The Detectability and Constraints of Biosignature Gases in the Near & Mid-Infrared from Transit Transmission Spectroscopy

Abstract: The James Webb Space Telescope (JWST) is expected to revolutionize our understanding of Jovian worlds over the coming decade. However, as we push towards characterizing cooler, smaller, "terrestrial-like" planets, dedicated next-generation facilities will be required to tease out the small spectral signatures indicative of biological activity. Here, we evaluate the feasibility of determining atmospheric properties, from near-to-mid-infrared transmission spectra, of transiting temperate terrestrial M-dwarf companions. Specifically, we utilize atmospheric retrievals to explore the trade space between spectral resolution, wavelength coverage, and signal-to-noise on our ability to both detect molecular species and constrain their abundances. We find that increasing spectral resolution beyond R=100 for near-infrared wavelengths, shorter than 5$\mu$m, proves to reduce the degeneracy between spectral features of different molecules and thus greatly benefits the abundance constraints. However, this benefit is greatly diminished beyond 5$\mu$m as any overlap between broad features in the mid-infrared does not deconvolve with higher resolutions. Additionally, our findings revealed that the inclusion of features beyond 11$\mu$m did not meaningfully improve the detection significance nor abundance constraints results. We conclude that an instrument with continuous wavelength coverage from $\sim$2-11$\mu$m, spectral resolution of R$\simeq$50-300, and a 25m$^2$ collecting area, would be capable of detecting H$2$O, CO$_2$, CH$_4$, O$_3$, and N$_2$O in the atmosphere of an Earth-analog transiting a M-dwarf (mag${K}=8.0$) within 50 transits, and obtain better than an order-of-magnitude constraint on each of their abundances.