On Detecting Biospheres from Chemical Thermodynamic Disequilibrium in Planetary Atmospheres

Brief

Krissansen-Totton, Bergsman, and Catling (2016) quantify available Gibbs free energy, the difference between an observed atmosphere's Gibbs energy and its fully reacted equilibrium, across all Solar System planets and Titan. For gas-phase calculations alone, Earth is unremarkable: Mars leads at 136.3 J/mol, driven by abiotic CO and O3. Only when the ocean is included in a multiphase calculation does Earth emerge as singular, with its atmosphere-ocean system carrying 20 to 2×10^6 J/mol more disequilibrium than competing bodies. Critically, the dominant source of that signal is not the O2-CH4 redox pair but the thermodynamically unstable three-way coexistence of N2, O2, and liquid water, which should react to form nitrate.

Metadata

Category

Search

Venue

Astrobiology

Type

Peer-reviewed

Year

2016

Authors

Joshua Krissansen-Totton, David S. Bergsman, David C. Catling

DOI

10.1089/ast.2015.1327

Access

Open access

Length

504.4 K

Instruments

Aspen Plus v8.6 (independent validation), Matlab fmincon interior-points optimizer

Data sources

NASA thermodynamic database (Burcat and Ruscic 2005), SUPCRT SPRONS96 aqueous species database, US Standard Atmosphere, Curiosity rover Mars atmospheric composition (Mahaffy et al. 2013; Baines et al. 2014), Pilson (2012) average seawater ion abundances, Gruber (2008) seawater nitrate abundances

Tags

biosignature, astrobiology, thermodynamics, atmospheric chemistry, exoplanets, Gibbs free energy

Key points

• Gas-phase available Gibbs energy for Mars is 136.3 J/mol, higher than Earth's gas-phase value, driven by abiotic CO and O3 photochemistry, demonstrating that gas-phase disequilibrium alone is insufficient as a life-detection metric.p.5
• Available Gibbs energy for other Solar System bodies (gas-phase): Venus 0.0596 J/mol, Titan 1.21 J/mol, Jupiter 0.00103 J/mol, Uranus 0 J/mol at 1 bar with basic composition.p.5
• Earth's atmosphere-ocean disequilibrium is 20 to 2×10^6 times larger than all other Solar System bodies surveyed; Mars ranks second even in this multiphase comparison.p.1
• The dominant biogenic disequilibrium source is the coexistence of N2, O2, and liquid water, which should react to nitrate, not the O2-CH4 redox couple historically emphasized since Lovelock (1965).p.1
• The O2-CH4 redox contribution is quantitatively minor, though sustaining even that signal requires a large, continuous CH4 flux into the atmosphere.p.1
• The methodology uses constrained Gibbs free energy minimization via an interior-points method (Matlab fmincon), validated independently against Aspen Plus commercial chemical engineering software; results agree closely in every case.p.4
• For multiphase systems the Gibbs energy landscape is non-convex, so no global minimum is guaranteed; the authors address this by iterating over a large ensemble of initial conditions and selecting the minimum of minima.p.4
• Lovelock (1975) is the only prior author to report Earth's atmosphere-ocean disequilibrium magnitude, but without sufficient methodological detail to reproduce; the paper infers he conflated 'reaction to completion' with thermodynamic equilibrium, which yields incorrect results.p.2

Verbatim

• “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.”

  p.1

• “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.”

  p.1

• “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.”

  p.1

Most interesting

• Judged by gas-phase chemistry alone, Mars is more anomalous than Earth: abiotic photochemistry there produces 136.3 J/mol of available Gibbs energy, making it the Solar System's gas-phase disequilibrium leader, yet it is lifeless as far as is known.
• Uranus at 1 bar with its basic atmospheric composition registers exactly zero available Gibbs energy (F = 0 J/mol), making it the most chemically inert atmosphere in the survey.
• The textbook O2-CH4 pair, cited as the canonical atmospheric biosignature since Lovelock (1965), is quantitatively secondary to the N2-O2-H2O → nitrate disequilibrium as an energy source, a result that reframes six decades of framing around methane detection.
• Lovelock's 1975 Earth atmosphere-ocean disequilibrium estimate was never reproduced because he did not publish his method; the authors infer his calculation treated reaction-to-completion as equivalent to true thermodynamic equilibrium, which it is not.
• The metric is theoretically computable from exoplanet spectroscopy alone, it requires only bulk atmospheric composition, temperature, and pressure, making it potentially applicable to JWST and ELT observations of terrestrial exoplanets.
• The complete Matlab source code and thermodynamic databases were released as open-source post-publication software, making the first rigorous Solar System disequilibrium survey fully reproducible by any research group.