Perspectives on the Comet-Asteroid-Meteorite Link

We discuss the possibility that CI and CM carbonaceous chondrites are fragments of extinct cometary nuclei. Theoretical and observational work suggests that comets evolve into asteroids, and several extinct cometary nuclei are now suspected to be among the near Earth object population. This population is the most likely source of meteorites and consequently, we may expect that some meteorites are from extinct comets in this population. The mineralogy and chemistry of CI and CM chondrites is consistent with the view that they originate from asteroidal objects of carbonaceous spectral classes, and these objects in turn may have a cometary origin. We do not suggest that CI or CM chondrites are directly delivered by active comets during perihelion passage or that these chondrites come from cometary debris in meteor streams. Instead, we summarize arguments suggesting that CI and CM chondrites represent fragments of cometary nuclei which evolved into near Earth asteroids after losing their volatiles. This revised version was published online in June 2006 with corrections to the Cover Date.     

An Oxygen Isotope Mixing Model for the Accretion and Composition of Rocky Planets

The oxygen isotope systematics in planetary and nebular matter are used to constrain the types of nebular material accreted to form a planet. The basic assumption of this model is that the mean oxygen isotopic composition of a planet is determined by the weighted mean oxygen isotopic composition of nebular matter accreted by the planet. Chondrites are taken as representatives of nebular matter. The chemical composition (which determines core size, mantle oxidation state, density, moment of inertia) of a planet results from the weighted mean compositions of the accreted nebular material, once the mass fractions of the different types of accreting matter are known. Here some results for Earth, Moon, Mars, and Vesta are discussed. The model implies that loss of volatile elements, such as alkalis and halogens, occurs during accretion and early planetary differentiation (e. g., by catastrophic impacts). The possible depletion mechanisms of moderately volatile elements are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.