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CH₃Br: A New Capstone Biosignature for Exoplanets

Presentation #234.07 in the session Exoplanet Atmospheres and Habitability.

Published onJun 29, 2022
CH₃Br: A New Capstone Biosignature for Exoplanets

Biosignatures are one of the most exciting areas that will be illuminated by the JWST and other ground and space-based telescopes in the near future. However, many biosignature candidates, such as O2 and CH4, have potential false positives that would require additional observations to eliminate. To address this issue, we propose the use of methylated halogens as capstone biosignatures. CH3Cl has previously been established as a biosignature candidate (Segura et al., 2005) and other methylated halogen 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. Here, we explore CH3Br as a new potential capstone biosignature. Because methylation is a detoxification process and Br is highly toxic, the production of methyl bromide, CH3Br, is decoupled from Br’s crustal abundance and the gas can become concentrated in certain environments. We present flux-abundance relationships for CH3Br, simulated for Earth-like planets orbiting FGKM host stars. We find that for a surface flux of ~9 * 106 molec/cm2/s, the globally averaged CH3Br flux, an Earth-like planet orbiting a late-type M dwarf can maintain an atmospheric concentration of ~0.5 ppm (comparable to pre industrial methane on Earth). For surface fluxes comparable to the most productive environments on Earth, ~10 ppm can build up in the atmosphere. We have simulated high and modest resolution spectra for favorable targets, find a co-additive effect from the presence of multiple methylated gases which nearly halves the number of transits necessary to detect a methylated gas feature. These capstone biosignatures have low false positive potential and would provide strong evidence for life in conjunction with other well established biosignature candidates.

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