On Detecting Biospheres from Chemical Thermodynamic Disequilibrium in Planetary Atmospheres
Open accessAbstract
Atmospheric chemical disequilibrium has been proposed as a method for detecting extraterrestrial biospheres from exoplanet observations. Chemical disequilibrium is potentially a generalized biosignature since it makes no assumptions about particular biogenic gases or metabolisms. Here, we present the first rigorous calculations of the thermodynamic chemical disequilibrium in Solar System atmospheres, in which we quantify the available Gibbs energy: the Gibbs free energy of an observed atmosphere minus that of atmospheric gases reacted to equilibrium. The purely gas phase disequilibrium in Earth's atmosphere is mostly attributable to O2 and CH4. The available Gibbs energy is not unusual compared to other Solar System atmospheres and smaller than that of Mars. However, Earth's fluid envelope contains an ocean, allowing gases to react with water and requiring a multiphase calculation with aqueous species. The disequilibrium in Earth's atmosphere-ocean system (in joules per mole of atmosphere) ranges from ∼20 to 2 × 10(6) times larger than the disequilibria of other atmospheres in the Solar System, where Mars is second to Earth. Only on Earth is the chemical disequilibrium energy comparable to the thermal energy per mole of atmosphere (excluding comparison to Titan with lakes, where quantification is precluded because the mean lake composition is unknown). Earth's disequilibrium is biogenic, mainly caused by the coexistence of N2, O2, and liquid water instead of more stable nitrate. In comparison, the O2-CH4 disequilibrium is minor, although kinetics requires a large CH4 flux into the atmosphere. We identify abiotic processes that cause disequilibrium in the other atmospheres. Our metric requires minimal assumptions and could potentially be calculated from observations of exoplanet atmospheres. However, further work is needed to establish whether thermodynamic disequilibrium is a practical exoplanet biosignature, requiring an assessment of false positives, noisy observations, and other detection challenges. Our Matlab code and databases for these calculations are available, open source.
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When Earth's entire fluid envelope (the atmosphere-ocean reservoir) is considered, the disequilibrium is very large (2326 J/mol) due to the coexistence of O2, N2, and liquid water.
“In contrast, when the Earth’s entire fluid envelope (the atmosphere-ocean reservoir) is considered, the disequilibrium is very large (2326 J/mol) due to the coexistence of O2, N2, and liquid water.”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
The biogenic disequilibrium from O2, N2, and liquid water is potentially detectable on exoplanets similar to the modern Earth.
“Krissansen-Totton et al. (25) discuss how this biogenic disequilibrium is potentially detectable on exoplanets similar to the modern Earth.”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
Earth's purely gas-phase disequilibrium, as measured by available Gibbs free energy, is 1.5 J/mol of atmosphere and largely attributable to the O2-CH4 thermodynamic disequilibrium.
“Earth’s purely gas-phase disequilibrium, as measured by available Gibbs free energy, is 1.5 J/mol of atmosphere and largely attributable to the O2-CH4 thermodynamic disequilibrium.”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
O2, N2, and liquid water should react to form nitric acid in thermodynamic equilibrium.
“These three species should react to form nitric acid in thermodynamic equilibrium.”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
The abiotically produced disequilibria of all other solar system planets are much less than 1 J/mol.
“The abiotically produced disequilibria of all the other solar system planets are ≪1 J/mol (25).”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
The Phanerozoic Earth range was determined by varying initial oxygen mixing ratios from 0.1 to 0.3, the range inferred for the Phanerozoic.
“the Phanerozoic Earth range was determined by using the abundances from (25) but varying initial oxygen mixing ratios from 0.1 to 0.3, which is the range inferred for the Phanerozoic (30).”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
The modern Earth's N2-O2-H2O disequilibrium is relatively insensitive to ocean composition and ocean volume.
“In the study of Krissansen-Totton et al. (25), we showed the modern Earth’s N2-O2-H2O disequilibrium is relatively insensitive to these variables.”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
There is a large disequilibrium between organic carbon and ferric iron in the crust that far exceeds the disequilibrium in the fluid reservoir.
“There is also a large disequilibrium between organic carbon and ferric iron in the crust, both of which have accumulated over time from photosynthesis and hydrogen escape, that far exceeds the disequilibrium in the fluid reservoir (25).”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
Atmospheric N2 persists on Venus because it is in equilibrium with the CO2-dominated atmosphere and there is no mechanism to draw down N2 into the crust because of the lack of water.
“Atmospheric N2 persists on Venus because it is in equilibrium with the CO2-dominated atmosphere (25) and there is no mechanism to draw down N2 into the crust because of the lack of water.”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
The biogenic disequilibrium in Earth's crustal reservoir is several orders of magnitude larger than that of the atmosphere-ocean system.
“We therefore expect the biogenic disequilibrium in Earth’s crustal reservoir to be several orders of magnitude larger than that of the atmosphere-ocean system (25).”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
If life disappeared from Earth, lightning would convert atmospheric N2 and O2 to nitrate, depleting atmospheric oxygen in 20 to 200 Ma.
“In (25), we presented a calculation to show that if life disappeared from Earth, lightning would convert atmospheric N2 and O2 to nitrate, depleting atmospheric oxygen in 20 to 200 Ma.”
From Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life (2018) · cites this paper as [25]
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- Authors
- Krissansen-Totton, J., Bergsman, D. S., & Catling, D. C.
- Affiliations
- Department of Earth and Space Sciences/Astrobiology Program, University of Washington, Seattle, Washington.
- Stanford University (US)
- University of Washington (US)
- Venue
- Astrobiology
- Publisher
- SAGE Publications
- Published
- 2016-01
- Volume
- 16
- Issue
- 1
- Pages
- 39-67
- ISSN
- 1531-1074 (ISSN-L)
- 1557-8070
- OpenAlex
- W1566045705
- Cited by
- 132
- References
- 146
- Citations by year
- 2026: 5 · 2025: 9 · 2024: 15 · 2023: 10 · 2022: 16 · 2021: 11 · 2020: 9 · 2019: 14 · 2018: 25 · 2017: 11 · 2016: 5 · 2015: 1
- FWCI
- 8.50
- Type
- journal article
- Language
- en
- Open access
- green
- arxiv.org
- Locations
- Astrobiology (publishedVersion)
- PubMed (publishedVersion)
- arXiv (Cornell University) (submittedVersion)
- arXiv (Cornell University)
- Topics
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Space Science and Extraterrestrial Life
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Planetary Science and Exploration
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astro and Planetary Science
- Keywords
- chemical nonequilibrium, Gibbs free energy, biosignatures, exoplanet atmospheres, planetary atmospheres, Earth's atmosphere, Mars atmosphere, Titan, aqueous chemistry, abiotic processes, astrobiology
- MeSH terms
- Atmosphere, Extraterrestrial Environment, Models, Theoretical, Thermodynamics, Reproducibility of Results, Planets, Exobiology
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