could provide support for the presence of liquid water in the past and corroborate such climate modeling predictions.

Shawn Domagal-Goldman (NASA/GSFC, Greenbelt, Maryland, USA) discussed the significance of understanding Venus to learn more about exoplanet habitability, extraterrestrial planet evolution, and biosignature detection. He proposed a systems-based approach for Venus exploration that would seek to understand the planet as a global entity, an approach that has been proven to be highly effective for Earth.

Sanjay Limaye (University of Wisconsin, Madison, Wisconsin, USA) discussed the mysterious solar radiation absorbers present in the venusian clouds and the possibility that such absorbers could be biotic. Though several abiotic explanations have been postulated, none can explain the observed radiation absorption. Further understanding of terrestrial aerobiology and the chemical composition of Venus' atmosphere and clouds are necessary to assess the habitability of the venusian cloud layer.

Oleg Kotsyurbenko (Yugra State University, Khanty-Mansiysk, Russia) presented a state-of-the-art review of habitability-relevant parameters in the venusian cloud layer and the types of microorganisms that could survive in such conditions. Extremophilic microbes that thrive in various extreme environments on Earth may contain metabolic, biochemical, and physiological properties similar to those that would be required to survive in the clouds of Venus.

David Grinspoon (Planetary Science Institute, Washington, DC, USA) discussed the potential for habitable conditions in the venusian cloud layer within the context of what is known about Venus' planetary history. He also discussed terrestrial microbe capabilities to survive and reproduce in aerosolized droplets despite the presence of similar stressors to those found on Venus.

Vladimir Kompanichenko (Institute for Complex Analysis, Russian Academy of Sciences, Birobidzhan, Russia) presented the concept of “thermodynamic inversion” for the origin of life and its applications for Venus. He argued that plate tectonics and volcanism could introduce environmental oscillations needed to maintain a venusian cloud-based ecosystem.

Valeriy Snytnikov (Boreskov Institute of Catalysis, Russian Academy of Sciences, Novosibirsk, Russia) presented a potential chemical basis for life at the surface of Venus, based upon the formation of complex organic compounds with iron-containing catalysts.

Rakesh Mogul (CalPoly, Pomona, California, USA) outlined a putative photosynthetic system that is consistent with Venus' environmental conditions, along with the minimum constraints for photosynthesis in the venusian cloud layer.

Arif Ansari (Birbal Sahni Institute of Palaeosciences, Lucknow, India) illustrated how the concept of habitability has expanded via the study of extremophiles in a wide variety of environments on Earth and provided justification for why the venusian cloud layer is a compelling target in the search for life.

Tetyana Milojevic (University of Vienna, Vienna, Austria) discussed the potential for metallophilic extreme thermo-acidophiles to exist on Venus and the ways in which laboratory-based experiments investigating such terrestrial species can inform future interpretations of samples collected on Venus.

Anatoli Pavlov (Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia) discussed the potential for a panspermic transfer of microbes from Earth to Venus to have established a venusian biosphere. Though terrestrial microbes would have encountered multiple severe environmental conditions during transfer, it is possible that some could have survived the journey and adapted to a niche on Venus.

Margarita Kruchkova (Lomonosov Moscow State University, Moscow, Russia) described an experiment that investigated microbial (prokaryotic and fungal) survivorship under Venus-like conditions and showed evidence that prokaryotic species exhibit resistance to such conditions, while fungal species exhibit greater stability under such conditions. Results from the study indicate that living organisms could survive the environmental conditions of the simulated venusian cloud layer.

Grzegorz Słowik (University of Zielona Gora, Zielona Gora, Poland) argued that acidophilus bacteria may have the capability to survive conditions within the venusian cloud layer and could be sustained by ferrous iron and pyrite oxidation.

Mark Bullock (Science and Technology Corp., Boulder, Colorado, USA) discussed the need to further characterize Venus' gas and aerosol chemistry, arguing that a great deal of Venus astrobiology work could be done by understanding these characteristics.

Mary Voytek (NASA HQ, Washington, DC, USA) outlined NASA's astrobiology strategy and delineated the environmental requirements for known life and knowledge gaps inherent in assessing the habitability of the venusian cloud layer, both of which are important considerations for any potential life-detection missions to the planet.

David J. Smith (NASA/Ames, Moffett Field, California, USA) presented an overview of the current methods in bioaerosol collection and characterization in Earth's troposphere and stratosphere. At the present time, it is difficult to study life in Earth's clouds, and all analyses rely on sample return to the lab. With sustained investment and development, in situ aerobiology techniques could be ready for implementation on Earth and Venus in 10–15 years.

Diana Gentry (NASA/Ames, Moffett Field, California, USA) outlined habitability analysis and life-detection considerations for applications on a putative Venus astrobiology mission based on lessons learned from terrestrial analogs (notably, Earth's stratosphere). She also presented recommendations for enhancing life-detection strategies for Venus.

Satoshi Sasaki (Tokyo University of Technology, Tokyo, Japan) presented plans for a Life Signature Detection Microscope, a novel bioaerosol sampling and imaging approach under development for potential use on Venus. The microscope is designed to collect bioaerosols on an impactor plate and image them with fluorescence microscopy.

Kevin Baines (Jet Propulsion Laboratory, Pasadena, California, USA) presented an overview of the Quadrupole Ion Trap Mass Spectrometer (QITMS), an instrument that could characterize aerosols and gases of the venusian cloud layer.

Jaime Cordova (University of Wisconsin, Madison, Wisconsin, USA) discussed potential laboratory experiments that could advance the design of investigative approaches for biosignature detection in the venusian clouds.

Anastasia Kosenkova (Lavochkin Association, Moscow, Russia) presented the current Venera-D mission architecture, and Jason Rabinovitch (Jet Propulsion Laboratory, Pasadena, California, USA) presented aerobot concepts for atmospheric studies on Venus.

4. Findings from the Venus Cloud Habitability Workshop

The Venera-D workshop attendees' general consensus was that ancient Venus indeed may have been habitable for a long time. If life arose and evolved on Venus, fossils would most likely have been buried and destroyed under extensive lava flows that occurred in the last few hundred million years. Though there are a few locations where ancient crust is exposed at the surface of Venus, such as in the tesserae formations (oldest crustal remains), evidence of fossil life may be unrecognizable in such regions due to the extensive deformation they may have experienced. The venusian cloud layer, extending from 48 to about 70 km above the surface, is a survivable environment for extremophiles (Cockell, 1999). Even though the water availability is low in the acidic middle cloud layer on Venus, on Earth bacteria have been found to survive in relatively similar environments. The attendees concluded that the possibility of life finding a niche in the clouds after the surface became uninhabitable cannot be excluded. It was further suggested that future missions should search for biosignatures and that technology developments will need to be considered for capable aerial platforms that are maneuverable and long-lived so as to investigate the clouds. If this aerial platform were to be coupled with long-lived platforms for surface measurements, such a mission architecture would address some of these intriguing science questions.

5. The Venus Collection

This issue of Astrobiology comprises eight papers on topics discussed in this workshop, two of which consider recent reports of phosphine on Venus. The collection is ordered not by publication date but follows a progressive investigative approach.

Limaye et al. (2021b) present arguments for including Venus among the Solar System objects considered for astrobiology exploration as an overview. Many of these are discussed in the subsequent papers.

Kotsyurbenko et al. (2021) discuss the habitable environment of the cloud layer, based on the presence of a solvent for biochemical reactions, appropriate physicochemical conditions, available energy, and biologically relevant elements.

Seager et al. (2021) expand on the possibility of extant microorganisms (Limaye et al., 2018) in the cloud layer and propose a life cycle that suggests that the haze layer detected below the lower cloud layer is formed from desiccated spores and supports the life cycle as some spores get lofted back into the cloud layer.

Cockell et al. (2021) discuss the biologically available chemical energy in the clouds, which are deemed to be uninhabitable due to extremely low water activity at present. Based on a comparison with Earth, these authors explore how far venusian clouds are from habitability under physical and chemical considerations in light of terrestrial experiences.

Mogul et al. (2021a) report that phototrophy is possible in the venusian clouds based on radiative transfer calculations for scattering and absorption of downwelling solar radiation in the clouds. They also consider alternative interpretations of refractive index and radio occultation data that are supportive of the clouds being habitable.

Milojevic et al. (2021) discuss the bioavailability of phosphorous, based on the elemental detection from X-ray fluorescence measurements from the VeGa probes. Phosphorous is a key element for life on Earth, but fluorescence measurements have only indicated its presence on Venus in some unknown chemical molecule, which is also of interest due to a report of the possible presence of phosphine in the venusian atmosphere and its potential biological origins (Greaves et al., 2020c). This prompted two papers in the collection immediately after the phosphorous-related report, as follows. Omran et al. (2021) consider two abiotic sources for the phosphine—external (meteoritic) and geologic (volcanoes). This is countered, however, by Bains et al. (2021b), who discuss the results of a photochemical model of the venusian clouds and argue that the presence of phosphine in the atmosphere reported by Greaves et al. (2020c) cannot be a result of known conventional processes. Milojevic et al. (2021) also consider sources of phosphorous in the atmosphere.

Izenberg et al. (2021) present an equation for calculating the probability of life on Venus using heuristic arguments that indicate a nonzero probability for life on present-day Venus. The collection ends with a paper by Baines et al. (2021), who discuss an aerosol and gas composition with an aerosol sampling instrument that can be deployed from an aerial platform in the venusian cloud layer and can potentially detect water activity and trace species.

6. Recent Developments

Since the 2019 Venera-D Venus Cloud Habitability workshop and concurrent with the development of this collection of papers, new relevant analyses and observations have been reported. Phosphine, proposed as a potential biosignature for exoplanets (Sousa-Silva et al., 2020), was reported as having been discovered in the venusian atmosphere (Greaves et al., 2020a, 2020b, 2020c, 2021). A spirited debate about the detection (Encrenaz et al., 2020; Snellen et al., 2020; Akins et al., 2021; Lincowski et al., 2021) has ensued. Much of the debate has centered on the analysis of the observations, the altitude range over which phosphine was detected, the instability of phosphine in the hydrogen-poor venusian atmosphere, and the lack of known mechanisms of its production (Bains et al., 2021b). Meanwhile, re-interpretation of the Pioneer Venus Large Probe Neutral Mass Spectrometer (LNMS) data (Mogul et al., 2021b) also indicates the presence of phosphine in the middle cloud layer, along with other species such as nitrous acid, hydrogen cyanide, and ethane that indicate local chemical disequilibrium. Chemical disequilibrium conditions are compatible with the presence of life (Baum, 2018; Krissansen-Totton et al., 2016, 2018), and many disequilibria have been found in the venusian atmosphere. Molecular oxygen was discovered by the Pioneer Venus Large Probe (Oyama et al., 1980), and numerous nitrogen-bearing compounds have been inferred from reanalysis of the LNMS data (Mogul et al., 2021b).

The presumed low water activity in the venusian clouds has been considered a challenge for life (Cockell et al., 2021), though Bains et al. (2021a) argue that concentrated sulfuric acid is a possible solvent for life. The reported detection of phosphine by Greaves et al. (2020c) and the suggestions that it may have biological origins prompted Hallsworth et al. (2021) to consider the water activity from the scant measurements of water vapor abundances and assumed cloud droplet composition (dilute sulfuric acid). They concluded that some Earth life, which has been detected in water-scarce environments with water activity of 0.585, is not likely to be present in the venusian clouds where the inferred water activity based on the cloud model is much too low, which suggests that any extant life on Venus must be quite different from that on Earth.

Several issues relevant to their calculation of water activity on Venus are worth noting. First, the assumed water vapor is not likely to be representative of niches in the clouds since water vapor cannot be globally homogeneous (neither are the absorbers) and is likely temporally variable. Second, the sulfuric acid concentration in the middle and lower clouds has never been directly measured, but only inferred, from available measurements of sulfuric acid vapor and assumptions of cloud droplets. It has been suspected for some time that other chemicals are present in the cloud droplets based on other observations, such as the glory feature (Markiewicz et al., 2014; Petrova, 2018) and from interpretations of the SO₂ altitude profile (Rimmer et al., 2021) and mass spectrometer data (Mogul et al., 2021a). Rimmer et al. (2021) suggest the presence of hydrides to explain the depletion of sulfur in the clouds, while Mogul et al. (2021a) suggest the presence of ammonia salts. The effect of these “contaminants” in the cloud droplets is to lower the sulfuric acid concentration.

Recent selections of new missions to Venus by NASA and ESA, along with the announcement of Rocketlab to send a small probe to sample a small layer of the venusian clouds, are indicative of a growing interest in Venus. As of now, the missions to be launched between the years 2023 and 2032 include orbiters with surface mapping radars (ISRO Venus orbiter, VERITAS, and EnVision), two atmospheric probes (Rocketlab-Venus Life Finder and DAVINCI), and an atmospheric orbiter with large and long-lived small landers (Venera-D). More missions are likely to follow them in this decade and beyond (Rocketlab, Akatsuki 2 from JAXA) as interest in past and present life on Venus grows.

7. Summary

The quest for evaluating the habitability of the present-day cloud cover stems not just from the possible presence of life in the past but also because we do not have enough information about the clouds and their myriad of mysteries. They include the cloud contrasts caused by unknown absorbers that have been observed over the last half century for which two dozen candidates have been proposed, and rejected, for the ultraviolet absorption alone (Limaye et al., 2021a). This initial collection of papers from the first workshop to discuss the potential habitability of venusian clouds encompasses our current understanding of key issues and lays the foundation for further research. Besides an assessment of atmospheric trace constituents; cloud/aerosol properties; and confirmation of the past presence, duration, and amount of liquid water in the past, the transition from clement to inclement conditions should be explored along with the potential for life to have existed during the 1.5 billion years that liquid water has been inferred to be on Venus. If life did not originate or exist in the early history of Venus, an understanding of “why not?” is relevant, not only to elucidate Venus' history but also for understanding exoplanets that orbit beyond our solar system in the “Venus zone” (Kane et al., 2019).

A second workshop on Venus cloud habitability will be held in 2021, from November 29 through December 3, in a hybrid virtual and in-person format. This workshop will be hosted by the Space Research Institute in Moscow, Russia.