Characteristics of large terrestrial impact structures as revealed by remote sensing studies

Abstract

Remote sensing data sets of a spectrum of impact structures on Earth have been analyzed in an effort to determine if such structures have identifiable signatures which could be used as an aid in searching broad areas for the “missing craters” predicted by current estimates of the cratering flux. Meteor Crater, Arizona has been used as a “control” site, due to the extensive data available for this feature. Laboratory thermal infrared reflectance spectra of shocked sandstones indicate that shock processes alter the appearance of fundamental absorption features enough to permit such effects to be observed in airborne thermal infrared imagery collected with the NASA TIMS instrument. Analysis of this Tims imagery suggests that shocked and displaced target rocks can be readily identified. Aircraft multi-frequency imaging radar data collected with the NASA/JPL SAR polarimeter clearly reveals the textural pattern of the preserved components of the ejecta blanket. SPOT multi-spectral imagery of Bosumtwi, Ghana, illustrates the difficulties associated with remote sensing in highly vegetated terrains. Nevertheless, the imagery reveals the subtle expression of an outer tectonic ring at the crater, and may be sensitive to geobotanical variations associated with impact-related suevite deposits. In contrast, the Zhamanshin impact feature on the arid, sedimentary plains of Kazakhstan is ideal for multi-spectral remote sensing, although much of the expected morphologic expression of the complex crater is missing or subdued. Virtually all of the field-mapped impact-related deposits display a unique spectral pattern in decorrelated Landsat Thematic Mapper imagery. The extensive deposits of allogenic breccias, which cap the hills defining the inner ring of the crater, are very distinctive, as are certain of the impact glass deposits. A second, or outer, ring at 13–13.5 km is apparent in the decorrelated principal component TM imagery. The suggestion is that spectral disruption patterns for large, multi-ring impact features in arid settings should be identifiable from orbit. By examining a pristine 144 km diameter multi-ring impact feature on the Venus rolling plains, an impression of the original morphology of a continental impact crater of the magnitude of the Cretaceous-Tertiary (K/T) boundary impact structure can be deduced. The 35 Ma Popigai structure is the only known 100 km class impact crater on Earth approximately equal in age or younger than the K/T event, and its morphology is poorly expressed in single-band Landsat imagery. Thus, spectral anomalies associated with key impact deposits, such as allogenic breccias, suevite, melt, and glasses may have to form the basis for identifying candidate K/T impact sites in the absence of preserved morphologic elements of the crater. It is apparent that remote sensing is not a panacea for the discovery and validation of new impact craters, but it can contribute by serving as a pathfinder both for the identification of promising structures and the possible locations of key deposits for detailed fieldwork and sampling.