Abstract EANA2024-8

Introduction: As JWST and other advanced instrumentation usher in a new era of exoplanet observations, terrestrial exoplanet atmospheres will soon be available for observation and interpretation, including for analysis of potential signs of life. Many biosignature candidates, such as O2 and CH4, have significant potential false positives generated by abiotic processes. To address this issue, we propose using methylated gases as “capstone biosignatures.” The process of biomethylation can utilize and subsequently volatilize a broad range of substrates including halogens, chalcogens, and metalloids (e.g., CH3Cl, CH3Br, CH3I, (CH3)2S, (CH3)2Se, (CH3)2Hg, etc.). Methylated gases are not produced as equilibrium products in planetary atmospheres and have extremely limited pathways for abiotic production; therefore, they are signs of life with low ambiguity. Underlying biological processes such as detoxification support the application of these gases as biosignature candidates. Here, we apply the same methods to additional methyl halide and polyhalomethane gases, including CH3I, CHBr3, CHBr2Cl, etc. 

    Methods: Building on modifications used to simulate CH3Br, we use a combination of photochemical (atmos; Arney et al., 2016), spectral (SMART; Meadows & Crisp, 1996), and instrumental (PSG; Villanueva et al., 2018, 2022) models to perform vertically integrated simulations from surface fluxes to synthetic observations for various instruments and observing modes. 

    Results: We present flux-abundance relationships for CH3I, simulated for Earth-like planets orbiting FGKM host stars. Our preliminary results show that for surface fluxes comparable to the most productive known environments, an Earth-like planet orbiting a late-type M dwarf can maintain atmospheric concentrations 100 times (CH3I) greater than those in a solar-type photochemical environment.  Despite comparable surface fluxes, this is a more modest increase than seen for CH3Cl and CH3Br, likely due to the dominance of highly effective hydroxyl (OH-) loss reactions throughout the range of stars considered for CH3I. We also examine simulated transmission and emission spectra for planets with high levels of CH3I, and consider the impact of other methylated gases. 

   Previous work has shown a co-additive photochemical and spectral effect for multiple methylated gases, increasing the overall detectability of the ensemble of these related gaseous species. Our tests suggest that the photochemical effect continues for CH3I, presenting a unique advantage to this addition to the toolbox of potential biosignatures. Future work will consider the impact of polyhalomethanes and other combinations of methylated halogen gases.