Ultraviolet spectrophotometry of comet Giacobini-Zinner during the ICE encounter

Abstract

A program of ultraviolet spectrophotometry using the International Ultraviolet Explorer (IUE) Observatory was carried out in support of the International Cometary Explorer (ICE) mission. The H ₂O production rate was monitored from 1985 June to October. Between 1985 September 9 and 12, the spatial and temporal variation and abundance (or upper limits) of the remotely detectable species, C, CO, CO ⁺, CO ₂⁺, CS, H, Mg ⁺, O, OH, and S, were obtained. These observations included the time of the ICE encounter (1985 September 11.46) when the H ₂O production rate was 3 × 10 ²⁸ sec ^-1 ± 50%. This rate is consistent with a number of gas production rates derived indirectly from the ICE experiments. The comet was in a nearly steady state around the time of encounter showing no evidence of short-term temporal fluctuations in brightness greater than 6%. A sunward-tailward asymmetry of the OH brightness was observed at 10,000 km from the nucleus. The absence of detected Mg ⁺ rules out this species as a possible ion of M/Q = 24 which was detected by the Ion Composition Instrument, part of the ICE complement of instruments. Comparison of the abundance of CO ₂⁺ ions with total electron density measured by the plasma electron and radio science experiments on ICE indicates a deficiency of ions relative to electrons. To satisfy charge balance criteria, a major population of ions not detected by remote sensing must be present.