Nitrogen abundance in comet Halley

An inventory of nitrogen compounds observed in the coma of comet Halley is presented, and the elemental nitrogen abundance in the nucleus is derived. The dust fraction of the coma is found to contain virtually all (90%) of the elemental nitrogen. In the gas coma we find that NH₃ and CN contain most of the nitrogen, and that (N/O)_ice ∼ 0.004, accounting for ∼10% of the nitrogen inventory in the comet. Molecular nitrogen accounts for only ∼7% of the nitrogen containing volatiles and less than 0.1% of the total volatile content of the comet. The elemental nitrogen abundance in the ice component of the nucleus is deficient by a factor ∼75 relative to the solar photosphere. For a mass ratio, dust/gas ∼2, we find that the total (dust + gas) elemental nitrogen abundance in comet Halley is depleted by a factor ∼6 relative to the Sun. A correction to the nitrogen inventory for undetected species (e.g., ammonium salts and polymers) would reduce the nitrogen deficiency in the comet gases by a factor of ∼2 at most, and not significantly affect the total (gas + dust) nitrogen inventory in the comet. The fact that the dust component contains most of the nitrogen-bearing compounds in comet Halley means that the volatile and refractory solids in the nucleus had different evolutionary histories and probably did not result from a simple condensation sequence in the solar nebula. If N₂ were the most abundant nitrogen-containing species in the early protosolar cloud, then the depletion of nitrogen in the gas coma of the comet relative to the Sun can probably be explained by physical fractionation of N₂ during the condensation process as originally suggested by Geiss, or by subsequent preferential diffusion of molecular nitrogen from the cometary ices, or by both. If, however, the low nitrogen abundance in comet Halley represents the actual N content of gas and dust in the comet-forming region, then the nucleus must be comprised of material with a different nucleosynthesis history from the rest of the solar system. The elemental nitrogen deficiency in the comet ices indicates that the chemical partitioning of N₂ into NH₃ and other nitrogen compounds during the evolution of the solar nebula cannot account completely for the low abundance ratio, N₂/NH₃ ∼ 0.1, observed in the comet. Moreover, the low and uniform ammonia/water abundance ratios among a small sample of comets indicates that comet nuclei were probably not subjected to significant episodic accretion of NH₃-rich material from the giant planet subnebulae. Rather, we suggest that the low N₂/NH₃ ratio in comet Halley may be explained simply by physical fractionation or thermal diffusion, or both.