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Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

Presentation #503.04 in the session Atmospheres 4.

Published onJun 20, 2022
Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

The detection of biologically-produced gases indicative of life (“biosignature gases”) in rocky exoplanet atmospheres is a key goal of exoplanet science. Yet, simulations predict that observations of main atmospheric gases will be challenging due to the small sizes of rocky planets and their atmospheres, and observations of trace biosignature gases even more so. On modern Earth, most biogenic gases do not accumulate to remotely-detectable concentrations, because they are too photochemically reactive (Kasting et al. 2014). O2 is a notable exception: despite its reactivity, O2 production by biology is so vigorous that it saturates its photochemical sinks, permitting it to “runaway” to high concentrations in Earth’s atmosphere until limited by surface processes. Here we argue that O2 may not be unique: diverse gases may undergo this runaway process. As an illustrative case study, we show that on a habitable exoplanet with an H2-N2 atmosphere and net surface production of NH3 orbiting an M-dwarf (the “Cold Haber World” scenario, Seager et al. 2013ab), the reactive biogenic gas NH3 can enter runaway at a high but biochemically plausible production flux. In runaway, a 1 order of magnitude increase in surface production flux yields a 3 order of magnitude increase in atmospheric concentration, permitting detection with JWST in just 2 transits. We quantitatively address past qualitative concerns regarding the physicality of runaway, and demonstrate that previous blanket dismissal of runaway on thermodynamic grounds is not justified. Our work suggests that diverse gases, including reactive gases, may be detectable in exoplanet atmospheres.

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