Solids and Volatiles in Comets

Abstract

Comets occupy a key position in the cycles of ices and dust in the Universe. Their nuclei have mostly kept intact the composition and morphology they had at their formation. The return of a comet close to the Earth and Sun is thus an appreciated opportunity to investigate the original state of the material which built up our Solar System. The physical processes occurring in cometary nuclei and cometary atmospheres, as well as the interface between the two media, are reviewed. The status of our present knowledge of the chemical composition and physical state of comets is presented. The recent observations of comets C/1996 B2 (Hyakutake) and C/1995 01 (Hale-Bopp), which provided a bonanza of new cometary molecules, are outlined. There are strong analogies between interstellar and cometary ices. In contrast, the silicates of cometary dust (for comet Hale-Bopp) are akin to circumstellar, rather than interstellar dust. Different families of comets may have different composition and history, however. These clues will have to be taken into consideration to achieve a consistent scenario of cometary formation.