15 September 2005

Speaker: Renu Malhotra (University of Arizona)

Title: The Origin of Planetary Impactors in the Inner Solar System

Abstract: The Moon and all the terrestrial planets were resurfaced during a period of intense impact cratering that occurred between the time of their accretion, ~4.5 billion years ago, and ~3.85 billion years ago: the crater record and radiometric dating of lunar rocks attests to this conclusion. However, identifying the source(s) of those planetary impactors has proven elusive; speculations have included comets, asteroids, and fragments of a shattered 'large planetesimal'. I will describe compelling new evidence that the source of the impactors was the main asteroid belt, and that the dynamical mechanism that caused the so-called 'Late Heavy Bombardment' ~3.9 billion years ago was unique in the history of the Solar System and distinct from the processes producing the flux of objects that currently hit planetary surfaces. The Late Heavy Bombardment was a rain of asteroids dynamically ejected from the main asteroid belt due to the effects of orbital migration of Jupiter and Saturn.