Isotopic Heterogeneities in the Solar System

The nature of isotopic anomalies in solar system material is discussed with emphasis on correlated anomalies in refractory elements. Evidence for the existence of short-lived radioactive nuclides in the early solar system is given and is used to estimate the time scale for the last injection of freshly synthesized nuclear material. It is shown that the early solar nebula was incompletely mixed and contained debris which was injected from a stellar source at most a few million years prior to the formation of the solar system. It appears that the average solar system material is made up of ambient interstellar material which was deficient in certain nuclear species to which was added a small fraction of freshly synthesized material. It is this mixture which now makes up the bulk solar system. At the present time there may still remain both substantial and subtle isotopic differences between the sun, the terrestrial planets, and volatile-rich planetary bodies including comets. The isotopic variations which are observed reflect slightly different proportions of nuclei from different stellar sources which were locally well mixed and homogenized prior to or during the formation of the early solar nebula condensates. The process of local homogenization apparently destroyed most pre-solar dust grains but preserved the distinctive average isotopic character of the local regions. Substantial isotopic differences existed between the cool gas and some condensed matter. This is manifest in major chemical and some associated isotopic alteration of the early condensates