IMPACT CRATERING AND PROCESSES: RECENT PROGRESS

Over the past decades, the study of impact craters and their formation has reached a degree of maturity that has led to a reasonably good understanding of the processes involved in their formation, and their importance for the evolution of planetary bodies in the solar system and, especially, on Earth. Over 30 years ago, the general geological community greeted evidence for a major asteroid impact on Earth at the end of the Cretaceous with disbelief. On the other hand, planetary scientists and cosmochemists, who were familiar with lunar samples and meteorites that show abundant evidence of impact and collisional processes, readily accepted the interpretation of an end-Cretaceous impact event. Research initiated by this controversy led to a better definition and understanding of impact evidence, particularly in terms of shock metamorphism and the identification of meteoritic components. One key result for the K-T impact studies was also the acknowledgment that impacts may have influenced biological evolution on Earth. It also became clear that remote sensing investigations of planetary surfaces, especially on Earth, do not suffice to obtain the information required for understanding the ultra-high strain rate, high-pressure, and high-temperature impact process. We now recognize that impact processes were of great importance on the early Earth, even though the oldest currently known impact craters on Earth are about 2 billion years old, and earlier impact evidence is restricted to a variety of spherule beds up to about 3.4 billion years in age, the formation of which is still incompletely understood. The development of new analytical tools has also greatly helped our understanding impact processes. High-resolution electron microscopic methods have allowed to properly describe shock metamorphic products and helped to distinguish them from endogenically formed features. In terms of geochemistry, methods with improved detection limits and high spatial resolution, and new isotopic systems (e.g., Os and Cr), as well as improvement of previously used methods, has led to an unprecedented amount of information, in particular regarding the identification of extraterrestrial components within impact breccias and melt rocks. Dating of impact events has improved, but still requires more attention.