On Detecting Biospheres from Chemical Thermodynamic Disequilibrium in Planetary Atmospheres

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 O 2 and CH 4 . 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 N 2 , O 2 and liquid water instead of more stable nitrate. In comparison, the O 2 -CH 4 disequilibrium is minor, although kinetics requires a large CH 4 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 using 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.

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

“…This result is consistent with Van Cleemput and Baert (), who concluded that in anoxic soils should decay, generally to N 2 . The situation on reducing prebiotic Earth is very different from the situation on oxidizing modern Earth, where in equilibrium is thermodynamically favored (Krissansen‐Totton et al, ). Our thermochemical analysis confirms our kinetic analysis that previous work has overestimated prebiotic [ ], increasing our confidence in this conclusion.…”

Alien Mindscapes—A Perspective on the Search for Extraterrestrial Intelligence

“…In situ biosignatures are physicochemical, geological, morphological, and mineralogical in nature (Summons et al , 2011). Remotely detectable biosignatures include gases in planetary atmospheres (Pilcher, 2004; Segura et al , 2005; Domagal-Goldman et al , 2011; O'Malley-James et al , 2014; Seager, 2014; Krissansen-Totton et al , 2016). Given an environmental analogy, it is conceivable that alien biospheres presenting similarities with ours may have generated and left traces we could recognize.…”

A Maximum Subsurface Biomass on Mars from Untapped Free Energy: CO and H2as Potential Antibiosignatures

“…Historically, it was argued that the martian atmosphere is essentially at equilibrium and therefore Mars is unlikely to support life (Lovelock 1988;Lovelock 1975;Lovelock 1979). However, with modern observations, it is understood that Mars' atmosphere has the largest thermodynamic disequilibrium in the Solar System aside from Earth, with an available free energy of ~136 J per mole of atmosphere (Krissansen-Totton et al 2016). This free energy is predominantly attributable to the CO-O2 redox pair produced abiotically by the photolysis of CO2 and H2O in a thin, dry atmosphere.…”