The CO and N2 abundance in comet P/Halley

Abstract

A preliminary evaluation of the mass 28 amu/e signal observed in the neutral mode of the Giotto neutral gas mass spectrometer (NMS) is presented. At 1000 km from the nucleus we obtain for the CO number density n(CO)/n(H2 0);::;;0.07. The production rate of CO as a parent molecule directly from the nucleus is thus less than 7 % of the H 2 0 production rate. However, CO is also produced from an extended source in the inner coma (R < 20 000 km) and at 20 000 km from the nucleus we obtain for the total equivalent CO production rate 0.05;::;; Q(CO)/Q(H2 0) ;::;;0.15. For N 2 an upper limit Q(N2)/Q(H2 0) ;::;;0.1 is derived. No parent molecule for the CO could be identified which would be in agreement with the NMS measurements. It is proposed that CO or a very short-lived parent is relased in the coma from cometary dust grains, such as the "CHON" particles. The Neutral Gas Mass Spectrometer (NMS) on the Giotto spacecraft observed the first peaks due to neutral gas species at distances from the nucleus > 150 000 km. The last neutral mass spectrum before the NMS was damaged, probably by a large-amplitude electrical transient, was obtained in a distance of-850 km. The radial profile of the H2 0 density and upper limits of the C02, NH3, and CH4 abundances have been published (Krankowsky et al., 1986b). In this paper we give a preliminary evaluation of the CO and N 2 densities and related parameters (cf. also Eberhardt et al., 1986). A description of the NMS has been published (Krankowsky et al., 1986a). The instrument had two independent analyzers: the M-analyzer and the E-analyzer. The data reported in this paper are based essentially on the M-analyzer, which is a double focussing mass spectrometer covering the mass range from 1 amu/e to 37 amu/e. The half peak width at the 1 % level is-0.5 amu essentially independent of mass and the abundance sensitivity for adjacent integer mass number is > 10 3 • An electron impact ion source with fly-through geometry and two electron energies of-90 e V (He V mode) and-17 e V (Le V mode) was used. Cometary ions were suppressed by electrostatic deflection plates and a potential barrier. A focal plane detector allowed Send offprint requests to: P. Eberhardt simultaneous measurement of all masses over the whole mass range of the instrument.