Abstract

More than 140 different molecules have been identified in inter- and circumstellar clouds. These range from simple diatomic species such as H₂ and CO to exotic radicals (linear C₆H and HC₁₁N) and complex organic molecules like CH₃OCH₃ and CH₃CH₂CN.This chemical complexity has been explained for many years by gas-phase processes driven by cosmic-ray ionization but new models show that such processes only reproduce observed abundances of the smaller and open shell, i.e. unstable species. Exciting new evidence has now been found that icy dust grains with temperatures as low as 15 K act as catalytic sites for molecule formation and that the formation of the more complex species involves surface processes in ices on interstellar grains. Thermal and UV processing of the ices in combination with a permanent bombardment by H-, N-, O- and C-atoms subsequently trigger a so far barely understood chemical evolution.