Synthetic Spectra of Potential Exo-Earths: Quantifying Biotic Signatures with AROC

As exoplanet transmission spectra become possible for smaller planets, it becomes pertinent to question what we expect to observe from an exo-Earth. Previously, atmospheric and aqueous chemistry models have been used independently to investigate atmospheric and surface processes taking place on terrestrial exoplanets. However, these two models cannot be treated discretely. We present the AROC (Atmosphere-Rock-Ocean Chemistry) model, which couples KINETICS (the Caltech/JPL 1-D photochemical and transport model, Allen et al. 1981) with PHREEQC (the USGS ion-association aqueous chemistry model, Parkhurst and Appelo 2013) to incorporate atmosphere-ocean-rock interactions in near-surface exoplanet chemistry for terrestrial exoplanets. The KINETICS model currently set up for early Earth includes 50 (9 fixed, 41 varied) chemical species linked by 297 reactions, including nitrogen and sulfur chemistry and volcanic outgassing at the lower boundary. At each time step, PHREEQC calculates a gaseous flux input for KINETICS in order for AROC to achieve surface-atmospheric equilibrium. The output from this comprehensive model is then fed to petitRADTRANS (Mollière et al. 2019) to generate synthetic transmission spectra. We consider the following potential Earth-like exoplanet compositions: a pre-GOE (Great Oxidation Event), abiotic Earth; an abiotic Earth during the GOE with increasing oxygen fluxes; and a post-GOE, biotic Earth modeled with microbial outgassing. Our AROC results and derived spectra will therefore aid in quantifying the effect of biological processes on transmission spectroscopy and can be used to make predictions for future telescopes that would observe potentially habitable planets, including the James Webb Space Telescope (JWST), Origins Space Telescope (OST), LUVOIR, and HabEx.