• Impact crater formation in icy layered terrains on Mars

      Senft, L. E.; Stewart, S. T. (The Meteoritical Society, 2008-01-01)
      We present numerical simulations of crater formation under Martian conditions with a single near-surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects on observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H2O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well-preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near-surface icy layers are widespread on Mars.
    • Mid-sized complex crater formation in mixed crystalline-sedimentary targets: Insight from modeling and observation

      Collins, G. S.; Kenkmann, T.; Osinski, G. R.; Wünnemann, K. (The Meteoritical Society, 2008-01-01)
      Large impact crater formation is an important geologic process that is not fully understood. The current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets. Real targets are rarely uniform; for example, the majority of Earths surface is covered by sedimentary rocks and/or a water layer. The ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process. To advance understanding of the mechanics of crater collapse, and the effect of variations in target properties on crater formation, the first Bridging the Gap workshop recommended that geological observation and numerical modeling focussed on mid-sized (15-30 km diameter) craters on Earth. These are large enough to be complex; small enough to be mapped, surveyed and modelled at high resolution; and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables. In this paper, we compare observations and numerical models of three 18-26 km diameter craters formed in different target lithology: Ries, Germany; Haughton, Canada; and El'gygytgyn, Russia. Based on the first-order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid-sized complex crater formation in a subaerial, mixed crystalline-sedimentary target. We compared our results with interpreted geological profiles of Ries and Haughton, based on detailed new and published geological mapping and published geophysical surveys. Our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre-impact sedimentary cover in each case. We conclude that the presence of an inner ring at Ries, and not at Haughton, is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post-impact erosion. For constant impact energy, transient and final crater diameters increase with increasing sediment thickness.
    • The Dakhleh Glass: Product of an impact airburst or cratering event in the Western Desert of Egypt?

      Osinski, G. R.; Kieniewicz, J.; Smith, J. R.; Boslough, M. B. E.; Eccleston, M.; Schwarcz, H. P.; Kleindienst, M. R.; Haldemann, A. F. C.; Churcher, C. S. (The Meteoritical Society, 2008-01-01)
      Impact cratering is a ubiquitous geological process on the terrestrial planets. Meteorite impact craters are the most visible product of impact events, but there is a growing recognition that large aerial bursts or airbursts should occur relatively frequently throughout geological time. In this contribution, we report on an unusual impact glass--the Dakhleh Glass (DG)--which is distributed over an area of ~400 km^2 of the Dakhleh Oasis, Egypt. This region preserves a rich history of habitation stretching back to over 400,000 years before the emergence of Homo sapiens. We report on observations made during recent fieldwork and subsequent analytical analyses that strengthen previous suggestions that the DG formed during an impact event. The wide distribution and large size of DG specimens (up to ~50 cm across), the chemistry (e.g., CaO and Al2O3 contents up to ~25 and ~18 wt%, respectively), the presence of lechatelierite and burnt sediments, and the inclusion of clasts and spherules in the DG is inconsistent with known terrestrial processes of glass formation. The age and other textural characteristics rule out a human origin. Instead, we draw upon recent numerical modeling of airbursts to suggest that the properties of DG, coupled with the absence of a confirmed crater, can best be explained by melting of surficial sediments as a result of a large airburst event. We suggest that glass produced by such events should, therefore, be more common in the rock record than impact craters, assuming that the glass formed in a suitable preserving environment.
    • The effect of target lithology on the products of impact melting

      Osinski, G. R.; Grieve, R. A. F.; Collins, G. S.; Marion, C.; Sylvester, P. (The Meteoritical Society, 2008-01-01)
      Impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system. Impact events generate pressures and temperatures that can melt a substantial volume of the target; however, there remains considerable discussion as to the effect of target lithology on the generation of impact melts. Early studies showed that for impacts into crystalline targets, coherent impact melt rocks or sheets are formed with these rocks often displaying classic igneous structures (e.g., columnar jointing) and textures. For impact structures containing some amount of sedimentary rocks in the target sequence, a wide range of impactgenerated lithologies have been described, although it has generally been suggested that impact melt is either lacking or is volumetrically minor. This is surprising given theoretical constraints, which show that as much melt should be produced during impacts into sedimentary targets. The question then arises: where has all the melt gone? The goal of this synthesis is to explore the effect of target lithology on the products of impact melting. A comparative study of the similarly sized Haughton, Mistastin, and Ries impact structures, suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets, regardless of target properties. Furthermore, using advanced microbeam analytical techniques, it is apparent that, for the structures under consideration here, a large proportion of the melt is retained within the crater (as crater-fill impactites) for impacts into sedimentary-bearing target rocks. Thus, it is suggested that the basic products are genetically equivalent but they just appear different. That is, it is the textural, chemical and physical properties of the products that vary.
    • Validation of numerical codes for impact and explosion cratering: Impacts on strengthless and metal targets

      Pierazzo, E.; Artemieva, N.; Asphaug, E.; Baldwin, E. C.; Cazamias, J.; Coker, R.; Collins, G. S.; Crawford, D. A.; Davison, T.; Elbeshausen, D.; et al. (The Meteoritical Society, 2008-01-01)
      Over the last few decades, rapid improvement of computer capabilities has allowed impact cratering to be modeled with increasing complexity and realism, and has paved the way for a new era of numerical modeling of the impact process, including full, three-dimensional (3D) simulations. When properly benchmarked and validated against observation, computer models offer a powerful tool for understanding the mechanics of impact crater formation. This work presents results from the first phase of a project to benchmark and validate shock codes. A variety of 2D and 3D codes were used in this study, from commercial products like AUTODYN, to codes developed within the scientific community like SOVA, SPH, ZEUS-MP, iSALE, and codes developed at U.S. National Laboratories like CTH, SAGE/RAGE, and ALE3D. Benchmark calculations of shock wave propagation in aluminum-on-aluminum impacts were performed to examine the agreement between codes for simple idealized problems. The benchmark simulations show that variability in code results is to be expected due to differences in the underlying solution algorithm of each code, artificial stability parameters, spatial and temporal resolution, and material models. Overall, the inter-code variability in peak shock pressure as a function of distance is around 10 to 20%. In general, if the impactor is resolved by at least 20 cells across its radius, the underestimation of peak shock pressure due to spatial resolution is less than 10%. In addition to the benchmark tests, three validation tests were performed to examine the ability of the codes to reproduce the time evolution of crater radius and depth observed in vertical laboratory impacts in water and two well-characterized aluminum alloys. Results from these calculations are in good agreement with experiments. There appears to be a general tendency of shock physics codes to underestimate the radius of the forming crater. Overall, the discrepancy between the model and experiment results is between 10 and 20%, similar to the inter-code variability.