THE ABUNDANCE OF AMMONIA IN COMET-P HALLEY DERIVED FROM ULTRAVIOLET SPECTROPHOTOMETRY OF NH BY ASTRON AND IUE

Spectrophotometric spatial profiles of the ultraviolet emissions of NH and OH in comet P/Halley (1986 III) derived from observations made by the ASTRON satellite on 1986 April 9 are used to determine the total content of these radicals in the coma and consequently the ratio of ammonia-to-water production rates. This ratio is found to lie in the range of 0.44%-0.94%, where the limits accommodate the possible values of the NH and OH photochemical lifetimes based on both recent theoretical work and the observed spatial profiles. In order to compare this result with those based on both ground-based and in situ observations made on other dates during the 1985-1986 apparition of P/Halley, the IUE observational data base for 1985 December and 1986 March-April (which includes nearly simultaneous observations on April 9) is used to evaluate the ratio of NH to OH column density in the IUE field of view and thus constrain the long-term behavior of this ratio. Although our results are more compatible with the ammonia to water abundance ratio derived by Allen et al. from in situ Giotto ion mass spectrometer measurements than with the lower values derived from a number of ground-based observations of NH2, cumulative uncertainties in the latter do not preclude consistency. The IUE data base also indicates that, to within a factor of 2, the ammonia-to-water production rate ratio is the same for a small sample of moderately bright comets observed recently.