Doctor of Philosophy
Planetary Science and Exploration
Osinski, Gordon R.
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 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.
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.
Hopkins, Ryan, "Numerical Simulations of Complex Crater Formation in Layered and Mixed Targets" (2019). Electronic Thesis and Dissertation Repository. 6738.
Available for download on Monday, June 01, 2020