Giotto IMS measurements of the production rate of hydrogen cyanide in the coma of comet Halley.
Schwenn, R.; Balsiger, H.; Geiss, J.; Ip, W. -H.; Goldstein, B. E.; Lazarus, A. J.; Meier, A.; Rosenbauer, H.; Shelley, E.; Kettmann, G.
Germany, Switzerland, United States
Abstract
The ion composition measurements in the ionosphere of comet Halley by the ion mass spectrometer (IMS) experiment on the Giotto spacecraft are used to estimate the relative abundance of HCN. From a comparison of the normalized number density of ions with mass-to-charge (M/q) ratio of 28 AMU/e with steady-state photochemical models, it can be determined that the production rate of HCN directly from the central nucleus is Q(HCN) ⪉ 2×10-4Q(H2O) at the time of Giotto encounter. The related photochemical model calculations also indicate that Q(NH3)/Q(H2O) ≡ 5×10-3 in agreement with recent determinations from ground-based observations. The estimated value of Q(HCN) is lower than the relative abundance of Q(HCN)/Q(H2O) ≈ 10-3 as derived from radio observations of the 88.6 GHz emission of the J = 1-0 transition of HCN. The difference may be the result of time-variations of the coma deposition and dynamics as well as other model-dependent effects.