Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres.

Ammonia (NH) in a terrestrial planet atmosphere is generally a good biosignature gas, primarily because terrestrial planets have no significant known abiotic NH source. The conditions required for NH to accumulate in the atmosphere are, however, stringent. NH's high water solubility and high biousability likely prevent NH from accumulating in the atmosphere to detectable levels unless life is a net source of NH and produces enough NH to saturate the surface sinks. Only then can NH accumulate in the atmosphere with a reasonable surface production flux. For the highly favorable planetary scenario of terrestrial planets with hydrogen (H)-dominated atmospheres orbiting M dwarf stars (M5V), we find that a minimum of about 5 ppm column-averaged mixing ratio is needed for NH to be detectable with JWST, considering a 10 ppm JWST systematic noise floor. When the surface is saturated with NH (., there are no NH-removal reactions on the surface), the required biological surface flux to reach 5 ppm is on the order of 10 molecules/(cm·s), comparable with the terrestrial biological production of methane (CH). However, when the surface is unsaturated with NH, due to additional sinks present on the surface, life would have to produce NH at surface flux levels on the order of 10 molecules/(cm·s) (∼4.5 × 10 Tg/year). This value is roughly 20,000 times greater than the biological production of NH on the Earth and about 10,000 times greater than Earth's CH biological production. Volatile amines have similar solubilities and reactivities to NH and hence share NH's weaknesses and strengths as a biosignature. Finally, to establish NH as a biosignature gas, we must rule out mini-Neptunes with deep atmospheres, where temperatures and pressures are high enough for NH's atmospheric production.