Ammonia abundances in four comets

Ammonia released from the nuclei of comets photodissociates with a branching ratio ∼97% into NH₂, which produces a moderately strong fluorescence spectrum in the 4500-8000 Å region in comets. Therefore, the determination of the NH₂ production rate in comets gives essentially a direct measure of the ammonia content. The NH₂ emission band strengths were measured in four comets (Halley, Borrelly, Hartley-Good, and Thiele), and the NH₂ column densities determined using improved fluorescence efficiencies. We demonstrate from an intercomparison of three comet coma models (collisional Monte Carlo, vectorial, and Haser) that there are differences of ∼25% among the models for the same assumed input parameters. The overall accuracy of the ammonia production rates is presently limited by uncertainties in the NH₂ photoabsorption cross sections. The mean ammonia/water abundance ratio derived for the four comets is found to be Q(NH₃)/Q(H₂O) = (0.13 ± 0.06)%, with no significant variation among the comets. The uniformity of the ammonia abundance among the four comets attests to a remarkable degree of chemical homogeneity over large scales (>1 AU) in the comet-forming region of the primordial solar nebula, and contrasts with the CO abundance variations found previously in comets. The N₂ and NH₃ abundances indicate a condensation temperature in the range, T ∼ 20-160 K, consistent with virtually all comet-formation hypotheses.