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.
|Original language||English (US)|
|Number of pages||8|
|State||Published - Feb 10 1991|
- Molecular processes
ASJC Scopus subject areas
- Space and Planetary Science