Constraints on the Origin of the Solar System from Meteorites

Abstract

Primitive meteorites have preserved material that was present in the presolar nebula and record processes that occurred as evolution proceeded from the earliest solids. The discovery of isotopic anomalies in these samples led to the isolation of presolar grains and allowed the presence of short-lived radionuclides in the early solar system to be inferred. Isotopic anomalies in oxygen may reflect non-linear chemical fractionation rather than a nuclear effect, but the theory is as yet insufficiently developed to be rigorously assessed.Analyses of individual SiC and refractory oxide presolar grains reveal that a large number of distinct nucleosynthetic sites contributed material to the solar nebula, and much progress has been made in identifying the various environments in which they formed. Isotopic anomalies associated with nanometre-size diamonds are best explained by supernova nucleosynthesis but it is clear that several sub-populations exist.The extinct nuclides 26Al, 53Mn and 129I have each been used to establish the relative timing of events in the formation of the solar system. Calibrations of the Mn-Cr and I-Xe systems against the Pb-Pb system (based on decay of uranium isotopes) have been proposed, and Al-Mg data can be included through a calibration with the I-Xe scheme. Assuming these calibrations to be valid allows a tentative chronology of the early solar system to be developed, the plausibility of which can be seen as a test of the calibrations. In this chronology, the first solids to form in the solar system were refractory inclusions. Chondrules (rapidly cooled silicate droplets) appear to have formed later than CAIs over a period of a few million years. Parent body processing began early in solar system history and was ongoing as chondrules formed.