Using atmospheric composition as a metric for detecting life on habitable planets

The extent to which a planet’s atmospheric composition is in chemical disequilibrium is a possible diagnostic tool for detecting life on habitable exoplanets. It is generally recognized that departure from chemical equilibrium of the Earth’s atmosphere reflects the prodigious release and uptake of gases by the biosphere (1). But abiotic free energy sources can promote disequilibrium too, e.g., through photochemistry and volcanic gases. Consequently, inferring life from thermodynamic disequilibrium is a question of degree. Nonetheless, a simple disequilibrium metric is the amount of energy per mole of air that would be released if gases in the air were taken to thermodynamic equilibrium. We calculated this quantity rigorously for Solar System planetary atmospheres using numerical Gibbs free energy minimization, an up-to-date database of thermodynamic data parameterized as a function of temperature, and current estimates of bulk composition. The metric was calculated at the surface for rocky planets and at the 1 bar level for gas giants. The method involves constrained nonlinear solution of algebraic equations, where the constraint of atomic mass balance allows identification of the principal gases responsible for the disequilibrium in each atmosphere. Earth’s atmosphere is at the high end of spread of in chemical potential energy (J/mol) of atmospheres, some ~102-107 greater than other atmospheres. For Earth, the key disequilibrium is between N2, liquid water and O2. Mars’s atmosphere (with a potential energy of ~200 J/mol) is largely a result of photochemical disequilibrium of CO and NO and O2, whereas other atmospheres (e.g., Jupiter at 0.003 J/mol and Venus at 0.08 J/mol) are much closer to thermodynamic equilibrium. Such a metric could be applied to exoplanets for comparison with Solar System planets and to infer life. Of course, uncertainty would depend upon obtaining accurate knowledge of bulk atmospheric constituents and physical conditions. (Refs: 1. Lovelock, J. E., 1975. Proc. Roy. Soc. Lond. B. 189, 167).