  Molecular nitrogen is the most commonly assumed background gas that supports
habitability on rocky planets. Despite its chemical inertness, nitrogen
molecule is broken by lightning, hot volcanic vents, and bolide impacts, and
can be converted into soluble nitrogen compounds and then sequestered in the
ocean. The very stability of nitrogen, and that of nitrogen-based habitability,
is thus called into question. Here we determine the lifetime of molecular
nitrogen vis-a-vis aqueous sequestration, by developing a novel model that
couples atmospheric photochemistry and oceanic chemistry. We find that HNO, the
dominant nitrogen compounds produced in anoxic atmospheres, is converted to N2O
in the ocean, rather than oxidized to nitrites or nitrates as previously
assumed. This N2O is then released back into the atmosphere and quickly
converted to N2. We also find that the deposition rate of NO is severely
limited by the kinetics of the aqueous-phase reaction that converts NO to
nitrites in the ocean. Putting these insights together, we conclude that the
atmosphere must produce nitrogen species at least as oxidized as NO2 and HNO2
to enable aqueous sequestration. The lifetime of molecular nitrogen in anoxic
atmospheres is determined to be >1 billion years on temperate planets of both
Sun-like and M dwarf stars. This result upholds the validity of molecular
nitrogen as a universal background gas on rocky planets.
