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Abstract
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.