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Chemical disequilibrium in exoplanetary atmospheres (detectable with remotespectroscopy) can indicate life. The modern Earth's atmosphere-ocean system hasa much larger chemical disequilibrium than other solar system planets withatmospheres because of oxygenic photosynthesis. However, no analysis existscomparing disequilibrium on lifeless, prebiotic planets to disequilibrium onworlds with primitive chemotrophic biospheres that live off chemicals and notlight. Here, we use a photochemical-microbial ecosystem model to calculate theatmosphere-ocean disequilibria of Earth with no life and with a chemotrophicbiosphere. We show that the prebiotic Earth likely had a relatively largeatmosphere-ocean disequilibrium due to the coexistence of water and volcanicH2, CO2, and CO. Subsequent chemotrophic life probably destroyed nearly all ofthe prebiotic disequilibrium through its metabolism, leaving a likely smallerdisequilibrium between N2, CO2, CH4, and liquid water. So, disequilibrium fellwith the rise of chemotrophic life then later rose with atmospheric oxygenationdue to oxygenic photosynthesis. We conclude that big prebiotic disequilibriumbetween H2 and CO2 or CO and water is an anti-biosignature because these easilymetabolized species can be eaten due to redox reactions with low activationenergy barriers. However, large chemical disequilibrium can also be abiosignature when the disequilibrium arises from a chemical mixture withbiologically insurmountable activation energy barriers, and clearlyidentifiable biogenic gases. Earth's modern disequilibrium between O2, N2, andliquid water along with minor CH4 is such a case. Thus, the interpretation ofdisequilibrium requires context. With context, disequilibrium can be used toinfer dead or living worlds.

astrophysical journal/the astrophysical journal

When is chemical disequilibrium in Earth-like planetary atmospheres a  biosignature versus an anti-biosignature? Disequilibria from dead to living  worlds