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

TL;DR

This paper investigates how chemical disequilibrium in exoplanet atmospheres can indicate life or its absence, showing that certain disequilibria are anti-biosignatures while others signal biological activity, depending on context.

Contribution

It introduces a model comparing disequilibrium on lifeless and biosphere-bearing Earth-like planets, clarifying when disequilibrium indicates life or death.

Findings

Prebiotic Earth likely had large disequilibrium due to water and volcanic gases.

Chemotrophic life reduced disequilibrium by consuming reactive species.

Modern Earth's disequilibrium includes biogenic gases like O2, indicating life.

Abstract

Chemical disequilibrium in exoplanetary atmospheres (detectable with remote spectroscopy) can indicate life. The modern Earth's atmosphere-ocean system has a much larger chemical disequilibrium than other solar system planets with atmospheres because of oxygenic photosynthesis. However, no analysis exists comparing disequilibrium on lifeless, prebiotic planets to disequilibrium on worlds with primitive chemotrophic biospheres that live off chemicals and not light. Here, we use a photochemical-microbial ecosystem model to calculate the atmosphere-ocean disequilibria of Earth with no life and with a chemotrophic biosphere. We show that the prebiotic Earth likely had a relatively large atmosphere-ocean disequilibrium due to the coexistence of water and volcanic H2, CO2, and CO. Subsequent chemotrophic life probably destroyed nearly all of the prebiotic disequilibrium through its metabolism, leaving a likely smaller disequilibrium between N2, CO2, CH4, and liquid water. So, disequilibrium fell with the rise of chemotrophic life then later rose with atmospheric oxygenation due to oxygenic photosynthesis. We conclude that big prebiotic disequilibrium between H2 and CO2 or CO and water is an anti-biosignature because these easily metabolized species can be eaten due to redox reactions with low activation energy barriers. However, large chemical disequilibrium can also be a biosignature when the disequilibrium arises from a chemical mixture with biologically insurmountable activation energy barriers, and clearly identifiable biogenic gases. Earth's modern disequilibrium between O2, N2, and liquid water along with minor CH4 is such a case. Thus, the interpretation of disequilibrium requires context. With context, disequilibrium can be used to infer dead or living worlds.