When is chemical disequilibrium in Earth-like planetary atmospheres a biosignature versus an anti-biosignature? Disequilibria from dead to living worlds

Abstract

Chemical disequilibrium in exoplanetary atmospheres (detectable with remote spectroscopy) can indicate life. The modern Earth's atmosphere-ocean system has a much larger chemical disequilibrium than other solar system planets with atmospheres because of oxygenic photosynthesis. However, no analysis exists comparing disequilibrium on lifeless, prebiotic planets to disequilibrium on worlds with primitive chemotrophic biospheres that live off chemicals and not light. Here, we use a photochemical-microbial ecosystem model to calculate the atmosphere-ocean disequilibria of Earth with no life and with a chemotrophic biosphere. We show that the prebiotic Earth likely had a relatively large atmosphere-ocean disequilibrium due to the coexistence of water and volcanic H2, CO2, and CO. Subsequent chemotrophic life probably destroyed nearly all of the prebiotic disequilibrium through its metabolism, leaving a likely smaller disequilibrium between N2, CO2, CH4, and liquid water. So, disequilibrium fell with the rise of chemotrophic life then later rose with atmospheric oxygenation due to oxygenic photosynthesis. We conclude that big prebiotic disequilibrium between H2 and CO2 or CO and water is an anti-biosignature because these easily metabolized species can be eaten due to redox reactions with low activation energy barriers. However, large chemical disequilibrium can also be a biosignature when the disequilibrium arises from a chemical mixture with biologically insurmountable activation energy barriers, and clearly identifiable biogenic gases. Earth's modern disequilibrium between O2, N2, and liquid water along with minor CH4 is such a case. Thus, the interpretation of disequilibrium requires context. With context, disequilibrium can be used to infer dead or living worlds.

arXiv commentThis draft was accepted by Astrophysical Journal on 2/29/20

References in corpus (16)

• A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
• Biosignatures from Earth-Like Planets Around M Dwarfs
• Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model
• The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth
• Finding extraterrestrial life using ground-based high-resolution spectroscopy
• Exoplanet Biosignatures: A Framework for Their Assessment
• Abiotic O${}_2$ Levels on Planets around F, G, K, and M Stars: Possible False Positives for Life?
• Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life
• Exoplanet Biosignatures: Observational Prospects
• Habitable worlds with JWST: transit spectroscopy of the TRAPPIST-1 system?
• Detectability of biosignatures in anoxic atmospheres with the James Webb Space Telescope: A TRAPPIST-1e case study
• On detecting biospheres from chemical thermodynamic disequilibrium in planetary atmospheres
• False negatives for remote life detection on ocean-bearing planets: Lessons from the early Earth
• Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System
• Optimizing Ground-based Observations of O2 in Earth Analogs
• A Maximum Subsurface Biomass on Mars from Untapped Free Energy: CO and H2 as Potential Antibiosignatures

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• The Fundamental Connections Between the Solar System and Exoplanetary Science
• Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach
• Potential long-term habitable conditions on planets with primordial H-He atmospheres
• The Venus Life Equation
• Alternative Methylated Biosignatures I: Methyl Bromide, A Capstone Biosignature
• Inferring Chemical Disequilibrium Biosignatures for Proterozoic Earth-Like Exoplanets
• Controls of Atmospheric Methane on Early Earth and Inhabited Earth-like Terrestrial Exoplanets
• UV Spectral Characterization of Low-Mass Stars With AstroSat UVIT for Exoplanet Applications: The Case Study of HIP 23309