Structure and formation of shatter cones in experimental and natural impact craters

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Shatter cones are the only shock (impact) indicators that can be readily identified on the macroscopic scale. They are, therefore, an important diagnostic tool to confirm ancient eroded impact craters. This proposal is aimed at better constraining the formation conditions of shatter cones by rigorous experimental analysis and comparison with natural shatter cones from the Steinheim crater, Germany. There is still no consensus regarding the physical boundary conditions that are required for their formation. We intend to determine shock pressure, pressure length and profile, the stress and strain tensor, and strain rate by applying the numerical models developed in the MEMIN program. Moreover, there is no consensus on how shatter cones form and it has remained controversial whether the fracture surfaces are the result of shear or tensile failure. We will carry out a complete morphometric analysis of shatter cone surfaces by applying, for the first time, white light interferometry, and atomic force microscopy (AFM). This and rigorous microstructural analysis using SEM, EBSD, and TEM will be used to decipher the formation and failure mechanism of laboratory and natural shatter cones from the Steinheim crater. The high-energy MEMIN cratering experiments using solid rocks as target materials are uniquely suited to form and detect shatter cones, as the crater-forming process is monitored with highest possible precision, and the craters have decimeter sizes and allow a detailed spatial survey. It is intended to find laboratory conditions that allow a reproducible and predictable formation of shatter cones. It is likewise important to delimit the conditions at which shatter cones do not form. The final objective is to derive a unified model for the generation of shatter cones that has to be consistent with our macroscopic and microscopic observations and the physical boundary conditions obtained. None of the published formation models is universally accepted so far and provides an explanation for both the conical shape and the horsetail hierarchical striation pattern of shatter cones. Most of the models have in common that some sort of heterogeneity (inclusion, pore space, etc.) must exist at the apex of a shatter cone. This heterogeneity causes an impedance contrast with the surrounding rock and is the source point for a spherical wave that interferes with the standard shock wave. We question this prerequisite because shatter cones are particularly well developed in fine-grained micritic limestones, as in the Steinheim basin, in which any sort of large inclusion is presumably rare or absent.