Leiden Observatory

The birthplace of stars, planets, and small bodies are molecular clouds consisting mainly of H and He gas as well as tiny amounts of solid particles. Dust and gas from such an interstellar cloud collapsed to form a central condensation that became the Sun, as well as a surrounding disk — the solar nebula. In this protoplanetary disk innumerable submicrometer particles — icy and refractory in nature — agglomerated to larger planetesimals and subsequently into planets. Comets were formed from remnant inner disk material that was not incorporated into planets. Comets assembled beyond the orbit of Jupiter and may therefore provide a record of some pristine material from the parent interstellar cloud. Our knowledge of the composition of comets is predominantly based on evaporation of volatile species and thermal emission from siliceous and carbonaceous dust when bright comets approach the Sun. The investigation of outgassing curves from bright comets has provided a general link with abundances of ices and gas phase molecules detected in dense interstellar clouds. Theoretical models indicate that bulk material in cometary nuclei is stratified in density, porosity, and composition and contains coexisting ice phases, possibly clathrates and trapped gases. It is therefore apparent that the outgassing of species from cometary nuclei will not essentially mimic that expected from pure sublimation of ices of interstellar composition. The current lack of astronomical data related to solar-type star-forming regions, and the statistically small sample of comets studied to date, are the major obstacles to be overcome before a coherent link between interstellar and solar system material can be established. This chapter attempts to compile the current knowledge on the connection between interstellar and cometary material, based on observations of interstel-lar dust and gas, observations of cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter. 1