Remote observations of the composition of cometary volatiles

The volatile species released in the coma are an important clue to the comp osition of the cometary nucleus ices. Their identification and the measurem ent of their abundances is possible by remote sensing. Considerable progres s has been made recently using radio and infrared spectroscopy, especially with the observations of the two exceptional comets C/1996 B2 (Hyakutake) a nd C/1995 O1 (Hale-Bopp).) 24 molecules likely to be parent molecules outga ssed from the nucleus have now been identified. Significant upper limits ex ist for many other species, and the presence of unidentified lines suggests that further species are to be identified. In addition, isotopic varieties have been observed for hydrogen, carbon, nitrogen and sulphur. We will rev iew these results: with a special emphasis on the reliability of the identi fications and of the molecular production rate determinations. A critical p oint is to assess whether a given species is a genuine parent molecule outg assed from nuclear ices, or is a secondary product coming from grains or fr om gas-phase photochemistry. Ground-based spectral imaging, such as radio i nterferometry, may help resolving this problem.