In situ gas and ion measurements at comet Halley
Berthelier, J. J.; Eberhardt, P.; Krankowsky, D.; Hodges, R. R.; Schulte, W.; Dolder, U.; Lammerzahl, P.; Woweries, J.; Hoffman, J. H.; Illiano, J. M.; Herrwerth, I.; Herrmann, U.
Germany, Switzerland
Abstract
The neutral mass spectrometer experiment (NMS) carried by the Giotto spacecraft was designed to determine the abundances and the chemical, elemental and isotopic composition of the gases and low-energy ions in the coma of comet Halley. Its first results show the predominance of water vapour (~80% by volume). Preliminary evaluation of the data yields an H2O density of 4.7 × 107 molecules cm-3 at 1,000 km, a neutral gas expansion velocity of 0.9 km s-1 and a total gas production rate of 6.9 × 1029 molecules s-1. The photodestruction scale length for H2O was 3.9 × 104 km. Limits on the abundances of CO2, NH3 and CH4 relative to H2O are given. The contact surface was located at 4,700+/-200 km, as observed from a change in ion temperature. The water-group ions H3O+, H2O+ and OH+ have been unambiguously identified, along with the ions 12C+, 12CH+, 16O+, Na+, 12C2+, 32S+, 34S+ and 56Fe+. The 34S+/32S+ ratio is 0.045+/-0.010. General features of the low-energy ion flow at large distances from the nucleus are presented.