104 results found
Davison TM, Derrick JG, Collins GS, et al., Impact-induced compaction of primitive solar system solids: The need for mesoscale modelling and experiments, Procedia Engineering, ISSN: 1877-7058
Primitive solar system solids were accreted as highly porous bimodal mixtures of mm-sized chondrules and sub-μm matrix grains. To understand the compaction and lithification of these materials by shock, it is necessary to investigate the process at the mesoscale; i.e., the scale of individual chondrules. Here we document simulations of hypervelocity compaction of primitive materials using the iSALE shock physics model. We compare the numerical methods employed here with shock compaction experiments involving bimodal mixtures of glass beads and silica powder and find good agreement in bulk material response between the experiments and models. The heterogeneous response to shock of bimodal porous mixtures with a composition more appropriate for primitive solids was subsequently investigated: strong temperature dichotomies between the chondrules and matrix were observed (non-porous chondrules remained largely cold, while the porous matrix saw temperature increases of 100’s K). Matrix compaction was heterogeneous, and post-shock porosity was found to be lower on the lee-side of chondrules. The strain in the matrix was shown to be higher near the chondrule rims, in agreement with observations from meteorites. Chondrule flattening in the direction of the shock increases with increasing impact velocity, with flattened chondrules oriented with their semi-minor axis parallel to the shock direction.
Collins GS, 2017, Moon formation: Punch combo or knock-out blow?, Nature Geoscience, Vol: 10, Pages: 72-73, ISSN: 1752-0894
Collins GS, Lynch E, Mcadam R, et al., 2017, A numerical assessment of simple airblast models of impact airbursts, Meteoritics and Planetary Science, ISSN: 1086-9379
© 2017 The Meteoritical Society.Asteroids and comets 10-100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy-based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving-source airburst model where the meteoroid's energy is partitioned as two-thirds internal energy and one-third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving-source model predicts overpressures two times greater than the static-source model, whereas the cylindrical line-source model based on the pancake model predicts overpressures two times lower than the static-source model. Given other uncertainties associated with airblast damage predictions, the static-source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.
Forman LV, Bland PA, Timms NE, et al., 2017, Defining the mechanism for compaction of the CV chondrite parent body, Geology, Vol: 45, Pages: 559-562, ISSN: 0091-7613
© 2017 Geological Society of America.The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-μm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension < a >, yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body.
Muxworthy AR, Bland PA, Davison TM, et al., 2017, Evidence for an impact-induced magnetic fabric in Allende, and exogenous alternatives to the core dynamo theory for Allende magnetization, Meteoritics & Planetary Science, ISSN: 1086-9379
Rae ASP, Collins GS, Grieve RAF, et al., 2017, Complex crater formation: Insights from combining observations of shock pressure distribution with numerical models at the West Clearwater Lake impact structure, Meteoritics and Planetary Science, ISSN: 1086-9379
© 2017 The Meteoritical Society.Large impact structures have complex morphologies, with zones of structural uplift that can be expressed topographically as central peaks and/or peak rings internal to the crater rim. The formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization. Here, samples of shock-metamorphosed quartz-bearing lithologies at the West Clearwater Lake impact structure, Canada, are used to estimate the maximum recorded shock pressures in three dimensions across the crater. These measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift. The distribution of peak shock pressures, together with apparent crater morphology and geological observations, is compared with numerical impact simulations to constrain parameters used in the block-model implementation of acoustic fluidization. The numerical simulations produce craters that are consistent with morphological and geological observations. The results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse, and should be included in future implementations. Based on the comparison between observational data and impact simulations, we conclude that the West Clearwater Lake structure had an original rim (final crater) diameter of 35-40 km and has since experienced up to ~2 km of differential erosion.
Rutherford ME, Chapman DJ, Derrick JG, et al., 2017, Probing the early stages of shock-induced chondritic meteorite formation at the mesoscale, SCIENTIFIC REPORTS, Vol: 7, ISSN: 2045-2322
Smith R, 2017, Numerical modelling of tsunami generated by deformable submarine slides
Submarine slides can generate tsunami waves that cause significant damage and loss of life. Numerical modelling of submarine slide generated waves is complex and computationally challenging, but is useful to understand the nature of the waves that are generated, and identify the important factors in determining wave characteristics which in turn are used in risk assessments. In this work, the open-source, finite-element, unstructured mesh fluid dynamics framework Fluidity is used to simulate submarine slide tsunami using a number of different numerical approaches. First, three alternative approaches for simulating submarine slide acceleration, deformation and wave generation with full coupling between the slide and water in two dimensions are compared. Each approach is verified against benchmarks from experimental and other numerical studies, at different scales, for deformable submarine slides. There is good agreement to both laboratory results and other numerical models, both with a fixed mesh and a dynamically adaptive mesh, tracking important features of the slide geometry as the simulation progresses. Second, Fluidity is also used in a single-layer Bousinesq approximation in conjunction with a prescribed velocity boundary condition to model the propagation of slide tsunami in two and three dimensions. A new, efficient approach for submarine slide tsunami that accounts for slide dynamics and deformation is developed by imposing slide dynamics, derived from multi-material simulations. Two submarine slides are simulated in the Atlantic Ocean, and these generate waves up to 10 m high at the coast of the British Isles. Results indicate the largest waves are generated in the direction of slide motion. The lowest waves are generated perpendicular to the slide motion. The slide velocity and acceleration are the most important factors in determining wave height. Slides that deform generate higher waves than rigid slides, although this effect is of secondary importance f
Baker DMH, Head JW, Collins GS, et al., 2016, The formation of peak-ring basins: Working hypotheses and path forward in using observations to constrain models of impact-basin formation, ICARUS, Vol: 273, Pages: 146-163, ISSN: 0019-1035
Davison TM, Collins GS, Bland PA, 2016, MESOSCALE MODELING OF IMPACT COMPACTION OF PRIMITIVE SOLAR SYSTEM SOLIDS, ASTROPHYSICAL JOURNAL, Vol: 821, ISSN: 0004-637X
Forman LV, Bland PA, Timms NE, et al., 2016, Hidden secrets of deformation: Impact-induced compaction within a CV chondrite, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 452, Pages: 133-145, ISSN: 0012-821X
Johnson BC, Collins GS, Minton DA, et al., 2016, Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors, ICARUS, Vol: 271, Pages: 350-359, ISSN: 0019-1035
Kring DA, Kramer GY, Collins GS, et al., 2016, Peak-ring structure and kinematics from a multi-disciplinary study of the Schrodinger impact basin, NATURE COMMUNICATIONS, Vol: 7, ISSN: 2041-1723
Miljkovic K, Collins GS, Wieczorek MA, et al., 2016, Subsurface morphology and scaling of lunar impact basins, JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS, Vol: 121, Pages: 1695-1712, ISSN: 2169-9097
Monteux J, Collins GS, Tobie G, et al., 2016, Consequences of large impacts on Enceladus' core shape, ICARUS, Vol: 264, Pages: 300-310, ISSN: 0019-1035
Morgan JV, Gulick SPS, Bralower T, et al., 2016, The formation of peak rings in large impact craters, SCIENCE, Vol: 354, Pages: 878-882, ISSN: 0036-8075
Smith RC, Hill J, Collins GS, et al., 2016, Comparing approaches for numerical modelling of tsunami generation by deformable submarine slides, OCEAN MODELLING, Vol: 100, Pages: 125-140, ISSN: 1463-5003
Asphaug E, Collins GS, Jutzi M, 2015, Global Scale Impacts, Asteroids IV, Editors: Michel, DeMeo, Bottke, Publisher: University of Arizona Press, Pages: 661-678, ISBN: 9780816532131
Global scale impacts modify the physical or thermal state of a substantial fraction of a target asteroid. Specific effects include accretion, family formation, reshaping, mixing and layering, shock and frictional heating, fragmentation, material compaction, dilatation, stripping of mantle and crust, and seismic degradation. Deciphering the complicated record of global scale impacts, in asteroids and meteorites, will lead us to understand the original planet-forming process and its resultant populations, and their evolution in time as collisions became faster and fewer. We provide a brief overview of these ideas, and an introduction to models.
Forman LV, Bland PA, Timms NE, et al., 2015, RECOVERING THE PRIMORDIAL IMPACT HISTORY OF CHONDRITES IN UNPRECEDENTED DETAIL USING MASSIVE EBSD DATASETS, 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY-BLACKWELL, ISSN: 1086-9379
Jacobs CT, Goldin TJ, Collins GS, et al., 2015, An improved quantitative measure of the tendency for volcanic ash plumes to form in water: implications for the deposition of marine ash beds, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, Vol: 290, Pages: 114-124, ISSN: 0377-0273
Milbury C, Johnson BC, Melosh HJ, et al., 2015, Preimpact porosity controls the gravity signature of lunar craters, GEOPHYSICAL RESEARCH LETTERS, Vol: 42, Pages: 9711-9716, ISSN: 0094-8276
Miljkovic K, Wieczorek MA, Collins GS, et al., 2015, Excavation of the lunar mantle by basin-forming impact events on the Moon, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 409, Pages: 243-251, ISSN: 0012-821X
Muxworthy AR, Bland PA, Collins G, et al., 2015, MAGNETIC FABRICS IN ALLENDE: IMPLICATIONS FOR MAGNETIC REMANENCE ACQUISITION., 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY-BLACKWELL, ISSN: 1086-9379
Ormö J, Melero-Asensio I, Housen KR, et al., 2015, Scaling and reproducibility of craters produced at the Experimental Projectile Impact Chamber (EPIC), Centro de Astrobiología, Spain, Meteoritics and Planetary Science, Vol: 50, Pages: 2067-2086, ISSN: 1086-9379
© 2015 The Meteoritical Society.The Experimental Projectile Impact Chamber (EPIC) is a specially designed facility for the study of processes related to wet-target (e.g., "marine") impacts. It consists of a 7 m wide, funnel-shaped test bed, and a 20.5 mm caliber compressed N2 gas gun. The target can be unconsolidated or liquid. The gas gun can launch 20 mm projectiles of various solid materials under ambient atmospheric pressure and at various angles from the horizontal. To test the functionality and quality of obtained results by EPIC, impacts were performed into dry beach sand targets with two different projectile materials; ceramic Al2O3 (max. velocity 290 m s-1) and Delrin (max. velocity 410 m s-1); 23 shots used a quarter-space setting (19 normal, 4 at 53° from horizontal) and 14 were in a half-space setting (13 normal, 1 at 53°). The experiments were compared with numerical simulations using the iSALE code. Differences were seen between the nondisruptive Al2O3 (ceramic) and the disruptive Delrin (polymer) projectiles in transient crater development. All final crater dimensions, when plotted in scaled form, agree reasonably well with the results of other studies of impacts into granular materials. We also successfully validated numerical models of vertical and oblique impacts in sand against the experimental results, as well as demonstrated that the EPIC quarter-space experiments are a reasonable approximation for half-space experiments. Altogether, the combined evaluation of experiments and numerical simulations support the usefulness of the EPIC in impact cratering studies.
Potter RWK, Kring DA, Collins GS, 2015, Scaling of basin-sized impacts and the influence of target temperature, Special Paper of the Geological Society of America, Vol: 518, Pages: 99-113, ISSN: 0072-1077
© 2015 The Geological Society of America. All rights reserved.We produce a set of scaling laws for basin-sized impacts using data from a suite of lunar basin numerical models. The results demonstrate the importance of preimpact target temperature and thermal gradient, which are shown to greatly influence the modification phase of the impact cratering process. Impacts into targets with contrasting thermal properties also produce very different crustal and topographic profiles for impacts of the same energy. Thermal conditions do not, however, significantly influence the excavation stage of the cratering process; results demonstrate, as a consequence of gravity-dominated growth, that transient crater radii are generally within 5% of each other over a wide range of thermal gradients. Excavation depth-to-diameter ratios for the basin models (~0.12) agree well with experimental, geological, and geophysical estimates, suggesting basins follow proportional scaling. This is further demonstrated by an agreement between the basin models and Piscaling laws based upon first principles and experimental data. The results of this work should also be applicable to basin-scale impacts on other silicate bodies, including the Hadean Earth.
Bland PA, Collins GS, Davison TM, et al., 2014, Pressure-temperature evolution of primordial solar system solids during impact-induced compaction, NATURE COMMUNICATIONS, Vol: 5, ISSN: 2041-1723
Bray VJ, Collins GS, Morgan JV, et al., 2014, Hydrocode simulation of Ganymede and Europa cratering trends - How thick is Europa's crust?, ICARUS, Vol: 231, Pages: 394-406, ISSN: 0019-1035
Collins GS, 2014, Numerical simulations of impact crater formation with dilatancy, JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS, Vol: 119, Pages: 2600-2619, ISSN: 2169-9097
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