Asteroid break-ups and meteorite delivery to Earth the past 500 million years

Abstract

Significance The standard view of meteorite delivery to Earth is that of the cascading model where large asteroid break-ups generate new fragment populations that feed the inner solar system with material for extended time periods. Our investigated time windows, stretching from the Cambrian to the present, do not support this model. In fact, of 70 major family-forming break-ups the past ∼500 Ma, only 1 appears to have given rise to a strongly enhanced flux to Earth. We argue that meteorites and small asteroids delivered to Earth in deep time are not primarily linked to the sequence of asteroid family-forming events. Another, as yet unknown, delivery process appears to be associated with a very restricted region in the asteroid belt. The meteoritic material falling on Earth is believed to derive from large break-up or cratering events in the asteroid belt. The flux of extraterrestrial material would then vary in accordance with the timing of such asteroid family-forming events. In order to validate this, we investigated marine sediments representing 15 time-windows in the Phanerozoic for content of micrometeoritic relict chrome-spinel grains (>32 μm). We compare these data with the timing of the 15 largest break-up events involving chrome-spinel–bearing asteroids (S- and V-types). Unexpectedly, our Phanerozoic time windows show a stable flux dominated by ordinary chondrites similar to today’s flux. Only in the mid-Ordovician, in connection with the break-up of the L-chondrite parent body, do we observe an anomalous micrometeorite regime with a two to three orders-of-magnitude increase in the flux of L-chondritic chrome-spinel grains to Earth. This corresponds to a one order-of-magnitude excess in the number of impact craters in the mid-Ordovician following the L-chondrite break-up, the only resolvable peak in Phanerozoic cratering rates indicative of an asteroid shower. We argue that meteorites and small (<1-km-sized) asteroids impacting Earth mainly sample a very small region of orbital space in the asteroid belt. This selectiveness has been remarkably stable over the past 500 Ma.