Inferring Chemical Disequilibrium Biosignatures for Proterozoic Earth-Like Exoplanets

TL;DR

This study assesses how observational uncertainties affect the remote detection of chemical disequilibrium biosignatures, like oxygen-methane pairs, on Earth-like exoplanets, emphasizing the importance of signal quality and thermal data.

Contribution

It introduces a combined atmospheric retrieval and thermodynamics approach to evaluate the detectability of biosignatures under realistic observational conditions.

Findings

High abundance scenarios allow order-of-magnitude constraints at high SNR (50).

Moderate SNR (20-30) yields weak constraints for low-abundance cases.

Water vapor features improve disequilibrium energy constraints.

Abstract

Chemical disequilibrium quantified via available free energy has previously been proposed as a potential biosignature. However, exoplanet biosignature remote sensing work has not yet investigated how observational uncertainties impact the ability to infer a life-generated available free energy. We pair an atmospheric retrieval tool to a thermodynamics model to assess the detectability of chemical disequilibrium signatures of Earth-like exoplanets, emphasizing the Proterozoic Eon where atmospheric abundances of oxygen-methane disequilibrium pairs may have been relatively high. Retrieval model studies applied across a range of gas abundances revealed that order-of-magnitude constraints on disequilibrium energy are achieved with simulated reflected-light observations at the high abundance scenario and signal-to-noise ratios (50) while weak constraints are found at moderate SNRs (20\,--\,30) for med\,--\,low abundance cases. Furthermore, the disequilibrium energy constraints are improved by modest thermal information encoded in water vapor opacities at optical and near-infrared wavelengths. These results highlight how remotely detecting chemical disequilibrium biosignatures can be a useful and metabolism-agnostic approach to biosignature detection.