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Numerical Simulations of Complex Crater Formation in Layered and Mixed Targets Western University Scholarship@Western Electronic Thesis and Dissertation Repository 12-3-2019 2:00 PM Numerical Simulations of Complex Crater Formation in Layered and Mixed Targets Ryan Hopkins The University of Western Ontario Supervisor Osinski, Gordon R. The University of Western Ontario Graduate Program in Physics A thesis submitted in partial fulfillment of the equirr ements for the degree in Doctor of Philosophy © Ryan Hopkins 2019 Follow this and additional works at: https://ir.lib.uwo.ca/etd Part of the Fluid Dynamics Commons, Geology Commons, and the Geophysics and Seismology Commons Recommended Citation Hopkins, Ryan, "Numerical Simulations of Complex Crater Formation in Layered and Mixed Targets" (2019). Electronic Thesis and Dissertation Repository. 6738. https://ir.lib.uwo.ca/etd/6738 This Dissertation/Thesis is brought to you for free and open access by Scholarship@Western. It has been accepted for inclusion in Electronic Thesis and Dissertation Repository by an authorized administrator of Scholarship@Western. For more information, please contact [email protected]. Abstract Numerical simulations of hypervelocity impact events provide a unique method of analyzing the mechanics that govern impact crater formation. This thesis describes modifications that were made to the impact Simplified Arbitrary Lagrangian Eulerian (iSALE) shock-physics code in order to more accurately simulate meteorite impacts into layered target sequences and details several applications that were investigated using this improved strength model. Meteorite impacts occur frequently in layered targets but resolving thin layers in the target sequence is computationally expensive and therefore not often considered in numerical simulations. To address this limitation iSALE was modified to include an anisotropic yield criterion and rotation scheme to simulate the effect of thin, weak layers interspersed in the target. A comparison of ~4000 impact simulations shows that this method reduces computational cost while replicating the morphology of the craters formed in the high- resolution simulations with multiple weak layers modelled in the target geometry. Simulating layering via material anisotropy tends to increase the diameter and reduce the depth of the crater relative to a crater formed in an unlayered, isotropic target. In agreement with field observations at the Haughton and Ries impact structures, layering also appears to be partially responsible for suppressing central uplift formation during crater modification. Comparisons of terrestrial impact structures suggest those that formed in sedimentary or mixed targets tend to have a smaller depth-diameter ratio relative to craters formed in purely crystalline targets. Furthermore, several complex craters that formed in relatively thick sedimentary sequences (e.g., Haughton, Ries, Zhamanshin) do not have a central peak. An additional suite of ~60 simulations of impacts into mixed sedimentary-crystalline targets were created to further study the influence of the sedimentary layer on crater formation. A thick sedimentary layer changes the cratering flow field; the enhanced lateral motion of the weakened sedimentary material results in a crater that has a greater final diameter and reduced final depth relative to a crater formed in a purely crystalline target. Stratigraphic uplift tends ii to increase with in thicker sedimentary targets, but the most uplifted material tends to be found at further radial distances from the point of impact. Keywords Impact Cratering, Numerical Modelling, Shock-Physics code, Hydrocode, Layering, Material Anisotropy, Complex Crater Formation, Mixed Targets, Sedimentary Targets, Crystalline Targets, Central Uplift, Stratigraphic Uplift iii Summary for Lay Audience In computer simulations of meteorite impacts, the target is often simplified by removing details such as layering that may be present. Planetary bodies, such as Earth, the Moon, and Mars, are rarely so simple. This thesis highlights additions to numerical models of impact crater formation so that the target sequence can be more accurately represented. We first introduce an efficient method of simulating the inclusion of layers within the target. This new method, which treats the target as an anisotropic material (i.e., the strength of the target can be defined separately for different directions), accurately simulates the inclusion of weak layers in the target without the need to explicitly define these layers. Since the minimum thickness of target layers is dependent on the resolution of the models, the inclusion of an anisotropic model to replace these layers can significantly reduce the computational burden required to model layered targets. Using this new model, we address some of outstanding questions regarding complex crater (i.e., large craters, >5‐km diameter on Earth) formation in targets with thick sedimentary layers. Specifically, we examine the apparent suppression that layering causes on the uplift of the crater floor, known as the central uplift, and show that including layering in the model tends to produce a greater diameter and shallower crater relative to an unlayered target. We then examine the role of layering in mixed targets (targets with layered sedimentary material overlying uniform crystalline material) on complex crater formation. It was found that increasing the thickness of the sedimentary layer tends to increase lateral motion (i.e., radially outward, and then back inwards) of the sediments and reduce vertical motion of the crystalline material during crater formation. Thicker sedimentary layers result in a crater with greater diameter and reduced depth and tend to restrict the formation of central uplifts. Lastly, we track tracer particles (markers that flow with material during the simulation, but do not affect the motion of the material) within the simulation to show that thick sedimentary targets result in greater uplift of material in the target at further radial distances from the point of impact. iv Co-Authorship Statement Chapter 2: All numerical simulations were created, run, and processed by Ryan Hopkins. Modifications to the iSALE strength model were implemented by Ryan Hopkins. The manuscript was written by Ryan Hopkins, and editing and comments were provided by Dr. Gordon Osinski and Dr. Gareth Collins. This manuscript was published in January 2019 in the Journal of Geophysical Research: Planets. Reference: Hopkins R. T., Osinski G. R., and Collins G. S. 2019. Formation of complex craters in layered targets with material anisotropy. Journal of Geophysical Research: Planets 124(2): 349–373. Chapter 3: All numerical simulations were created, run, and processed by Ryan Hopkins. The manuscript was written by Ryan Hopkins, with editing and comments provided by Dr. Gordon Osinski and Dr. Gareth Collins. Chapter 4: All numerical simulations were created, run, and processed by Ryan Hopkins. The manuscript was written by Ryan Hopkins, and editing and comments were provided by Dr. Gordon Osinski. v Acknowledgments First, I would like to thank my supervisor, Dr. Gordon Osinski, for his mentorship and willingness to provide help and advice over the last seven years. I also extend my thanks to Dr. Livio Tornabene and Dr. Gareth Collins, both of whom helped me grow during my studies. The staff, students, and faculty of the Centre for Planetary and Science (CPSX) and the Physics and Astronomy department at Western have been great companions throughout the years and I thank them for their support. Funding from the Government of Ontario through the Ontario Graduate Scholarship program is also gratefully acknowledged. This work would not have been possible without the iSALE development team. A special thanks goes to Dr. Jay Melosh and his research group at Purdue University, as well as Dr. Gareth Collins and Dr. Tom Davison at Imperial College London, for helping to get me started using and developing iSALE. Lastly, I’d like to thank my friends and family from the bottom of my heart. Your relentless support helped me push through the tough times; this work would not have been possible without all of you. vi Table of Contents Abstract ............................................................................................................................... ii Summary for Lay Audience ............................................................................................... iv Co-Authorship Statement.................................................................................................... v Acknowledgments.............................................................................................................. vi Table of Contents .............................................................................................................. vii List of Tables ...................................................................................................................... x List of Figures .................................................................................................................... xi Chapter 1 ............................................................................................................................. 1 1 Introduction .................................................................................................................... 1 1.1 The Impact Cratering Process ................................................................................. 2 1.1.1 The Contact and Compression Stage .........................................................
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