Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

NH3 column-averaged mixing ratio as a function of net surface flux for an Earth-sized planet with an H2-dominated atmosphere for the Cold Haber World scenario (Seager et al. 2013a,b).NH3 surface deposition is assumed to be negligible due to surface saturation in this scenario. Our standard case (red solid line) corresponds to a planet orbiting an M dwarf star with low wet and dry deposition of atmospheric species (Table 3; Section 2.1), representing a planet with inefficient biological consumption of atmospheric species. We also show sensitivity test calculations for an otherwise-identical planet with high wet and dry deposition of non-NH3 species (red dashed line; Table 3), with elevated surface temperature (pink dashed line), and with elevated surface and stratospheric temperatures (hot pink dashed line) due to assumed warming by NH3 and/or its photochemical products. The yellow solid line shows NH3 accumulation for a Sun-like stellar host. Modern biological O2 production (net of biological consumption) is demarcated by a green line (Zahnle et al. 2006), and estimates of modern and pre-industrial NH3 flux to the atmosphere are represented by purple and blue shaded regions (Bouwman et al. 1997; Zhu et al. 2015). NH3 enters photochemical runaway at biochemically plausible surface production fluxes for M-dwarfs, but not Sunlike stars.

About 2.5 billion years ago, microbes learned to harness plentiful Solar energy to reduce CO2 with H2O, extracting energy and producing O2 as waste.

O2 production from this metabolic process was so vigorous that it saturated its photochemical sinks, permitting it to reach "runaway" conditions and rapidly accumulate in the atmosphere despite its reactivity.

Here we argue that O2 may not be unique: diverse gases produced by life may experience a "runaway" effect similar to O2. This runaway occurs because the ability of an atmosphere to photochemically cleanse itself of trace gases is generally finite. If produced at rates exceeding this finite limit, even reactive gases can rapidly accumulate to high concentrations and become potentially detectable. Planets orbiting smaller, cooler stars, such as the M dwarfs that are the prime targets for the James Webb Space Telescope (JWST), are especially favorable for runaway due to their lower UV emission compared to higher-mass stars.

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, whereupon an increase in surface production flux of 1 order of magnitude can increase NH3 concentrations by 3 orders of magnitude and render it detectable with JWST in just 2 transits. Our work on this and other gases suggests that diverse signs of life on exoplanets may be readily detectable at biochemically plausible production rates.

Sukrit Ranjan, Sara Seager, Zhuchang Zhan, Daniel D. B. Koll, William Bains, Janusz J. Petkowski, Jingcheng Huang, Zifan Lin

Comments: In review at ApJ. Comments & feedback welcome. Code available by request via GitHub
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2201.08359 [astro-ph.EP] (or arXiv:2201.08359v1 [astro-ph.EP] for this version)
Submission history
From: Sukrit Ranjan
[v1] Thu, 20 Jan 2022 18:47:00 UTC (792 KB)

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