WHEN AND WHY DO ASTEROIDS COLLIDE IN THE ASTEROID BELT? EVIDENCE FROM THE SEDIMENTARY RECORD

In the 1980s and 90s it was commonly believed that comet impacts were responsible for many, if not most, of Earth's large impact craters. Much has been written about possible perturbations of the Oort comet cloud, and a relation to the cratering record on Earth has been proposed. As the solar system in its orbit around the galactic center oscillates perpendicularly relative to the galactic equatorial plane, high concentrations of dark matter or interstellar dust in the plane may disturb the Oort cloud, sending comets to the inner solar system. More recent data, however, such as Cr isotopes and Cr-spinels in impact-related sediments, indicate that most of Earth's large craters instead formed by impacts of ordinary chondritic bodies from the asteroid belt. In our studies of extraterrestrial Cr-spinels in Earth's sedimentary strata we can determine with a very high resolution the time when major asteroid collisions in the asteroid belt took place. If our approach will show that such asteroid breakups tend to coincide with significant changes in Earth's climate, tectonism or life, it may be relevant to formulate models how large-scale astronomical processes can affect both the asteroid belt and Earth. Here I discuss the evidence for asteroid breakup events in the mid-Ordovician and in the late Eocene and their relation to earthly events.