The present special issue of Meteoritics & Planetary Science is a compilation of the research results gained in the MEMIN program during a 2 1/2 year period. It contains eleven articles that highlight different aspects of a number of hypervelocity cratering experiments with different techniques and scientific approaches. The articles are published together as they are thematically closely tied and cross-reference each other.
The MEMIN research unit (Multidisciplinary Experimental and Modeling Impact Research Network), established in 2009, is funded by the German Research Foundation DFG. The group pools geoscientists, physicists, and engineers to merge expertise in the field of experimental impact cratering; in total, seven groups located in Freiburg, Berlin, Muenster, Munich, and Potsdam participate in MEMIN. The rationale behind the research unit is the fact that understanding hypervelocity impacts of solid bodies—one of the most fundamental geological processes throughout the solar system—requires a multi- and interdisciplinary approach that includes laboratory experiments, numerical simulations, and the study of natural craters.
Newly designed powerful two-stage light-gas guns at the Fraunhofer Ernst-Mach-Institut, Freiburg, are central to the MEMIN program. These accelerators can produce decimeter-sized craters in solid rocks. The size of the experiments is essential for the success of the MEMIN research unit, as it allows monitoring of the impact process in real time, and provides enough material for detailed spatial postmortem analyses. Here, we report on the results of cratering experiments onto well-characterized sandstone targets. The data set comprises parametric studies of the role of water, porosity, and impact velocity on crater formation. We have quantified damaging and shock effects of the target rocks and the associated petrophysical changes, and we have studied the fate of the projectile in terms of geochemical and physical changes. The ejecta process and its distribution were constrained by the mineralogical data, and by data obtained from real-time recording. The experimental program is combined with numerical simulations, with a special focus on the behavior of porous media under shock. MEMIN was designed to yield a solid data base for the validation and refining of numerical cratering models; the data presented in this issue will help scale meso-scale observations to the size of natural craters.
To date, most cratering experiments in a geological context have been performed on loose materials. In the MEMIN program, however, we concentrate on “hard rocks” as they enable the study of pore collapse, shock metamorphism, target damaging, shear localization, fracture propagation, dilatancy, and petrophysical anomalies. The effects of porosity and interstitial target water on the cratering process are of particular importance for impacts on Earth where sedimentary targets play a dominant role. This is underlined by the recently discovered 45 m Kamil crater, Egypt (Folco et al. 2010, 2011; D’Órazio et al. 2011). Projectile and target material of this meteorite impact crater are nearly identical to the lithologies used in the cratering experiments of the initial MEMIN campaigns with Seeberger Sandstone as target and spheres of the iron meteorite Campo de Cielo as projectiles. Likewise, in a planetological context, our parameter studies are important for understanding cratering on bodies without or with just a tenuous atmosphere, where regolith as well as regolith breccias with considerable amounts of porosity cover the surface. Moreover, volatile bearing eolian and fluviatile sandstones are widespread on Mars.
The report by Lexow et al. (2013) outlines the principles of two-stage light-gas guns and gives an update of recent achievements in accelerator technology. A comparison of the performance of different facilities is provided. The article by Poelchau et al. (2013) provides an overview of the experimental campaigns and summarizes the results obtained by MEMIN. The authors derive scaling laws for porous target materials based on the volume of transient and final craters; such data will help us to understand natural strength-dominated impact craters like Kamil. Hoerth et al. (2013) describe in their paper the set-up of the cratering experiments and derive the dynamic ejecta behavior and crater growth from evaluation of high-speed videos. Sommer et al. (2013) focus on the ejecta of these experiments and assess data gathered with newly designed particle catchers. These authors document the strong dependency of ejecta and particle size distribution on the presence of water in the porous target.
Dufresne et al. (2013) present the morphometric properties of the experimental craters. They use digital elevation models to compare the crater efficiency for a variety of impact conditions. Moreover, they have derived a method to constrain transient crater sizes and spallation volumes. Buhl et al. (2013) perform a microstructural analysis of damage in the crater subsurface with a special emphasis on the effects of porosity and water saturation of the pore space. Moser et al. (2013) apply nondestructive testing methods to quantify the damage zone of the crater subsurface. Post-impact ultrasound tomography of the targets reveals a larger extent of damaging than identified visually or by microanalysis.
In addition to the cratering experiments, recovery experiments with sandstone and quartzite disks were carried out at 2.5–17.5 GPa to find shock effects at these low-shock pressures. These results are presented by Kowitz et al. (2013). Güldemeister et al. (2013) investigate the propagation of shock waves in porous sandstone by means of numerical simulation, and compare results of two meso-scale models using two different numerical codes. Using their models, these authors can explain pressure and temperature excursions frequently documented in shocked porous materials.
Last but not least two papers deal with the fate of the steel and iron meteorite projectiles upon impact. Ebert et al. (2013) analyze the chemical fractionation and mixing at the projectile–target interface, whereas Kenkmann et al. (2013) constrain deformation mechanisms by microstructural analysis of projectile fragments, and calculate the energy fraction consumed by plastic work and heating of the projectile.
The results presented here are part of ongoing research and evaluation of the MEMIN cratering experiments. Our intention is to expand on and to connect the aspects of the impact process presented here, and thus, in the future, to narrow the gap between experimental work, numerical modeling, and observation on natural impact structures. We look forward to developing an experimental database that will help the impact community to better understand details of the cratering process.