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Molecular nitrogen is the most commonly assumed background gas that supportshabitability on rocky planets. Despite its chemical inertness, nitrogenmolecule is broken by lightning, hot volcanic vents, and bolide impacts, andcan be converted into soluble nitrogen compounds and then sequestered in theocean. The very stability of nitrogen, and that of nitrogen-based habitability,is thus called into question. Here we determine the lifetime of molecularnitrogen vis-a-vis aqueous sequestration, by developing a novel model thatcouples atmospheric photochemistry and oceanic chemistry. We find that HNO, thedominant nitrogen compounds produced in anoxic atmospheres, is converted to N2Oin the ocean, rather than oxidized to nitrites or nitrates as previouslyassumed. This N2O is then released back into the atmosphere and quicklyconverted to N2. We also find that the deposition rate of NO is severelylimited by the kinetics of the aqueous-phase reaction that converts NO tonitrites in the ocean. Putting these insights together, we conclude that theatmosphere must produce nitrogen species at least as oxidized as NO2 and HNO2to enable aqueous sequestration. The lifetime of molecular nitrogen in anoxicatmospheres is determined to be >1 billion years on temperate planets of bothSun-like and M dwarf stars. This result upholds the validity of molecularnitrogen as a universal background gas on rocky planets.

Stability of Nitrogen in Planetary Atmospheres in Contact with Liquid Water