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Evaluating Maximum Plausible N₂O Biosignatures on ExoEarths orbiting FGKM Stars

Presentation #234.02 in the session Exoplanet Atmospheres and Habitability.

Published onJun 29, 2022
Evaluating Maximum Plausible N₂O Biosignatures on ExoEarths orbiting FGKM Stars

Nitrous oxide (N2O), a product of microbial nitrogen metabolism, is a potential exoplanet biosignature gas with small abiotic sources on Earth and spectral features in the near- and mid-infrared. The concentration of N2O in Earth’s atmosphere today is approximately ~0.3 ppmv, which would be challenging to detect with future space or ground-based telescopes. However, biological fluxes of N2O could be significantly higher than on Earth today due to a lack of enzymatic catalysts or if the last step of the denitrification metabolism never evolved. Here we use a global biogeochemical model (GENIE) in conjunction with photochemical and spectral models to quantify the limits of plausible N2O abundances and spectral detectability for Earth analogs orbiting main-sequence (FGKM) stars. Denitrification fluxes are maximized near pO2 ~50% present atmosphere levels with a shallow decrease to modern levels. Elevated pO2 moderately boosts N2O photochemical lifetimes due to UV shielding, ultimately favoring N2O near (or just below) modern oxygenation conditions. We find N2O fluxes of 10 [100] teramole/year would lead to maximum N2O abundances of ~5 [50] ppm for Earth-Sun analogs, 30 [300] ppm for an Earth-like TRAPPIST-1e, and 90 [1600] ppm for Earths around a K6V dwarf. We simulate emission and transmission spectra for intermediate and maximum N2O concentrations relevant to upcoming and future space-based telescopes and calculate the detectability of N2O spectral features. We evaluate the magnitude of potential false positives and conclude that N2O remains a promising infrared biosignature

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