Methanol and hydrogen sulfide in comet P/Halley.

Abstract

The Neutral Mass Spectrometer on the Giotto spacecraft measured the gas and ion composition in the coma of comet P/Halley. A detailed model of the ion chemistry inside the contact surface located at 4660 km is used to interpret the measured ion density profiles in the 32 to 35 amu/e mass range. The masses 33 and 35 amu/e are dominated by the protonated methanol and hydrogen sulfide ions CH_3_OH_2_^+^ and H_3_S^+^. Both profiles are essentially compatible with CH_3_OH and H_2_S originating from the nucleus only. The production rates relative to water are Y(CH_3_OH)=Q(CH_3_OH)/Q(H_2_O)=1.7% and Y(H_2_S)=0.41%. There is a hint in our data that the CH_3_OH mixing ratio may show a small temporal or spatial variation and that there could be a weak extended H_2_S source. The absolute errors in the CH_3_OH and H_2_S production rates due to the uncertainties in the measured ion densities and in the rate constants used in modelling the ion chemistry are estimated to be less than 30%. Our Y(CH_3_OH) agrees well with a determination from IR spectra obtained about 6 weeks after the Giotto encounter with P/Halley. In 7 other comets IR and microwave observations give Y(CH_3_OH) values between about 0.7 and 6%, indicating that the methanol abundance shows a strong variability from comet to comet. In three other comets Y(H_2_S) values between 0.2 and 0.5% have been reported. This limited number of observations seem to imply less variability in the H_2_S than in the CH_3_OH mixing ratio. In addition to H_2_S^+^ only ions containing minor isotopes of H, C, O and S contribute to mass 34 amu/e (e.g. ^34^S^+^, ^13^CH_3_OH_2_^+^, CH_4_DO^+^). These contributions can be calculated from the measured densities of the ions containing the major isotopes and the H_2_S^+^ contribution from the measured H_3_S^+^ density. Excellent agreement within a few percent between the predicted and the measured ion density at mass 34 amu/e are obtained over the whole distance range from 1740 km to the contact surface. This independent check verifies the validity of our ion chemical model. From mass 34 amu/e we can also derive an upper limit of 1% for the abundance of deuterated methanol. This limit is at most marginally compatible with a direct interstellar origin of the CH_3_OH in P/Halley as the measured interstellar abundance of deuterated methanol is 1 to 6%.