Chapter 4 - Planetary Impacts

Abstract

Impacts were a major geologic process in early planetary evolution and served to physically characterize the early upper crusts and surfaces of planetary bodies. The impact flux of very large bodies was still sufficiently high, as late as ∼4.0billion years ago, to produce impact basins with diameters measured in hundreds to thousands of kilometers. Impact structures vary in morphology depending on their size, planetary surface gravity and target materials. Impacts also produce irreversible changes (shock metamorphism) in the target materials. Although impacts producing craters 100–200km in diameter are relatively rare in more recent geologic time, they still occur on timescales of approximately 100million years. One such event on the Earth marks the boundary between the Cretaceous and Paleogene geologic periods and resulted in the mass extinction of approximately 75% of the species living on Earth 65million years ago.