Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres.

Ammonia (NH 3 ) in a terrestrial planet atmosphere is generally a good biosignature gas, primarily because terrestrial planets have no significant known abiotic NH 3 source. The conditions required for NH 3 to accumulate in the atmosphere are, however, stringent. NH 3 's high water solubility and high biousability likely prevent NH 3 from accumulating in the atmosphere to detectable levels unless life is a net source of NH 3 and produces enough NH 3 to saturate the surface sinks. Only then can NH 3 accumulate in the atmosphere with a reasonable surface production flux. For the highly favorable planetary scenario of terrestrial planets with hydrogen (H 2 )-dominated atmospheres orbiting M dwarf stars (M5V), we find that a minimum of about 5 ppm column-averaged mixing ratio is needed for NH 3 to be detectable with JWST, considering a 10 ppm JWST systematic noise floor. When the surface is saturated with NH 3 ( i.e ., there are no NH 3 -removal reactions on the surface), the required biological surface flux to reach 5 ppm is on the order of 10 10 molecules/(cm 2 ·s), comparable with the terrestrial biological production of methane (CH 4 ). However, when the surface is unsaturated with NH 3 , due to additional sinks present on the surface, life would have to produce NH 3 at surface flux levels on the order of 10 15 molecules/(cm 2 ·s) (∼4.5 × 10 6 Tg/year). This value is roughly 20,000 times greater than the biological production of NH 3 on the Earth and about 10,000 times greater than Earth's CH 4 biological production. Volatile amines have similar solubilities and reactivities to NH 3 and hence share NH 3 's weaknesses and strengths as a biosignature. Finally, to establish NH 3 as a biosignature gas, we must rule out mini-Neptunes with deep atmospheres, where temperatures and pressures are high enough for NH 3 's atmospheric production.