Preliminary work: the MEMIN pilot study
To test the feasibility of the MEMIN program, the Ernst Mach Institute Freiburg, the Museum für Naturkunde Berlin and the MEMIN team performed two experiments with the novel two-stage light gas gun (Thoma et al. 2005; Kenkmann et al. 2006, 2007a, b; Schäfer et al. 2006; Wünnemann et al. 2006). The accelerator configuration consists of a 22 m long pump tube with 150 mm diameter, in combination with a 12 m long launch tube, 50 mm in diameter. The projectiles, 1-cm-diameter CrMo-steel spheres (4.1 g) encapsulated in a cylindrical sabot, were launched against a vertically positioned sandstone target enclosed in a steel frame, and with ejecta catchers consisted of fiber boards mounted in appropriate positions. The impact velocity of the projectile was determined at a laser barrier to amount 5.300 m s-1 for both experiments. The impact process was documented by shadowgraph images using a high-speed framing camera. The target consisted of 1 x 1 x 0.5 m blocks of so-called Seeberger Sandstein, with precisely determined petrographic, geochemical and strength properties. This lithology has a porosity of 12-18 vol.%; one experiment was performed under dry (Exp. 2808), the other under wet target conditions (Exp. 2809).
For a quantitative analysis of crater shape and volume, gypsum replicas of the crater cavity were manufactured as a basis for 3D digital scanning (Fig. 1). Transient crater and crater volume were derived from the digital models. Non-intrusive geoelectrical measurements were carried out on the intact target blocks. Subsequently, the sandstone blocks were cut through the crater center to map crater floor fractures (Fig. 1). Bore hole profiles (15 mm ∅, 120 mm length) were drilled into the block to obtain samples from various distances from the crater floor surface. Ejected fragments were collected from the crater floor and fiber boards that were mounted at 50 cm distance parallel to the block surfaces.
The resulting craters had diameters of 24.3 (dry) and 28.7 cm (wet experiment), depths of 5.6 cm, and 4.5 cm, and volumes of 715 cm3, and 1099 cm3, repectively. The ejecta cone angles were 69.8° and 58° after 1.2 milliseconds (Fig. 1). Transient crater diameters were calculated to be 8.2 cm (dry) and 11.3 cm (wet experiment). In the dry experiment, 2.84 g of strongly deformed projectile were recovered. Further traces of the projectile indicating melting and mixing of target and projectile covered the crater floor (Fig. 2). Numerical modeling of the experiments provides good matches for the experimental craters and indicates peak pressures in excess of 60 GPa. An essential outcome for the MEMIN program is that geometry and size of the craters as well as ejecta dynamics clearly show a significant influence of fluids on the cratering process – the crater efficiency is seemingly higher in wet sandstone.
Microstructural analysis of ejected sandstone fragments and of samples from underneath the crater floor at varying distances from the crater center revealed three types of deformation in both experiments: (i) virtually undeformed sandstone with intact porosity and no shock deformation indicated, (ii) localized brittle fracturing and compaction in otherwise weakly-to-undeformed sandstone with intact porosity, and (iii) intensive and penetrative intragranular fracturing that led to comprehensive pore space crushing and shock metamorphism. Type (III) occurs dominantly in ejecta material.
The MEMIN feasibility study has demonstrated that the impact-affected volume of the target is sufficiently large to be resolved with geophysical methods. Having established the principle viability of the proposed project, we are convinced that use of the new high-energy facility at EMI and application of advanced measurement technologies will break unrivaled new ground in experimental impact studies.