From Dust to Planetesimals: Implications for the Solar Protoplanetary Disk from Short-lived Radionuclides

Since the publication of the Protostars and Planets IV volume in 2000, there have been significant advances in our understanding of the potential sources and distributions of shortlived, now extinct, radionuclides in the early solar system. Based on recent data, there is definitive evidence for the presence of two new short-lived radionuclides (10Be and 36Cl) and a compelling case can be made for revising the estimates of the initial solar system abundances of several others (e.g., 26Al, 60Fe, and 182Hf). The presence of 10Be, which is produced only by spallation reactions, is either the result of irradiation within the solar nebula (a process that possibly also resulted in the production of some of the other short-lived radionuclides) or of trapping of galactic cosmic rays in the protosolar molecular cloud. On the other hand, the latest estimates for the initial solar system abundance of 60Fe, which is produced only by stellar nucleosynthesis, indicate that this short-lived radionuclide (and possibly significant proportions of others with mean lives ?10 m.y.) was injected into the solar nebula from a nearby stellar source. As such, at least two distinct sources (e.g., irradiation and stellar nucleosynthesis) are required to account for the abundances of the short-lived radionuclides estimated to be present in the early solar system. In addition to providing constraints on the sources of material in the solar system, short-lived radionuclides also have the potential to provide fine-scale chronological information for events that occurred in the solar protoplanetary disk. An increasing number of studies are demonstrating the feasibility of applying at least some of these radionuclides as high-resolution chronometers. From these studies, it can be inferred that the millimeter- to centimeter-sized refractory calcium-aluminum-rich inclusions in chondritic meteorites are among the earliest solids to form (at 4567.2 ± 0.6 Ma). Formation of chondrules (i.e., submillimetersized ferromagnesian silicate spherules in chondrites) is likely to have occurred over a time span of at least ~3 m.y., with the earliest ones possibly forming contemporaneously with CAIs. Recent work also suggests that the earliest planetesimals began accreting and differentiating within a million years of CAI formation, i.e., essentially contemporaneous with chondrule formation. If so, it is likely that undifferentiated chondrite parent bodies accreted a few million years thereafter, when the short-lived radionuclides that served as the main heat sources for melting planetesimals (26Al and 60Fe) were nearly extinct.