Influence of Biomass Emissions on Habitability, Biosignatures, and Detectability in Earth-like Atmospheres

Abstract

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 ×1 to ×100 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.