Unresolved questions regarding the origins of Solar System solids

It is remarkable that we possess samples of Solar System solids which have retained a record of galactic and stellar events that occurred well before the birth of the Sun, as well as samples which contain evidence of Solar System processes that occurred during the earliest stages of planet building. The signatures of such processes are most commonly recognized in isotopic anomalies, identifiable against a pervasive background of isotopic homogeneity, and systematic trends in the elemental abundance patterns of primitive meteoritic materials. Although the significance of these anomalies and patterns is unmistakable, their interpretation in terms of specific processes is problematic. Central to such interpretations are questions of spatial scale: do cosmogonically significant isotopic and elemental compositions reflect processes that occurred on the grand scale of the proto–solar cloud, or do they result from many localized events within a nebular or planetary environment? The question is fundamental to our understanding of the formation of solid objects in the Solar System. This question will be examined here, with specific reference to theoretical models of nebular evolution and planet building, and evidence regarding the survival of presolar signatures, the origin of short–lived radionuclides and oxygen isotopic systematics.