2021ApJ...909..128G

We investigate the atmospheric responses of modeled hypothetical Earth-like planets in the habitable zone of the M-dwarf AD Leonis to reduced oxygen (O₂), removed biomass ("dead" Earth), and varying carbon dioxide (CO₂) and surface relative humidity (sRH). Results suggest large O₂ differences between the reduced-O₂ and dead scenarios in the lower but not the upper atmosphere layers. Ozone (O₃) and nitrous oxide (N₂O) also show this behavior. Methane depends on hydroxyl (OH), its main sink. Abiotic production of N₂O occurs in the upper layers. Chloromethane (CH₃Cl) decreases everywhere on decreasing biomass. Changing CO₂ (from x1 to x100 present atmospheric levels (PALs)) and sRH (from 0.1% to 100%) does not influence CH₃Cl as much as lowering biomass. Therefore, CH₃Cl can be considered a good biosignature. Changing sRH and CO₂ has a greater influence on temperature than changing O₂ or biomass alone. Changing the biomass produces an ∼6 km effective height in transmission compared with changing CO₂ and sRH (∼25 km). In transmission O₂ is discernible at 0.76 µm for >0.1 PAL. The O₃ 9.6 µm band is weak for the low-O₂ runs and difficult to discern from dead Earth; however O₃ at 0.3 µm could serve as an indicator to distinguish between reduced-O₂ and dead Earth. The spectral features of N₂O and CH₃Cl correspond to effective heights of a few kilometers. CH₄ could be detectable tens of parsecs away with the Extremely Large Telescope except for the 10^–4 and 10^–6 PAL O₂ scenarios. O₂ is barely detectable for the 1 PAL O₂ case and unfeasible at lower abundances.