Observational constraints on solar nebula nitrogen chemistry: N₂/NH₃

The abundance ratio N₂/NH₃ in comets and dense molecular clouds is predicted by chemical models to be critical diagnostic of physical and chemical conditions of the solar nebula. Observations of N⁺₂ and NH₂ in comet Halley and N₂H⁺ and NH₃ in nine Galactic star-forming regions are used to determine the average N₂/NH₃ abundance ratio in these objects. For comet Halley we find N₂/NH₃ ∼ 0.1, and for the quiescent gas in a sample of star-forming regions, N₂/NH₃ ∼ 170 ± 100. The cometary N₂/NH₃ ratio corrected for gasphase elemental nitrogen depletion in comet Halley indicates that the gas component in the comet-forming region of the solar nebula was N₂/NH₃ ∼4. Correcting the dense cloud value for depletion of ammonia onto grains yields a total (gas and grain) ratio, N₂/NH₃ ∼ 4. However, the depletion correction is highly uncertain because the N₂ and NH₃ abundances are not correlated with cloud temperature. We conclude that more realistic models which include condensation fractionation effects are required before the N₂ and NH₃ abundances in comet volatiles can be related to star-forming regions. We point out that the assumption N₂ ≫ NH₃ used as an initial condition in most models of solar nebula chemistry is consistent with the observations of dense molecular clouds but is not justified by observations of comet Halley.