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Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres

Presentation #1151 in the session “Open Engagement Session B”.

Published onMar 17, 2021
Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres

Ammonia (NH3) is a poor biosignature gas because of its extreme solubility in water. We simulate exoplanet atmosphere transmission spectra, with varying NH3 biological surface fluxes, considering both photochemistry and solubility (i.e., including surface deposition). We find that NH3 is detectable with JWST only for a very favorable case of an H2-dominated rocky planet orbiting an M dwarf star, and only if NH3 exists above about 5 ppm column-averaged mixing ratio. The most favorable scenario for the detection of ammonia has an unrealistic NH3 biological surface flux, at levels on the order of 1015 molecules cm-2 s-1 (~ 4.5×106 Tg year-1). This value is roughly 20,000 times greater than the biological production of NH3 on Earth, and about 10,000 times greater than Earth’s CH4 biological production. Nonetheless, NH3 could accumulate on planets with limited surface water. In summary, while one might envision NH3 accumulating in a planet atmosphere with intensive bioactivity, the planet surface would have to be covered with life that produces NH3 with far higher fluxes than life found in even the most highly NH3 producing niche environments on Earth.

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