Alternative Methylated Biosignatures I: Methyl Bromide, A Capstone Biosignature

TL;DR

This paper proposes methylated gases like CH3Br and CH3Cl as low false positive, corroborative biosignatures for exoplanet life detection, demonstrating their detectability through spectral modeling of Earth-like planets.

Contribution

It introduces methyl bromide as a new capstone biosignature and re-evaluates methyl chloride's potential using updated models and spectral analysis.

Findings

CH3Br and CH3Cl can accumulate in M dwarf atmospheres.

Spectral features of methylated gases can be enhanced by multiple gas effects.

Detectability of methylated biosignatures is feasible with current or upcoming instruments.

Abstract

The first potential exoplanet biosignature detections are likely to be ambiguous due to the potential for false positives: abiotic planetary processes that produce observables similar to those anticipated from a global biosphere. Here we propose a class of methylated gases as corroborative `capstone' biosignatures. Capstone biosignatures are metabolic products that may be less immediately detectable, but have substantially lower false positive potential, and can thus serve as confirmation for a primary biosignature such as O$_2$. CH$_3$Cl has previously been established as a biosignature candidate, and other halomethane gases such as CH$_3$Br and CH$_3$I have similar potential. These gases absorb in the mid infrared at wavelengths that are likely to be captured while observing primary biosignatures such as O$_3$ or CH$_4$. We quantitatively explore CH$_3$Br as a new capstone biosignature through photochemical and spectral modeling of Earth-like planets orbiting FGKM stellar hosts. We also re-examine the biosignature potential of CH$_3$Cl over the same set of parameters using our updated model. We show that CH$_3$Cl and CH$_3$Br can build up to relatively high levels in M dwarf environments and analyze synthetic spectra of TRAPPIST-1e. Our results suggest that there is a co-additive spectral effect from multiple CH$_3$X gases in an atmosphere, leading to increased signal-to-noise and greater ability to detect a methylated gas feature. These capstone biosignatures are plausibly detectable in exoplanetary atmospheres, have low false positive potential, and would provide strong evidence for life in conjunction with other well established biosignature candidates.