2020AJ....160...11L

In the near future, extremely large ground-based telescopes may conduct some of the first searches for life beyond the solar system. High spectral resolution observations of reflected light from nearby exoplanetary atmospheres could be used to search for the biosignature oxygen. However, while Earth's abundant O₂ is photosynthetic, early ocean loss may also produce high atmospheric O₂ via water vapor photolysis and subsequent hydrogen escape. To explore how to use spectra to discriminate between these two oxygen sources, we generate high-resolution line-by-line synthetic spectra of both a habitable Earth-like and post-ocean-loss Proxima Centauri b. We examine the strength and profile of four bands of O₂ from 0.63 to 1.27 µm, and quantify their relative detectability. We find that 10 bar O₂ post-ocean-loss atmospheres have strong suppression of oxygen bands, and especially the 1.27 µum band. This suppression is due to additional strong, broad O₂-O₂ collisionally induced absorption (CIA) generated in these more massive O₂ atmospheres, which is not present for the smaller amounts of oxygen generated by photosynthesis. Consequently, any detection of the 1.27 µm band in reflected light indicates lower Earth-like O₂ levels, which suggests a likely photosynthetic origin. However, the 0.69 µm O₂ band is relatively unaffected by O₂-O₂ CIA, and the presence of an ocean-loss high-O₂ atmosphere could be inferred via detection of a strong 0.69 µm O₂ band, and a weaker or undetected 1.27 µm band. These results provide a strategy for observing and interpreting O₂ in exoplanet atmospheres, that could be considered by future ground-based telescopes.