# Inferring Chemical Disequilibrium Biosignatures for Proterozoic Earth-Like Exoplanets

**Authors:** Amber V. Young, Tyler D. Robinson, Joshua Krissansen-Totton, Edward W. Schwieterman, Nicholas F. Wogan, Michael J. Way, Linda E. Sohl, Giada N. Arney, Christopher T. Reinhard, Michael R. Line, David C. Catling, James D. Windsor  
**arXiv:** [2311.06083](https://arxiv.org/abs/2311.06083)  
**Date:** 2023-11-10  
**Summary page:** https://www.summarizepaper.com/en/paper/2311.06083v1/

## Key points

- Chemical disequilibrium is a potential biosignature and is quantified through available free energy.
- The impact of observational uncertainties on inferring life-generated available free energy from remote sensing studies of exoplanets has not been explored.
- The authors combine an atmospheric retrieval tool with a thermodynamics model to assess the detectability of chemical disequilibrium signatures on Earth-like exoplanets during the Proterozoic Eon.
- Atmospheric abundances of oxygen-methane disequilibrium pairs may have been relatively high during this time period.
- Simulated reflected-light observations can achieve order-of-magnitude constraints on disequilibrium energy at high abundance scenarios and signal-to-noise ratios (SNRs) of 50.
- Weak constraints are found at moderate SNRs (20-30) for cases with medium to low gas abundances.
- Including thermal information encoded in water vapor opacities improves the constraints on disequilibrium energy.
- Remotely detecting chemical disequilibrium biosignatures can be an effective approach to biosignature detection that is independent of specific metabolic processes.
- Understanding the stellar photochemical context is essential when evaluating potential biosignatures for any exoplanet.
- Considering false negative scenarios is important in future research on chemical disequilibrium biosignatures.

## Summary

Chemical disequilibrium has been suggested as a potential biosignature and is quantified through available free energy. However, the impact of observational uncertainties on the ability to infer life-generated available free energy from remote sensing studies of exoplanets has not been explored. In this study, the authors combine an atmospheric retrieval tool with a thermodynamics model to assess the detectability of chemical disequilibrium signatures on Earth-like exoplanets during the Proterozoic Eon. It is believed that atmospheric abundances of oxygen-methane disequilibrium pairs may have been relatively high during this time period. The retrieval model studies conducted in this research consider a range of gas abundances and reveal that simulated reflected-light observations can achieve order-of-magnitude constraints on disequilibrium energy at high abundance scenarios and signal-to-noise ratios (SNRs) of 50. However, weak constraints are found at moderate SNRs (20-30) for cases with medium to low gas abundances. Furthermore, including modest thermal information encoded in water vapor opacities at optical and near-infrared wavelengths improves the constraints on disequilibrium energy. These findings demonstrate how remotely detecting chemical disequilibrium biosignatures can be an effective approach to biosignature detection that is independent of specific metabolic processes. The authors emphasize that understanding the stellar photochemical context will be essential when evaluating potential biosignatures for any exoplanet and highlight the importance of considering false negative scenarios in future research. This study contributes to our understanding of how observational uncertainties affect the detection and interpretation of chemical disequilibrium biosignatures on Earth-like exoplanets during different geological eras.