EXPERIMENTAL IMPACT CRATERING: THE MEMIN-PROJECT

The DFG-funded MEMIN research unit is focused on impact cratering experiments into geological materials. Several sets of experiments have been performed at the two-stage acceleration facilities of the EMI in Freiburg, Germany. Each campaign was set up to focus on different aspects of impact cratering, including studies on reproducibility and target heterogeneity, lithology, scaling, and pore space saturation. The innovative aspects of these experiments are diverse instrumentation and detailed numerical modeling based on experimental observations. The instrumentation includes (i) high-speed cameras to observe and quantify the ejection process, (ii) different catchment assemblies to collect ejecta with high spatial resolution, and (iii) ultrasound sensor systems to record pressure waves and fragmentation processes in the target during and after the experiment.

In the experimental setup, spherical aluminum, steel and meteoritic iron projectiles between 2.5 and 10 mm diameter were accelerated to velocities ranging from 2.5 to 8 km/s, impacting into dry and water-saturated sandstone targets. Pressure in the target chamber was varied between 100 and 0.1 mbar. Atmospheric effects on ejecta formation were observed with high speed cameras at up to 5*10⁵ frames per second.

Morphological evaluation of the impact craters with digital scanning methods reveals that cratering efficiency is reduced by open pore space in comparison to non-porous geological materials. Saturation of the pore space with water on the other hand, leads to an increase in crater volume and cratering efficiency. Evaluation of the ejecta catchment assemblies shows a bimodal distribution of ejecta. A well defined outer ring of fine particles is distributed in relation to the early-stage formation of an ejecta cone, while larger debris particles are focused at the center of the catchment assembly, reflecting either late stage atmospheric interaction or relaxation processes. Analysis of ejecta particles also reveals varying degrees of shock metamorphism, including PDF formation. SEM measurements of the crater subsurface show that several different mechanisms of fracturing and grain comminution take place during cratering. Future campaigns will additionally focus on highly porous lithologies to better constrain porosity effects on cratering.