Disequilibrium Biosignatures over Earth History and Implications for Detecting Exoplanet Life

Brief

Using proxy- and model-constrained atmospheric and oceanic compositions, the authors calculated available Gibbs free energy, the difference between observed and thermodynamic equilibrium states, for the Archean, Proterozoic, and Phanerozoic atmosphere-ocean systems. The modern Earth's atmosphere-ocean disequilibrium is 2326 J/mol, driven by the unstable coexistence of O2, N2, and liquid water; by contrast, the purely gas-phase disequilibrium is only 1.5 J/mol. Maximum estimates for the Proterozoic and Archean reach 884 J/mol and 234 J/mol respectively, both in principle remotely detectable. The paper establishes that methane mixing ratios exceeding 10⁻³ are potentially biogenic and those exceeding 10⁻² are likely biogenic, and proposes simultaneous CH4 + CO2 detection as a biosignature for anoxic exoplanets.

Metadata

Category

Search

Venue

Science Advances

Type

Peer-reviewed

Year

2018

Authors

Joshua Krissansen-Totton, Stephanie Olson, David C. Catling

DOI

10.1126/sciadv.aao5747

arXiv

1801.08211

Access

Open access

Length

1.7 M

Instruments

JWST, Very Large Telescope, European Extremely Large Telescope, Thirty Meter Telescope, Giant Magellan Telescope, Wide-Field Infrared Survey Telescope

Data sources

Gibbs free energy minimization (MATLAB), Aspen Plus v8.6 (validation), NIST thermodynamic database, published Precambrian proxy estimates (atmospheric and oceanic compositions)

Tags

biosignature, astrobiology, exoplanet, atmospheric chemistry, thermodynamics, SETI-adjacent, Archean geochemistry

Key points

• Modern Earth's atmosphere-ocean disequilibrium is 2326 J/mol (coexistence of O2, N2, and liquid water), versus only 1.5 J/mol when the ocean is excluded, demonstrating that ocean inclusion is essential to quantifying remotely observable disequilibrium.p.2
• Maximum Proterozoic atmosphere-ocean disequilibrium is 884 J/mol; nitric acid formation from N2 + O2 + H2O accounts for ~640 J/mol (72%) of that total.p.4
• Maximum Archean atmosphere-ocean disequilibrium is 234 J/mol, with ~170 J/mol (74%) attributable to the unstable coexistence of CH4, N2, CO2, and liquid water; even the minimum Archean case yields 5.1 J/mol.p.5
• In the maximum Archean scenario, 99.8% of initial atmospheric CH4 is consumed when the system is reacted to thermodynamic equilibrium, confirming that CH4 should not coexist with N2-CO2-H2O across a broad range of initial conditions.p.5
• Methane mixing ratios greater than 10⁻³ are potentially biogenic; those exceeding 10⁻² are likely biogenic, because abiotic fluxes cannot plausibly sustain such concentrations in anoxic atmospheres.p.1
• Atmosphere-only disequilibrium decreases with time rather than tracking the rise of oxygen, illustrating that excluding the ocean systematically undercounts biogenic disequilibrium.p.6
• Earth's disequilibrium was smallest in the Archean, increased with the Great Oxidation Event, and increased again after the Neoproterozoic oxygen rise, a secular trend correlated with biogenic oxygen history.p.4
• No organism has evolved a metabolism to exploit the N2-O2-H2O disequilibrium, plausibly because the enzymatic machinery required to split the N2 triple bond and couple nitrification under aerobic conditions presents insurmountable kinetic barriers.p.6

Verbatim

• “The available Gibbs energy for the maximum Archean case is 234 J/mol, which is dominated by the coexistence of CO2, N2, CH4, and liquid water.”

  p.5

• “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.”

  p.1

• “The atmosphere-only disequilibrium decreases with time and thus does not reflect the rise of oxygen or the growth of primary productivity since 3.5 Ga.”

  p.6

Most interesting

• Despite Earth having the largest atmosphere-ocean disequilibrium in the solar system (2326 J/mol), no organism has ever evolved a metabolism to exploit the N2-O2-H2O free energy, the authors attribute this to kinetic barriers in splitting the N2 triple bond under aerobic conditions.
• Oxygenic photosynthesis is so enzymatically complex that it evolved only once in Earth's entire history, making it an uncertain template for exoplanet biospheres.
• The Archean atmosphere-only disequilibrium (dominated by H2-CO2) is not large compared to the abiogenic disequilibria of Mars and Titan, meaning early Earth could have looked less 'alive' by gas-phase-only metrics than some lifeless solar system bodies.
• Even the minimum Proterozoic disequilibrium scenario (9.5 J/mol) and minimum Archean scenario (5.1 J/mol) retain thermodynamic disequilibrium, bounded by conservative proxy estimates, suggesting biogenic signal persists across a wide uncertainty range.
• The absence of abundant CO in a CH4-CO2-rich atmosphere would strengthen biogenicity inference, because CO and CH4 should not coexist in biological scenarios.
• Lewis and Randall first noted in their classic thermodynamics text (pp. 567–568) that N2, O2, and water are out of equilibrium in Earth's atmosphere, and speculated that if life had evolved to catalyze their reaction, the oceans would have turned to dilute nitric acid.