Ice Formation on the Surfaces of Interstellar Dust Grains: Chemical Processing of the Ice
Ice mantles on the surfaces of interstellar dust grains are detected in denser, cooler gas clouds in the interstellar medium. The ices are predominantly water ice, with significant amounts of other fairly simple molecular species. The parameters controlling the onset of ice deposition are not well understood. Laboratory experiments show that mixed ices of simple molecular species irradiated by UV, X-rays, ions, or electrons may generate new and more complex chemical species, many of which are detected in very dense gas in star-forming regions. It is difficult to identify reaction schemes that form some of these species through gas phase chemistry, so it is now frequently assumed that the origin of the more complex interstellar molecules arises in part or entirely through ice processing. This chemical change may occur when irradiation creates radicals within the ice. In suitable conditions, the radicals may become mobile and associate to form more complex species. The predicted range of complex species formed in this way has a striking similarity to the list of detected interstellar large molecules.