Results – Overview
The general objective of MEMIN is to comprehensively quantify impact processes on the meso-scale in porous rocks in order to understand the (i) dynamics of cratering, (ii) impact damaging, and (iii) geophysical properties of impact craters in sedimentary targets.
In order to reach these objectives we have performed parameter studies on impact energy, the effects of porosity, and the presence of water in sandstone targets. Our new data set helps to narrow the gap between experiment and nature.
Impact Cratering Experiments
In total, we have carried out 24 cratering experiments at the facilities of the Ernst Mach Institut (Table 1). Projectiles ranging in diameter from 2.5 to 12 mm were accelerated to velocities ranging from 2.5 to 7.8 km/s, resulting in impact energies between 0.7 and 82 kJ. Craters were formed with diameters between 3.9 and 40 cm.
Target and Projectile Material
Seeberger Sandstein (sandstone) was picked based on its composition
and the small and homogeneous grain size. Taunus
Quarzit (a low-grade metamorphic quartzite) was chosen as a non-porous quartz-rich
end member, and Weiberner Tuff (a
volcanic tuff) as highly porous material. All materials were analyzed
petrographically, geochemically, petrophysically, and by ultrasound techniques.
Several materials were evaluated for the projectiles. The Campo del Cielo iron meteorite turned out as the best proxy for a natural impactor based on its homogeneity, and lack of large inclusions or fractures. The D290-1 steel and a high Ni-Cr aluminum alloy (55X G28J1) were selected for their high content in trace elements.
Seeberger Sandstein and Campo del Cielo meteorite were used as targets and projectiles.
For each set of experiments a long lead time was needed to test recording systems, and to prepare projectiles and target blocks. After basic preparations, ejecta catchers, ultrasound sensors, high-speed video cameras and illumination, pressure sensors, and sensors that registered the impact flash had to be installed and tested.
Top: Target chamber at the "Space gun" with a quartzite target. Acoustic emission sensors are attached to the target. Vaseline and phenolic foam ejecta catchers are behind the target, and a high-speed camera and flash are visible on the left. Bottom: A sandstone target is being prepared in the target chamber at the "XL gun".
Impact of a 1 cm steel sphere onto a sandstone target at 5.3 km/s, ~10 µs into the impact process.
Post-impact analysis was carried out in part directly after each experiment, e.g. the search for remnants of the projectile. After the campaign, ejecta and target block material was distributed between the research groups. First, non-invasive measurements were performed, beginning with a 3D-laser scan of the craters, followed by ultrasound tomography, vibrational analysis and computer tomography. Then, blocks were sawed for microanalysis of crater profile segments.
The speaker examining a post-impact projectile fragment.
Parallel to the cratering experiments, planar shock recovery experiments were performed on the same sandstone target material, and gave important constraints on the behavior of sandstone in the low-shock pressure regime.
Numerical models of the impact cratering process play a crucial role within MEMIN. The experimental results are used to validate numerical models of crater formation. These models in turn give us important insights into crater formation that can be extrapolated to planetary-scale impact phenomena.
Future course of the program
We have now achieved a detailed knowledge on the influence of porosity, water saturation of the pore space, experimental parameters (size, velocity, material of the projectile) on specific aspects of the cratering process. The state of knowledge acquired so far is thoroughly documented in a special issue of Meteoritics & Planetary Science (now in print) with contributions of each MEMIN project.
We have established intense collaboration and networking between the projects, with a special benefit resulting from combination of experimental results with numerical models. These models help to quantify and evaluate the processes in question, while experimental data serve as benchmarks to validate the improved numerical models, thus helping to “bridge the gap” between experiments and models. Work on this synthesis is currently underway, and will be published in the near future.