The Detectability of CH4/CO2/CO and N2O Biosignatures through Reflection Spectroscopy of Terrestrial Exoplanets

Although molecular oxygen and ozone are the usual biosignatures of choice when searching for 7 extrasolar life and a potential Earth twin, it is useful to consider alternative biogenic gases present 8 throughout Earth’s history. For example, the chemical makeup of the Earth’s atmosphere during 9 the Archean eon (4 Ga - 2.5 Ga) and the Proterozoic eon (2.5 Ga - 0.5 Ga) contrast considerably 10 with the present-day state, where the Archean was dominated by carbon dioxide and methane and 11 the Proterozoic was marked by increasing oxygen levels and potentially higher amounts of nitrous 12 oxide. The disequilibrium between CO2 and CH4 in an Archean Earth analog may be a compelling 13 biosignature because their coexistence implies methane replenishment at rates unlikely to be abiotic. 14 However, CH4 can also be produced through volcanism and other geological processes, and setting 15 constraints on volcanic molecules like CO may help address this ambiguity. The detection of N2O 16 in a Proterozoic Earth analog, on the other hand, may be evidence for the presence of life because 17 the production of N2O on Earth is mostly biological. Motivated by these ideas, we use the code 18 ExoReLℜ to generate forward models and simulate spectral retrievals of an Archean Earth-like planet 19 and Proterozoic Earth-like planet to determine the detectability of CH4, CO2, CO, and N2O in their 20 reflected light spectrum in the wavelength range 0.25 - 1.8 μm. We show that it is challenging to 21 detect CO in an Archean atmosphere for volume mixing ratio ≤ 10%. We also show that CH4 is 22 readily detectable for both the full wavelength span and truncated ranges cut at 1.6μm and 1.7μm, 23 but for the latter two cases, the dominant gas of the atmosphere was not correctly identified and 24 the constrained mixing ratio of CH4 was slightly biased. Meanwhile, an N2O mixing ratio of 10−3 is 25 detectable for the long wavelength cutoff ≥ 1.4μm, but a mixing ratio of ≤ 10−4 is undetectable on a 26 Proterozoic-Earth-like planet. The results presented here will be useful for the strategic design of the 27 future Habitable Worlds Observatory (HWO) and the components needed to potentially distinguish 28 between inhabited and lifeless planets