Chemical evolution in space

The bulk of complex molecules in the space between the stars is probably contained in small frozen interstellar dust grains. A typical grain is about as old as the earth and has, as a result of photochemical processing, converted a large fraction of its oxygen, carbon and nitrogen bearing mantle into large organic molecules whose maximum molecular weights are limited only by the grain size of about 0.1 m. Laboratory and theoretical methods provide the basis for explaining the evolution of interstellar grains from the time they are formed as seedlings in the atmospheres of cool evolved stars to the time they are destroyed by being incorporated into the material of new stars. The organic dust constitutes about one tenth of a percent of the total mass of the Milky Way and far outweighs any estimates of the total mass of all the planets. A planet like the earth is continually and directly accreting interstellar dust from space. Primitive carbonaceous meteorites show evidence of containing interstellar dust. Since comets are possibly almost pure aggregated interstellar dust they also provide a source of interstellar organic material on the earth.