Mixing and Transport of Isotopic Heterogeneity in the Early Solar System

Isotopic abundances of short-lived radioisotopes such as Al-26 appear to provide precise chronometers of events in the early Solar System, assuming that they were initially homogeneously distributed. However, both Fe-60 and Al-26 were likely formed in a supernova and then injected into the solar nebula in a highly heterogeneous manner. Conversely, the abundances in primitive meteorites of the three stable oxygen isotopes exhibitmass-independent fractionations that somehow survived homogenization in the solar nebula. Both the presence of refractory particles in Comet 81P/Wild 2 and the anomalously high crystallinity observed in protoplanetary disks may require large-scale outward radial transport from the hotter inner disk regions, even as disk gas accretes onto the central protostar. We examine here theoretical efforts to solve these seemingly disparate cosmochemical puzzles and conclude that the mixing and transport produced by a phase of marginal gravitational instability appears to meet all of these constraints.