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 (O 2 ), removed biomass (“dead” Earth), and varying carbon dioxide (CO 2 ) and surface relative humidity (sRH). Results suggest large O 2 differences between the reduced-O 2 and dead scenarios in the lower but not the upper atmosphere layers. Ozone (O 3 ) and nitrous oxide (N 2 O) also show this behavior. Methane depends on hydroxyl (OH), its main sink. Abiotic production of N 2 O occurs in the upper layers. Chloromethane (CH 3 Cl) decreases everywhere on decreasing biomass. Changing CO 2 (from ×1 to ×100 present atmospheric levels (PALs)) and sRH (from 0.1% to 100%) does not influence CH 3 Cl as much as lowering biomass. Therefore, CH 3 Cl can be considered a good biosignature. Changing sRH and CO 2 has a greater influence on temperature than changing O 2 or biomass alone. Changing the biomass produces an ∼6 km effective height in transmission compared with changing CO 2 and sRH (∼25 km). In transmission O 2 is discernible at 0.76 μ m for >0.1 PAL. The O 3 9.6 μ m band is weak for the low-O 2 runs and difficult to discern from dead Earth; however O 3 at 0.3 μ m could serve as an indicator to distinguish between reduced-O 2 and dead Earth. The spectral features of N 2 O and CH 3 Cl correspond to effective heights of a few kilometers. CH 4 could be detectable tens of parsecs away with the Extremely Large Telescope except for the 10 −4 and 10 −6 PAL O 2 scenarios. O 2 is barely detectable for the 1 PAL O 2 case and unfeasible at lower abundances.