Recent results and future prospects for the spectroscopy of comets

The links between cometary science and molecular spectroscopy date back as far as ca. 1860, with the beginnings of astronomical spectroscopy and astrophysics. In these days where the real nature of cometary material was quite unknown, cometary spectra were compared to laboratory spectra. By now, although the space exploration of comets is on the verge of performing direct chemical analysis of cometary nuclei, this spectroscopic approach is still topical. Spectroscopic observations of comets allow us to determine the physical conditions (temperature, velocity field, density) of the cometary environment as well as the chemical composition of these bodies. This information is precious for understanding how comets are working and how they interact with the solar environment. It gives us clues to the origin and first steps of the formation of the Solar System. It also allows us to evaluate the possible role played by comets in the origin of life, by bringing complex molecules to the early Earth. Observations of the exceptional comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp) and other recent bright comets—taking advantage of the availability of efficient new instrumentation (both ground-based and space-based) in the radio, infrared and UV—revealed the molecular complexity of comets as well as their diversity. After the study of secondary products (‘daughter molecules’) which have spectroscopic signatures in the visible, we are now able to study molecules directly sublimated from cometary ices (‘parent molecules’), using radio and infrared spectroscopy. More than two dozen such molecules are now known (not counting isotopic species). The last, most complex species identified to date is ethylene glycol, which has a remarkably high abundance relative to water. The most abundant compounds are now probably all known, but a wealth of minor components are still to be identified and studied. Many unidentified lines are present in cometary spectra, from radio wavelengths to the UV. Future progress relies on the extensive study of cometary spectra, for which the use of comprehensive and reliable spectroscopic databases is crucial. †Based on a review presented at the 19th Colloquium on High Resolution Molecular Spectroscopy, 12–16 September 2006, Salamanca, Spain.