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.