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Project VI

Crater structure and excavation: the effect of lithology and target layering on crater formation in experimental and natural impact craters

continuation of TP-6 

Project directors:

Kenkmann, Thomas, Albert-Ludwigs-Universität Freiburg (ALU)
Trullenque, Ghislain, Albert-Ludwigs-Universität Freiburg (ALU)
Deutsch, Alexander, Westfälische-Wilhelms-Universität Münster (WWU)
Thoma, Klaus, Fraunhofer Institut für Kurzzeitdynamik Freiburg (EMI)

Research staff:

Frank Sommer (PhD student, ALU Freiburg)
Rebecca Winkler (PhD student, ALU Freiburg)

Summary

This proposal is the continuation of MEMIN I project 6 (KE 732-16/1). This application focuses on crater formation in limestone targets through the performance of sophisticated cratering experiments at various impact energies. Moreover, we consider the effects of mixed-layer targets with a rheological stratification on impact cratering processes and finalize the parameter study on porosity and water saturation. Post-impact investigations comprise (i) rigorous analysis of crater morphologies, (ii) ejection characteristics and deformation of recovered ejecta, and (iii) the systematic evaluation of target damaging in the crater’s sub-surface. The determination of crater volumes of transient and final craters, as well as crater efficiencies, allows us to compare impact cratering in CaCO3-dominated targets with mixed-layer targets and those in SiO2-dominated that were the subject of investigation in MEMIN I. This project includes a detailed pre-impact petrographic and mechanical characterization of the target material. The strength properties of the target rocks and fracture characteristics at failure are investigated for various loading rates. Fractures developed during the impact experiments will be compared with those obtained by the confining pressure release technique by means of a detailed microstructure and surface analysis to gain insights into the fracture mechanisms. Our accurate morphometric and structural analysis of the experimentally produced craters will yield a thorough data base for limestone and mixed-target craters. With these data at hand, current scaling laws for impact cratering will be refined and provide important input data for the improvement of material models and numerical computation. The up-scaled experimental results are tested and validated in a field campaign at the 3.8 km diameter Steinheim crater, Germany. This project yields the structural geology aspects of the field survey that will be merged with the geophysical data detailed in proposal VIII.
The realisation of this proposal is possible due to the experience our team has gained during the first application period. We conducted 24 impact cratering experiments to study the effects of porosity, pore water, and impact energy on the cratering process. We found that a higher degree of water saturation of the target yields an increase of total ejecta mass (up to 400% with respect to dry targets), higher ejecta velocity, and a steeper ejecta cone angles. Morphometric data showed that the depth/diameter ratio of the craters strongly correlates with projectile/target density ratios and porosity, with impacts in low density targets forming deeper craters. Saturation counteracts the effect of porosity and increases crater volumes when other impact conditions are kept the same. Crater volume scaling of experiments in dry sandstone over a range of velocities is in good agreement with first results of numerically modeled craters.