TRACING THE TRANSIENT CRATER IN IMPACT EXPERIMENTS

Small-scale impact cratering experiments into sandstone targets were performed at the Fraunhofer Ernst-Mach Institute in Freiburg, Germany. Steel, iron meteorite and aluminium projectiles were accelerated to velocities of 2.5 up to 7.9 km/s, resulting in impact energies between 0.8 and 56 kJ. Morphological analyses of the resulting craters reveal an outer, shallow-dipping area, two central depressions and a fragile, white-coloured centre as characteristic features of all experiments into dry targets. However, late spallation of the brittle target material induces a certain degree of variability in crater shape and total crater volume. For a sensible comparison to experiments into other target materials and for scaling to natural impact craters, a comparable feature that is independent of late-stage spallation is needed. During the early cratering process, a transient crater forms with a shape that is initially unaffected by spallation effects. First results indicate that this transient crater can be well described by quadratic parabola fits to crater profiles. The results are compared to independently calculated geometric markers using the following methods: (i) extrapolating parabola radii to ejecta imprints on catcher systems, (ii) comparing the parabola slope to ejecta curtain angles and tracking ejecta particle trajectories on high-speed video images, and (iii) comparing weight fractions of grain size analyses to calculated transient crater volumes. As a first approximation, the transient crater volume is constrained to a volume of about 10-40% of the final crater volume, depending on the method applied. Definite values and the application of these methods for a comparison to experiments in other brittle and non-brittle target materials will be presented.