Introduction: Impact cratering and planetary studies—a fifty-year perspective

Abstract

*Emails: Gibson: roger.Gibson@wits.ac.za; Reimold: uwe.reimold@mfn-berlin.de. Of all the geological processes having—and still— affecting the planets in our solar system, it is only impact cratering that can lay claim to being ubiquitous. Today, some 50 years after the emergence of impact cratering studies as a distinct discipline within the geosciences, it is worth refl ecting briefl y on the various stages through which the discipline has evolved to its present, multi-facetted, form. In the 1960s, systematic studies of lunar surface images and of terrestrial craters prompted by the race for the Moon delivered the fi rst quantitative morphological datasets on impact craters. Building on the intensive studies of shock-wave phenomena, also seen around nuclear explosions, from the 1950s and 1960s, scientists studying the fi rst lunar samples and the growing geological and mineralogical fi eldand sample-based dataset from terrestrial craters turned their attention to understanding the process of cratering and the peculiar rock deformation features and microscopic mineral phenomena associated with impacts (e.g., French and Short, 1968). Simultaneous technological advances in the military sphere provided the opportunity to glimpse the exceptional, transient pressure and temperature conditions generated by high-velocity collisions. Consequently, by the late 1970s, scientists were armed with a wide array of experimentally constrained shock deformation features with which to test the origin of circular structures on Earth (e.g., Stoffl er and Langenhorst, 1994; Grieve et al., 1996). At the other end of the scale, the 1960s and 1970s also saw the fi rst systematic remote sensing investigations of other planetary surfaces in the solar system. Studies of the lunar surface were complemented by investigations of the samples returned to Earth by the Apollo and, to a smaller degree, the Luna missions, whilst the Pioneer and Voyager probes provided fundamental insights on crater forms on bodies of different target composition, size, and gravitational strength compared to those on Earth. Hands-on studies of natural shock metamorphism were complemented by shock experiments with a range of rock-forming minerals and lithologies, and by detailed analysis of meteorites (e.g., Stoffl er et al., 2006, and references therein). The detailed shock-metamorphism experimental database, as well as cataloging of fi eld and microscopic shock features, led to the identifi cation of new terrestrial impact structures at a rate of two to three per year during the 1970s to 1990s, with the consequence that the database grew from a mere dozen in 1960 to more than 150 structures by the mid-1990s. By this time, the identifi cation of the globally-distributed extraterrestrial iridium anomaly in the Cretaceous-Tertiary (K-T) boundary layer and the establishment of a link between the end-Cretaceous mass extinction event and the large Chicxulub impact structure in Mexico (e.g., Koeberl and MacLeod, 2002, and references therein) provided global awareness of the devastating planetary-scale environmental effects of large impacts. Earlier work that established the impact origin of the Sudbury structure in Canada (Grieve, 2006 and refs. therein), and the fi nal steps in confi rming the impact origin of the Vredefort structure in South Africa (Gibson