The chemistry of comets

Abstract

In comets, most elements seem to be present in their cosmic abundances. This includes the metals whose abundances are the same as in chondrites, but also the light elements C, N, O, S that are the same as in the Sun; only hydrogen (and presumably helium and neon) is depleted by a factor close to 1000. In the bright comets of the 1970s, three-quarters of the cosmic abundance of carbon was found to be missing from the gaseous fraction. The missing carbon has now been found in Comet Halley: it was in the large organic fraction representing 33 % of the cometary dust. A part of this fraction vaporizes slowly out of the dust grains: it is the origin of an extended source of gas discovered around the nucleus of Comet Halley. Water remains the major constituent being 80% of the volatile ices. Formic acid, formaldehyde, carbon dioxide and carbon monoxide explain together more than 13% of the rest of the volatiles. The last 7% include the parent molecules of the radicals excited by fluorescence and observed in the traditional spectra, like hydrogen cyanide HCN (for CN), probably acetylene C₂C₂(for C₂) and cyclopropadiene C₃H 2 (for C₃). The inorganic fraction of the dust contains mainly silicates and some iron sulphide, whereas the organic fraction also contains unsaturated hydrocarbons and probably hydrogen cyanide, acetonitrile, aminoethylene, pyrrole, pyridine, pyrimidine and possibly purines including adenine. Some prebiotic precursors of the nucleic bases are present, but no traces of any amino acids have been found.