Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life

Abstract

Chemical disequilibrium in planetary atmospheres has been proposed as a generalized method for detecting life on exoplanets through remote spectroscopy. Among solar system planets with substantial atmospheres, the modern Earth has the largest thermodynamic chemical disequilibrium due to the presence of life. However, how this disequilibrium changed over time and, in particular, the biogenic disequilibria maintained in the anoxic Archean or less oxic Proterozoic eons are unknown. We calculate the atmosphere-ocean disequilibrium in the Precambrian using conservative proxy- and model-based estimates of early atmospheric and oceanic compositions. We omit crustal solids because subsurface composition is not detectable on exoplanets, unlike above-surface volatiles. We find that (i) disequilibrium increased through time in step with the rise of oxygen; (ii) both the Proterozoic and Phanerozoic may have had remotely detectable biogenic disequilibria due to the coexistence of O₂, N₂, and liquid water; and (iii) the Archean had a biogenic disequilibrium caused by the coexistence of N₂, CH₄, CO₂, and liquid water, which, for an exoplanet twin, may be remotely detectable. On the basis of this disequilibrium, we argue that the simultaneous detection of abundant CH₄ and CO₂ in a habitable exoplanet's atmosphere is a potential biosignature. Specifically, we show that methane mixing ratios greater than 10^-3 are potentially biogenic, whereas those exceeding 10^-2 are likely biogenic due to the difficulty in maintaining large abiotic methane fluxes to support high methane levels in anoxic atmospheres. Biogenicity would be strengthened by the absence of abundant CO, which should not coexist in a biological scenario.

Fig. 1. Schematic of methodology for calculating atmosphere-ocean disequilibrium.

We quantify the disequilibrium of the atmosphere-ocean system by calculating the difference in Gibbs energy between the initial and final states. The species in this particular example show the important reactions to produce equilibrium for the Phanerozoic atmosphere-ocean system, namely, the reaction of N₂, O₂, and liquid water to form nitric acid, and methane oxidation to CO₂ and H₂O. Red species denote gases that change when reacted to equilibrium, whereas green species are created by equilibration. Details of aqueous carbonate system speciation are not shown.

Fig. 2. The evolution of Earth’s atmosphere-ocean disequilibrium through time, as measured by available Gibbs free energy.

The blue shaded regions show the evolution of Earth’s atmosphere-ocean disequilibrium. The wide ranges in the Archean and Proterozoic span our minimum and maximum disequilibrium scenarios. The large ranges are attributable to uncertainties in the atmospheric composition in each eon, mainly uncertain Pch₄ in the Archean and uncertain Po₂ in the Proterozoic. The two shadings for the Proterozoic represent different assumptions about atmospheric oxygen levels that represent divergent views in the current literature. Darker blue denotes Po₂ > 2% PAL (present atmospheric level), whereas lighter blue denotes Po₂ < 2% PAL. We calculate a secular increase in Earth’s atmosphere-ocean disequilibrium over Earth history, correlated with the history of atmospheric oxygen. The black dashed line shows the upper bound of the Earth’s atmosphere-only disequilibrium through time. We also include the modern (photochemically produced) disequilibria of Mars (red dashed) and Titan (blue dashed) for comparison (25). The abiotically produced disequilibria of all the other solar system planets are ≪1 J/mol (25).

Fig. 3. Atmosphere-ocean disequilibrium in the Proterozoic (maximum disequilibrium scenario).

Blue bars denote assumed initial abundances from the literature, and green bars denote equilibrium abundances calculated using Gibbs free energy minimization. Subplots separate (A) atmospheric species and (B) ocean species. The most important contribution to Proterozoic disequilibrium is the coexistence of atmospheric oxygen, nitrogen, and liquid water. These three species are lessened in abundance by reaction to equilibrium to form aqueous H⁺ and ${\text{NO}}_3^{-}$. Changes in carbonate speciation caused by the decrease in ocean pH also contribute to the overall Gibbs energy change.

Fig. 4. Atmosphere-ocean disequilibrium in the Archean (maximum disequilibrium scenario).

Blue bars denote assumed initial abundances from the literature, and green bars denote equilibrium abundances calculated using Gibbs free energy minimization. Subplots separate (A) atmospheric species and (B) ocean species. The most important contribution to Archean disequilibrium is the coexistence of atmospheric CH₄, N₂, CO₂, and liquid water. These four species are lessened in abundance by reaction to equilibrium to form aqueous ${\text{HCO}}_3^{-}$ and ${\text{NH}}_4^{+}$. Oxidation of CO and H₂ also contributes to the overall Gibbs energy change.

Fig. 5. Sensitivity of Archean disequilibrium to bicarbonate molality and ammonium molality in ocean, quantities that are probably impossible to directly observe for exoplanets.

Colors shows fraction of methane depleted in equilibrium, as determined by semianalytic calculations. In this case, Pco₂ = 0.49 bar, Pn₂ = 0.5 bar, and Pch₄ = 0.01 bar. As can be seen, most parts of parameter space have high CH₄ depletion, that is, CH₄ is in disequilibrium. Thus, unless both bicarbonate and ammonium molalities are extremely large, detectable quantities of methane are out of equilibrium with an N₂-CO₂ atmosphere and ocean. The white dashed line box denotes the plausible Archean range.

Fig. 6. Probability distribution for maximum abiotic methane production from serpentinization on Earth-like planets.

This distribution was generated by sampling generous ranges for crustal production rates, FeO wt %, maximum fractional conversion of FeO to H₂, and maximum fractional conversion of H₂ to CH₄, and then calculating the resultant methane flux 1 million times (see the main text). The modern biological flux (58) and plausible biological Archean flux (59) far exceed the maximum possible abiotic flux. These results support the hypothesis that the co-detection of abundant CH₄ and CO₂ on a habitable exoplanet is a plausible biosignature.