Book Review: The Universe Unveiled: Instruments and Images through History, B. Stephenson, M. Bolt, A. F. Friedman (Eds.)
AuthorKelley, M. S.
MetadataShow full item record
CitationKelley, M. S. (2002). Book Review: The Universe Unveiled: Instruments and Images through History, B. Stephenson, M. Bolt, A. F. Friedman (Eds.). Meteoritics & Planetary Science, 37(1), 143-144.
PublisherThe Meteoritical Society
JournalMeteoritics & Planetary Science
DescriptionBook Review: The Universe Unveiled: Instruments and Images through History, B. Stephenson, M. Bolt, A. F. Friedman (Eds.). The Adler Planetarium and Astronomy Museum, Chicago, Illinois, USA, and Cambridge University Press, New York, New York, USA (2000).
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Boltysh, another end-Cretaceous impactKelley, S. P.; Gurov, E. (The Meteoritical Society, 2002-01-01)The Chixculub impact occurred at the Cretaceous/Tertiary (K/T) boundary, and although several other Late Cretaceous and Paleogene impact craters have, at times, been linked with the K/T boundary, isotope geochronology has demonstrated that all have significantly different ages. The currently accepted age of the 24 km diameter Boltysh crater, a K-Ar whole-rock age, places it in the Coniacian at 88 +/- 3 Ma. However, comprehensive Ar-Ar dating of a range of melt samples yields a mean age of 65.17 +/- 0.64 Ma, within errors of the K/T boundary. Several of the fresh samples exhibit signs of excess argon, but this seems to be concentrated in rapidly crystallized glass-rich samples. The Ar-Ar age confirms an earlier fission track measurement and thus two dating techniques have yielded an age within errors of the K/T boundary for this crater. Crucially, although the ages of Boltysh and Chixculub are within errors, they may not have formed synchronously. Craters of 24 km diameter occur much more commonly than impacts of Chixculub dimensions, but their proximity does raise the important question of how many impacts there might have been close to the K/T boundary.
Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formationGrieve, R. A. F.; Reimold, W. U.; Morgan, J.; Riller, U.; Pilkington, M. (The Meteoritical Society, 2008-01-01)The structural, topographic and other characteristics of the Vredefort, Sudbury, and Chicxulub impact structures are described. Assuming that the structures originally had the same morphology, the observations/interpretations for each structure are compared and extended to the other structures. This does not result in any major inconsistencies but requires that the observations be scaled spatially. In the case of Vredefort and Sudbury, this is accomplished by scaling the outer limit of particular shock metamorphic features. In the case of Chicxulub, scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring. The observations/interpretations are then used to construct an integrated, empirical kinematic model for a terrestrial peak-ring basin. The major attributes of the model include: a set of outward-directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor, some of which are pre-existing structures that have been reactivated during transient cavity formation; inward-directed motions along the same outermost structures and along a set of structures, at intermediate radial distances, during transient cavity collapse; structural uplift in the center followed by a final set of radially outward-directed thrusts at the outer edges of the structural uplift, during uplift collapse. The rock displacements on the intermediate, inward and innermost, outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift.
Critical crater diameter and asteroid impact seismologyAsphaug, Erik (The Meteoritical Society, 2008-01-01)If impact stress reverberation is the primary gradational process on an asteroid at global scales, then the largest undegraded crater records an asteroids seismological response. The critical crater diameter Dcrit is defined as the smallest crater whose formation disrupts all previous craters globally up to its size; it is solved for by combining relationships for crater growth and for stress attenuation. The computation for Dcrit gives a simple explanation for the curious observation that small asteroids have only modest undegraded craters, in comparison to their size, whereas large asteroids have giant undegraded craters. Dcrit can even exceed the asteroid diameter, in which case all craters are local and the asteroid becomes crowded with giant craters. Dcrit is the most recent crater to have formed on a blank slate; when it is equated to the measured diameter of the largest undegraded crater on known asteroids, peak particle velocities are found to attenuate with the 1.2-1.3 power of distance--less attenuative than strong shocks, and more characteristic of powerful seismic disturbances. This is to be expected, since global degradation can result from seismic (cm s^(-1)) particle velocities on small asteroids. Attenuation, as modeled, appears to be higher on asteroids known to be porous, although these are also bodies for which different crater scaling rules might apply.