In situ gas and ion measurements at comet Halley

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.