Meteorite Evidence for the Accretion and Collisional Evolution of Asteroids

Meteorites contain a record of impacts during all stages of asteroid origin and evolution: the formation and accretion of chondritic particles; the alteration, metamorphism and melting of asteroids; and the erosion and disruption of asteroids by hypervelocity impacts. A review of meteorite classification shows that numerous meteorites are not readily classified because they do not fit simple models for asteroid formation and evolution that assume impacts were only important during the final stage of asteroid evolution and because of inadequate understanding of asteroidal impacts. Chronological, textural, and thermal constraints allow us to identify meteorite impact breccias that formed during accretion (e.g., Kaidun), when asteroids were partly molten (e.g., mesosiderites), and during the subsequent disruption of asteroids (e.g., L chondrites). Studies of chondrites including Kaidun suggest that chondrules accreted with similarsized fragments of preexisting bodies that formed at greater heliocentric distances. In the inner solar system, chondrules appear to have been crucial for initiating accretion. Without chondrules and rock fragments, dry dust failed to accrete to the nebular midplane because of nebular turbulence and spiraled into the protosun. The tiny mass of asteroids may be partly due to inefficient chondrule formation beyond 2 AU or less-efficient delivery of chondrules from near the protosun.