High-Resolution Spectral Discriminants of Ocean Loss for M Dwarf Terrestrial Exoplanets

Abstract

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 O${}_2$is photosynthetic, early ocean loss may also produce high atmospheric O${}_2$ 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 O${}_2$ from 0.63 to 1.27 $\mu$m, and quantify their relative detectability. We find that 10 bar O${}_2$ post-ocean-loss atmospheres have strong suppression of oxygen bands, and especially the 1.27$\mu$m band. This suppression is due to additional strong, broad O${}_2$-O${}_2$ collisionally-induced absorption (CIA) generated in these more massive O${}_2$atmospheres, which is not present for the smaller amounts of oxygen generated by photosynthesis. Consequently, any detection of the 1.27$\mu$m band in reflected light indicates lower Earth-like O${}_2$ levels, which suggests a likely photosynthetic origin. However, the 0.69 $\mu$m O${}_2$ band is relatively unaffected by O${}_2$-O${}_2$ CIA, and the presence of an ocean-loss high-O${}_2$ atmosphere could be inferred via detection of a strong 0.69 $\mu$m O${}_2$ band, and a weaker or undetected 1.27 $\mu$m band. These results provide a strategy for observing and interpreting O${}_2$ in exoplanet atmospheres, that could be considered by future ground-based telescopes.

14 pages, 11 figures

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