Ion composition and chemistry

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The gas and ion measurements by the VEGA and Giotto probes in the coma of comet Halley have provided much new information on the composition and chemistry of the volatile ices at the surface of the cometary nucleus. While it has been well-established that water vapour is the most dominant species followed by CO (≈ 15%) and CO2 (≈ 4%) in terms of sublimation rates relative to that of H2O, several major issues remain unresolved. In the present report, we consider the "missing" parent molecules of the C+ and S+ ions which were observed to be very abundant in the outer coma beginning at a radial distance of 105 km. The large abundance of singly-ionized helium ions is unexplained. The uncertainties involved in the determination of the relative abundance (0-1%) of ammonia are outlined. While ground-based optical observations provided an independent estimate of Q(NH3) < 3 × 10-3 Q(H2O), radio observations also permitted a derivation of the production rate of HCN to be Q(HCN) ≈ 10-3Q(NH3). The published result from the Giotto ion mass spectrometer (IMS) experiment pointed to a smaller value of Q(HCN) ≈ 3 × 10-4 Q(H2O). These production rates of HCN and NH3 together with an upper limit of Q(N2) ≤ 1.5% Q(H2O) as obtained from the IMS measurements on Giotto, could imply that the nitrogen abundance is highly reduced. The deficiency of N2 and NH3, and the abundance of CO and CO2 with a significant amount of H2CO could all be results of the condensation process of volatile ices in the solar nebula. If a conservative approach is adopted, the presence of formaldehyde polymers (POM's) and polycylic aromatic hydrogen carbons (PAH's), could be just a very minor component in the coma. Their formation might be directly linked to interstellar grain origin or could have been produced in the condensed icy grains through cosmic ray and/or UV irradiation after agglomeration in the solar nebula. On the other hand, if the generation of the CN and C2 jets observed in the coma of comet Halley should be related to the unzipping of PAH's and/or POM's connected with a monovalent radicals like CN and OH, the production of these complex polymers could be as high as 0.1 - 0.3 Q(H2O). The total mass of the detected population of submicron CHON grains is far too small to account for the POM's and PAH's. In fact, the organic CHON grains have difficulties in being the parent bodies of the CN and C2 jets from the point of veiw of mass budget - let alone the distributed source of the CO molecules. Thus there exists yet no consistent picture for a variety of gas phenomena and compositional variation observed in the coma of comet Halley. The questions as stated above should nevertheless permit a better focus of our attentions in the near future.

Original languageEnglish
Pages (from-to)141-150
Number of pages10
JournalAdvances in Space Research
Issue number3
StatePublished - 1989


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