Atomic and molecular abundances in comet Giacobini-Zinner

Abstract

Spectrophotometric observations of comet Giacobini-Zinner were obtained on 11 September 1985. C 2 and NH 2 were found to be depleted in Giacobini-Zinner relative to an average comet by factors of 10 and 5, respectively. The ratio of the production rates, Q( NH 2)/ Q( H 2 O) = 2 × 10 −4 indicates a very low NH 3/H 2O abundance ratio if NH 2 comes mostly from photodissociation of NH 3. The water production rate for Giacobini-Zinner at the time of the ICE encounter was found to be 2.4 × 10 28 molecules s −1, in good agreement with spacecraft results. Neither neutral Na nor C 2 + ion was detected in the ground-based spectrum at the time of the ICE encounter. Therefore, Na + and C 2 + can be excluded from the candidates for the ions in the mass range 23 to 24 amu detected by the ICE ion composition instrument.