Observational constraints on solar nebula nitrogen chemistry: N2/NH3

Maria Womack, Susan Wyckoff, L. M. Ziurys

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29 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)728-735
Number of pages8
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 20 1992


  • Comets: general
  • ISM: molecules
  • Molecular processes
  • Solar system: formation

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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