Evolution of the Solar Nebula. VIII. Spatial and Temporal Heterogeneity of Short‐lived Radioisotopes and Stable Oxygen Isotopes

Isotopic abundances of short-lived radionuclides such as 26Al provide themost precise chronometers of events in the early solar system, provided thatthey were initially homogeneously distributed. On the other hand, theabundances of the three stable isotopes of oxygen in primitive meteorites showa mass-independent fractionation that survived homogenization in the solarnebula. As as result of this and other cosmochemical evidence, the degree ofspatial heterogeneity of isotopes in the solar nebula has long been a puzzle.We show here that based on hydrodynamical models of the mixing and transport ofisotopic anomalies formed at, or injected onto, the surface of the solarnebula, initially high levels of isotopic spatial heterogeneity are expected tofall to steady state levels (~10%) low enough to validate the use of 26Al forchronometry, but high enough to preserve the evidence for mass-independentfractionation of oxygen isotopes. The solution to this puzzle relies on themixing being accomplished by the chaotic fluid motions in a marginallygravitationally unstable disk, as seems to be required for the formation of gasgiant planets and by the inability of alternative physical processes to drivelarge-scale mixing and transport in the planet-forming midplane of the solarnebula. Such a disk is also capable of large-scale outward transport of thethermally annealed dust grains found in comets, and of driving the shock frontsthat appear to be responsible for much of the thermal processing of thecomponents of primitive meteorites, creating a self-consistent picture of thebasic physical processes shaping the early solar nebula.Comment: 25 pages, 10 figure