Overview of the rocky component of Wild 2 comet samples: Insight into the early solar system, relationship with meteoritic materials and the differences between comets and asteroids

The solid 210 mu m samples of comet Wild 2 provide a limited but direct view of the solar nebula solids that accreted to form Jupiter family comets. The samples collected by the Stardust mission are dominated by high-temperature materials that are closely analogous to meteoritic components. These materials include chondrule and CAI-like fragments. Five presolar grains have been discovered, but it is clear that isotopically anomalous presolar grains are only a minor fraction of the comet. Although uncertain, the presolar grain content is perhaps higher than found in chondrites and most interplanetary dust particles. It appears that the majority of the analyzed Wild 2 solids were produced in high-temperature rock forming environments, and they were then transported past the orbit of Neptune, where they accreted along with ice and organic components to form comet Wild 2. We hypothesize that Wild 2 rocky components are a sample of a ubiquitously distributed flow of nebular solids that was accreted by all bodies including planets and meteorite parent bodies. A primary difference between asteroids and the rocky content of comets is that comets are dominated by this widely distributed component. Asteroids contain this component, but are dominated by locally made materials that give chondrite groups their distinctive properties. Because of the large radial mixing in this scenario, it seems likely that most comets contain a similar mix of rocky materials. If this hypothesis is correct, then properties such as oxygen isotopes and minor element abundances in olivine, should have a wider dispersion than in any chondrite group, and this may be a characteristic property of primitive outer solar system bodies made from widely transported components.