Alternative Methylated Biosignatures. I. Methyl Bromide, a Capstone Biosignature

The first potential exoplanetary 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 O₂. CH₃Cl has previously been established as a biosignature candidate, and other halomethane gases such as CH₃Br and CH₃I have similar potential. These gases absorb in the mid-infrared at wavelengths that are likely to be captured while observing primary biosignatures such as O₃ or CH₄. We quantitatively explore CH₃Br as a new capstone biosignature through photochemical and spectral modeling of Earthlike planets orbiting FGKM stellar hosts. We also reexamine the biosignature potential of CH₃Cl over the same set of parameters using our updated model. We show that CH₃Cl and CH₃Br 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 coadditive spectral effect from multiple CH₃X gases in an atmosphere, leading to an 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.