The Chemical and Physical Evolution of Interstellar Dust

Abstract

The chemical and physical evolution of interstellar dust is studied by theoretical interpretations combining observations and laboratory experiments. Following the various stages of evolution leads to a picture of grains as consisting of basically two distinct size populations: tenth micron and hundredth (or less) micron. The larger particles are core-mantle structures with cores of silicates, inner mantles of a complex organic refractory material and outer mantles of various ices dominated by frozen H2O. The key to the evolution of grain mantles is solid state photoprocessing by ultraviolet radiation while fully taking into account the gas phase and surface reactions of interstellar atoms and molecules. After a mean lifetime of several thousand million years most grains are consumed by star formation. Some, however, survive essentially intact during aggregation into comets. It is shown that the ultimate observation of interstellar dust particles will have to await the recovery of a sample of a comet nucleus which has preserved the interstellar dust as it was 4½ thousand million years ago.