Evaluating the Plausible Range of N₂O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach

Abstract

Nitrous oxide (N2O)—a product of microbial nitrogen metabolism—is a compelling exoplanet biosignature gas with distinctive spectral features in the near- and mid-infrared, and only minor abiotic sources on Earth. Previous investigations of N2O as a biosignature have examined scenarios using Earthlike N2O mixing ratios or surface fluxes, or those inferred from Earth’s geologic record. However, biological fluxes of N2O could be substantially higher, due to a lack of metal catalysts or if the last step of the denitrification metabolism that yields N2 from N2O had never evolved. Here, we use a global biogeochemical model coupled with photochemical and spectral models to systematically quantify the limits of plausible N2O abundances and spectral detectability for Earth analogs orbiting main-sequence (FGKM) stars. We examine N2O buildup over a range of oxygen conditions (1%–100% present atmospheric level) and N2O fluxes (0.01–100 teramole per year; Tmol = 1012 mole) that are compatible with Earth’s history. We find that N2O fluxes of 10 [100] Tmol yr−1 would lead to maximum N2O abundances of ∼5 [50] ppm for Earth–Sun analogs, 90 [1600] ppm for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e. We simulate emission and transmission spectra for intermediate and maximum N2O concentrations that are relevant to current and future space-based telescopes. We calculate the detectability of N2O spectral features for high-flux scenarios for TRAPPIST-1e with JWST. We review potential false positives, including chemodenitrification and abiotic production via stellar activity, and identify key spectral and contextual discriminants to confirm or refute the biogenicity of the observed N2O.

Large Interferometer For Exoplanets (LIFE). XII. The Detectability of Capstone Biosignatures in the Mid-infrared—Sniffing Exoplanetary Laughing Gas and Methylated Halogens

“…To maintain consistency, simplify reproducibility, and focus solely on examining the impact of changing surface molecular fluxes for the respective gases, we assume an Earthlike bulk atmosphere (78 % N 2 , 21% O 2 by volume) along with the Earth's temperature-pressure profile, which has a globally averaged surface temperature of 288 K. We also assume a planetary radius and surface gravity identical to that of Earth (R = 6371 km; g = 9.8 m s −1 ). We provide further details in the subsections below and the original sources for these atmospheric simulations (Leung et al 2022;Schwieterman et al 2022). O signatures in the directly imaged thermal infrared spectra of terrestrial exoplanets with the understanding that detectability at lower production rates enhances the probability such a signature may be found.…”

“…Nitrous oxide (N 2 O) and methylated halogens (e.g., CH 3 Cl, CH 3 Br) are part of the group of molecules that have been proposed as potential biosignatures 9 for detecting life in exoplanetary contexts (see, e.g., Leung et al 2022;Schwieterman et al 2022). A large inferred flux of these biogenic gases is most consistent with a productive global photosynthetic biosphere, which could be revealed in tandem with O 3 .…”

Double Trouble: Two Transits of the Super-Earth GJ 1132 b Observed with JWST NIRSpec G395H

“…In Visit 1, an uptick in transit depth at ∼4.5 μm is also noticeable. Multiple molecules, including O 3 , CS 2 , and N 2 O, have an absorption band around this wavelength (e.g., Schwieterman et al 2022), but of these N 2 O has the best-matching feature center and width. Therefore, in addition to our pure H 2 O atmosphere, we also generate atmospheric models with H 2 O as the background gas and either 10% N 2 O or 10% CH 4 .…”

Star-exoplanet interactions: A growing interdisciplinary field in heliophysics

“…Particles could have enough energy to split the nitrogen molecule, N 2 , producing NO x (NO and NO 2 ), which can destroy the potential biosignature O 3 (Segura et al, 2005;Segura et al, 2010;Tilley et al, 2019) or could contribute to create biosignature "false positives" by catalyzing the abiotic generation of biosignature molecules such as N 2 O (Airapetian et al, 2016;Airapetian et al, 2020). Potential false positive scenarios could be predicted by extensive characterization of the host star or identified by searching for the spectrally active non-biosignature gases that are predicted to form in combination with the putative biosignatures (e.g., Tabataba-Vakili et al, 2016;Schwieterman et al, 2022). For example, abiotic production of N 2 O would be accompanied by more robust production of NO 2 , HNO 3 , and/or HCN (Ibid.).…”