Planetary compositions

We review the chemical and mineralogical properties of primitive meteorites and chemical data for the Sun, Comet Halley and interplanetary dust particles. Regardless of where meteorites formed, concentrations of rock-forming elements in solar nebular solids could not have varied simply with distance from the Sun. Thus compositional differences between neighboring planets and the chemical and mineralogical diversity of chondritic asteroids may have been caused by local variations in the compositions of planetesimals, rather than transport of planetesimals over large heliocentric distances. Chemical variations were partly caused by differential transport and preferential agglomeration of various presolar and solar grains and aggregates, and the production from these aggregates of diverse types of chondrules, refractory inclusions and other chondritic components in brief, local high temperature events in the nebula. These processes were just as important in controlling solar system chemistry as effects due to changes in ambient nebular temperatures and pressures. Differences between the Fe/Si ratios of the Sun, CI chondrites, interplanetary dust particles and Comet Halley suggest that planetesimals in the outer solar system had diverse relative concentrations of rock-forming elements. (orig.)