Biosignatures & Paleobiology

Alternative Methylated Biosignatures I: Methyl Bromide, A Capstone Biosignature

By Keith Cowing
Press Release
August 16, 2022
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Alternative Methylated Biosignatures I: Methyl Bromide, A Capstone Biosignature
Alternative Methylated Biosignatures I: Methyl Bromide, A Capstone Biosignature

Comparison of emission features for atmospheres with methylated biogenic gases around a K6V, M3.5Ve and M5V star. From top to bottom: CH3Cl, CH3Br, combination of CH3Cl and CH3Br for a range of fluxes ranging from globally averaged Modern Earth fluxes to most productive local terrestrial and marine environments. Spectra are shown for a 50% clear sky, 50% cloudy with equal parts cirrus and stratocumulus clouds.

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 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 Earth-like planets orbiting FGKM stellar hosts. We also re-examine 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 co-additive spectral effect from multiple CH3X 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.

Michaela Leung, Edward W. Schwieterman, Mary N. Parenteau, Thomas J. Fauchez

Comments: 18 pages, 11 figures, appendix. Accepted at ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2208.07393 [astro-ph.EP] (or arXiv:2208.07393v1 [astro-ph.EP] for this version)
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Submission history
From: Michaela Leung
[v1] Mon, 15 Aug 2022 18:22:26 UTC (48,248 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻