THE CHEMICAL ROLE OF COSMIC DUST

Dust particles in interstellar gas appear to have an active chemical r ole, which may affect the physical state of the gas. Dust catalyses re actions al its surface, acts as a sink for molecules in the formation of interstellar ices, allows the processing of ices by electromagnetic or particle radiation, and may be a source of molecules as the ices a re either slowly or abruptly returned to the gas phase. Plausible desc riptions of many of these processes have been developed, and it appear s that in a variety of interstellar and circumstellar regions the cont ribution of dust to microscopic processes is essential. However, a fir m basis for understanding these processes is, in general, missing. Sho uld these mechanisms be shown to be inoperative, the consequences for our understanding of the physical evolution of diffuse matter in space would be severe. This paper describes the likely chemical roles of co smic dust, and highlights the need for further basic studies to place these roles on a more secure foundation.