Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres

Ammonia (NH₃) in a terrestrial planet atmosphere is generally a good biosignature gas, primarily because terrestrial planets have no significant known abiotic NH₃ source. The conditions required for NH₃ to accumulate in the atmosphere are, however, stringent. NH₃'s high water solubility and high biousability likely prevent NH₃ from accumulating in the atmosphere to detectable levels unless life is a net source of NH₃ and produces enough NH₃ to saturate the surface sinks. Only then can NH₃ accumulate in the atmosphere with a reasonable surface production flux. For the highly favorable planetary scenario of terrestrial planets with hydrogen (H₂)-dominated atmospheres orbiting M dwarf stars (M5V), we find that a minimum of about 5 ppm column-averaged mixing ratio is needed for NH₃ to be detectable with JWST, considering a 10 ppm JWST systematic noise floor. When the surface is saturated with NH₃ (i.e., there are no NH₃-removal reactions on the surface), the required biological surface flux to reach 5 ppm is on the order of 10¹⁰ molecules/(cm²·s), comparable with the terrestrial biological production of methane (CH₄). However, when the surface is unsaturated with NH₃, due to additional sinks present on the surface, life would have to produce NH₃ at surface flux levels on the order of 10¹⁵ molecules/(cm²·s) (∼4.5 × 10⁶ Tg/year). This value is roughly 20,000 times greater than the biological production of NH₃ on the Earth and about 10,000 times greater than Earth's CH₄ biological production. Volatile amines have similar solubilities and reactivities to NH₃ and hence share NH₃'s weaknesses and strengths as a biosignature. Finally, to establish NH₃ as a biosignature gas, we must rule out mini-Neptunes with deep atmospheres, where temperatures and pressures are high enough for NH₃'s atmospheric production.