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  • 1
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    Subsurface morphology and scaling of lunar impact basins (2016)
    Wiley
    Details
    Publication Date: 2016-08-21
    Description: Impact bombardment during the first billion years after the formation of the Moon produced at least several tens of basins. The GRAIL mission mapped the gravity field of these impact structures at significantly higher spatial resolution than previous missions, allowing for detailed subsurface and morphological analyses to be made across the entire globe. GRAIL-derived crustal thickness maps were used to define the regions of crustal thinning observed in centers of lunar impact basins, which represents a less unambiguous measure of a basin size than those based on topographic features. The formation of lunar impact basins was modeled numerically using the iSALE-2D hydrocode, with a large range of impact and target conditions typical for the first billion years of lunar evolution. In the investigated range of impactor and target conditions, the target temperature had the dominant effect on the basin subsurface morphology. Model results were also used to update current impact scaling relationships applicable to the lunar setting (based on assumed target temperature). Our new temperature-dependent impact-scaling relationships provide estimates of impact conditions and transient crater diameters for the majority of impact basins mapped by GRAIL. As the formation of lunar impact basins is associated with the first ∼ 700 Myr of the solar system evolution when the impact flux was considerably larger than the present day, our revised impact scaling relationships can aid further analyses and understanding of the extent of impact bombardment on the Moon and terrestrial planets in the early solar system.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Full waveform tomographic images of the peak ring at the Chicxulub impact crater (2011)
    Wiley
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    Publication Date: 2011-06-16
    Description: Peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation. The precise lithology and kinematics of peak ring formation, however, remains unclear. Previous work has revealed a suite of bright inward dipping reflectors beneath the peak ring at the Chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity. New two-dimensional, full waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin (∼100–200 m thick) layer of low-velocity (∼3000–3200 m/s) rocks. This low-velocity layer is most likely composed of highly porous, allogenic impact breccias. Our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors. Across the peak ring, velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation, where the rocks that form the peak ring originate from an uplifted basement that has been subjected to high shock pressures (10–50 GPa) and lie above downthrown sedimentary rocks that have been subjected to shock pressures of
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Estimating transient crater size using the crustal annular bulge: Insights from numerical modeling of lunar basin-scale impacts (2012)
    Wiley
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    Publication Date: 2012-09-27
    Description: The transient crater is an important impact cratering concept. Its volume and diameter can be used to predict impact energy and momentum, impact melt volume, and maximum depth and volume of ejected material. Transient crater sizes are often estimated using scaling laws based on final crater rim diameters. However, crater rim estimates, especially for lunar basins, can be controversial. Here, we use numerical modeling of lunar basin-scale impacts to produce a new, alternative method for estimating transient crater radius using the annular bulge of crust observed beneath most lunar basins. Using target thermal conditions appropriate for the lunar Imbrian and Nectarian periods, we find this relationship to be dependent on lunar crust and upper mantle temperatures. This result is potentially important when analyzing lunar basin subsurface structures inferred from the GRAIL mission.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Pre-Impact Porosity Controls the Gravity Signature of Lunar Craters (2015)
    Wiley
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    Publication Date: 2015-11-06
    Description: We model the formation of lunar complex craters and investigate the effect of pre-impact porosity on their gravity signatures. We find that while pre-impact target porosities less than ~7% produce negative residual Bouguer anomalies (BAs), porosities greater than ~7% produce positive anomalies whose magnitude is greater for impacted surfaces with higher initial porosity. Negative anomalies result from pore space creation due to fracturing and dilatant bulking, and positive anomalies result from destruction of pore space due to shock wave compression. The central BA of craters larger than ~215 km in diameter, however, are invariably positive because of an underlying central mantle uplift. We conclude that the striking differences between the gravity signatures of craters on the Earth and Moon are the result of the higher average porosity and variable porosity of the lunar crust.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Pressure–temperature evolution of primordial solar system solids during impact-induced compaction (2014)
    Springer Nature
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    Publication Date: 2014-12-05
    Description: Article Collisions between primordial planetesimals led to the formation of our asteroids and meteorites. Here, the authors use modelling to explore the compaction of planetsimals, tracking how pressure, temperature and porosity may have varied during the impacts, helping interpret early Solar System processes. Nature Communications doi: 10.1038/ncomms6451 Authors: P. A. Bland, G. S. Collins, T. M. Davison, N. M. Abreu, F. J. Ciesla, A. R. Muxworthy, J. Moore
    Electronic ISSN: 2041-1723
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 6
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    Numerical simulations of impact crater formation with dilatancy (2014)
    Wiley
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    Publication Date: 2014-11-16
    Description: Impact-induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater. This paper describes a simple model of dilatancy—the creation of porosity in a shearing geological material—and its implementation in the iSALE shock physics code. The model is used to investigate impact-induced dilatancy during simple and complex crater formation on Earth. Simulations of simple crater formation produce porosity distributions consistent with observations. Dilatancy model parameters appropriate for low-quality rock masses give the best agreement with observation; more strongly dilatant behaviour would require substantial post-impact porosity reduction. The tendency for rock to dilate less when shearingunder high pressure is an important property of the model. Pressure suppresses impact-induced dilatancy: in the shock wave; at depth beneath the crater floor; and in the convergent sub-crater flow that forms the central uplift. Consequently, sub-surface porosity distribution is a strong function of crater size, which is reflected in the inferred gravity anomaly. The Bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius; larger craters exhibit a central gravity high within a suppressed gravity low. Lower crustal pressures on the Moon relative to Earth imply that impact-induced dilatancy is more effective on the Moon than Earth for the same size impact in an initially non-porous target. This difference may be mitigated by the presence of porosity in the lunar crust.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Geochemistry. Moonstruck magnetism (2012)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2012-03-10
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Collins, Gareth S -- New York, N.Y. -- Science. 2012 Mar 9;335(6073):1176-7. doi: 10.1126/science.1217681.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK. g.collins@imperial.ac.uk〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22403376" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 8
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    The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary (2010)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2010-03-06
    Description: The Cretaceous-Paleogene boundary approximately 65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Schulte, Peter -- Alegret, Laia -- Arenillas, Ignacio -- Arz, Jose A -- Barton, Penny J -- Bown, Paul R -- Bralower, Timothy J -- Christeson, Gail L -- Claeys, Philippe -- Cockell, Charles S -- Collins, Gareth S -- Deutsch, Alexander -- Goldin, Tamara J -- Goto, Kazuhisa -- Grajales-Nishimura, Jose M -- Grieve, Richard A F -- Gulick, Sean P S -- Johnson, Kirk R -- Kiessling, Wolfgang -- Koeberl, Christian -- Kring, David A -- MacLeod, Kenneth G -- Matsui, Takafumi -- Melosh, Jay -- Montanari, Alessandro -- Morgan, Joanna V -- Neal, Clive R -- Nichols, Douglas J -- Norris, Richard D -- Pierazzo, Elisabetta -- Ravizza, Greg -- Rebolledo-Vieyra, Mario -- Reimold, Wolf Uwe -- Robin, Eric -- Salge, Tobias -- Speijer, Robert P -- Sweet, Arthur R -- Urrutia-Fucugauchi, Jaime -- Vajda, Vivi -- Whalen, Michael T -- Willumsen, Pi S -- New York, N.Y. -- Science. 2010 Mar 5;327(5970):1214-8. doi: 10.1126/science.1177265.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉GeoZentrum Nordbayern, Universitat Erlangen-Nurnberg, Schlossgarten 5, D-91054 Erlangen, Germany. schulte@geol.uni-erlangen.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20203042" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; *Extinction, Biological ; *Fossils ; Geologic Sediments ; Mexico ; *Minor Planets
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 9
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    Asymmetric distribution of lunar impact basins caused by variations in target properties (2013)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2013-11-10
    Description: Maps of crustal thickness derived from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission revealed more large impact basins on the nearside hemisphere of the Moon than on its farside. The enrichment in heat-producing elements and prolonged volcanic activity on the lunar nearside hemisphere indicate that the temperature of the nearside crust and upper mantle was hotter than that of the farside at the time of basin formation. Using the iSALE-2D hydrocode to model impact basin formation, we found that impacts on the hotter nearside would have formed basins with up to twice the diameter of similar impacts on the cooler farside hemisphere. The size distribution of lunar impact basins is thus not representative of the earliest inner solar system impact bombardment.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Miljkovicc, Katarina -- Wieczorek, Mark A -- Collins, Gareth S -- Laneuville, Matthieu -- Neumann, Gregory A -- Melosh, H Jay -- Solomon, Sean C -- Phillips, Roger J -- Smith, David E -- Zuber, Maria T -- New York, N.Y. -- Science. 2013 Nov 8;342(6159):724-6. doi: 10.1126/science.1243224.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Universite Paris Diderot, Case 7011, Lamarck A, 5, 35 rue Helene Brion, 75205 Paris cedex 13, France.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24202170" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 10
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    Positron lifetime measurements of hydrogen passivation of cation vacancies in yttrium aluminum oxide garnets (2013)
    American Physical Society (APS)
    Details
    Publication Date: 2013-11-05
    Description: Author(s): F. A. Selim, C. R. Varney, M. C. Tarun, M. C. Rowe, G. S. Collins, and M. D. McCluskey A charge compensation mechanism is proposed for cation vacancy defects in complex oxides based on positron lifetime measurements, infrared spectroscopy, and composition analysis. Defects were characterized in samples of yttrium aluminum garnet grown in O 2 or Ar. However, no positron trapping was det... [Phys. Rev. B 88, 174102] Published Mon Nov 04, 2013
    Keywords: Structure, structural phase transitions, mechanical properties, defects
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
    Published by American Physical Society (APS)
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