Impaktprozesse gehören zu den fundamentalsten Prozessen unseres Sonnensystems. Sie hatten dabei einen maßgeblichen Einfluss auf die Entstehung und Entwicklung der planetaren Körper sowie auf die Evolution des Lebens. Die enorme, auf verhältnismäßig kleinem Raum freigesetzte Energie bei Impakten unterscheidet diese von anderen geologischen Prozessen. Unser Verständnis von Impaktprozessen hat in den letzten Jahrzehnten erheblich zugenommen. Dennoch sind etliche Fragen noch nicht abschließend geklärt. Dies liegt vor allem an der schwierigen Beobachtbarkeit von Impaktprozessen. Zum einen sind die Zeiträume der Kraterbildung, selbst für große Krater, extrem kurz, zum anderen sind natürliche Kraterstrukturen häufig durch sekundäre Prozesse modifiziert. Aus diesem Grund werden seit Mitte des 20. Jahrhunderts Impaktexperimente genutzt, um Prozesse der Kraterbildung eingehender zu studieren. Es ist bekannt, dass sich die Porosität des beaufschlagten Körpers (Target) auf die Kratergröße und Morphologie auswirkt. Einfluss hat zudem der Grad der Porenraumsättigung des Gesteins mit Wasser. Die genauen Prozesse hinter diesen Veränderungen sind jedoch nur unzureichend untersucht. Aus diesem Grund beschäftigt sich die vorgelegte Arbeit mit der Auswirkung von Porosität auf die Impakt-induzierte Deformation. Hierzu wurde eine Reihe von experimentellen Impakten in trockenem und wassergesättigtem Sandstein sowie in Quarzit untersucht. Diese Experimente umfassen Impaktgeschwindigkeiten von 2,5 km s^-1 bis 5,3 km s^-1, Projektildurchmesser von 2,5 mm bis 12 mm und Impaktenergien von 773 J bis 81060 J. Der Fokus liegt dabei auf den Auswirkungen der Porosität und der Wassersättigung des Porenraums auf die Sprödbruchmechanismen. Mikrostrukturelle Analysen des Krateruntergrundes und der Auswurfmassen zeigen, dass sich der Porenraum auch auf die Größe, Art und räumliche Verteilung der im Krateruntergrund gefundenen Deformationsstrukturell auswirkt. Die Wassersättigung zeigt ebenfalls einen deutlichen Einfluss auf die Untergrundstruktur und die Partikelgrößenverteilung der ausgeworfenen Masse. In dieser .Arbeit werden diese Phänomene sowohl detailliert charakterisiert als auch quantifiziert und ihre Bedeutung und Auswirkungen diskutiert.

Impact processes have profoundly influenced the development and evolution of the planetary bodies in our solar system. Although our knowledge about impact mechanisms has grown over the last decades, several open questions remain. In this thesis hypervelocity impact experiments on sandstone were investigated to specify the effects of porosity and pore space saturation with water on impact-induced deformation. Microstructural analyses of the deformation of the crater's sub-surface and of the impact ejecta were performed for a total of 13 impacted targets. Twelve of these experiments were conducted on Seeberger Sandstein porous, fine grained sandstone. For one experiment a dense quartzite was chosen as target material, as a non-porous sample. To determine the effects of porosity and pore space saturation on the sub-surface deformation mechanisms two cratering experiments were investigated. The experiments were conducted on a dry and a water-saturated sandstone cube (20 x 20 x 20 cm), both impacted by 2.5 mm diameter steel spheres at 4.8 km s^-1 and 5.3 km s^-1, respectively. Microstructural analyses revealed differences in the sub-surface deformation between the experiments. Enhanced comminution and compaction in the dry experiment in contrast to a wider extent of localized deformation in the wet experiment suggest that pore water has a strong influence on the deformation. An explanation may be reduced peak stresses at graingrain contacts. This reduces grain comminution and compaction, but also decreases shock wave attenuation. It is likely that reduced shock wave attenuation is responsible for the deeper penetration of the localized deformation in the water saturated experiment. Detailed analyses of particle size distributions (PSDs) from an impact (4.8 km s^-1) and a shock recovery experiment (2.5 GPa) showed that the PSDs are affected by the deformation rate. Power-law fits were used to quantify the PSDs as power-law exponents (D-values). A close connection between the D-values and distinct microstructural features was found. The obtained values are in good agreement with the D-values reported for fault zones, impact sites and deformation experiments. Comparison with the shock recovery experiment showed that very high D-values can be developed at low strain when the strain-rate is high. To quantify strain rate dependent deformation, numerical impact modeling was used to estimate strain rates for the analyzed impact experiment. To investigate the size scaling of impact damage two impact experiments with different projectile sizes on dry sandstone were compared. The experiments were performed with a 2.5 mm steel projectile at 4.8 km s^-1 impact velocity (773 J), and a 10 mm iron meteorite projectile at 4.6 km s^-1 impact velocity (42627 J). Both samples revealed similar zoned deformation microstructures in the sub-surface. The deformation zones were mapped for both experiments and their volumes were calculated. The results, normalized to the projectile diameter, revealed that the damage is very similar in size and geometry for both experiments. Analysis of the long axes of deformation bands found in both experiments showed that the bands are developed under shear deformation. PSDs were measured to quantify the impact damage. Comparison showed that the decay of the power-law exponents with increasing distance from the impact point source is similar for both experiments. Reconstruction of the shock wave loading path allowed to infer the stresses at which different deformation microstructures were developed. To analyze the effects of porosity, pore space saturation, impact velocity, and projectile size on the PSD of the excavated material 11 impacts into sandstone and one impact into quartzite were investigated. The experiments performed at impact velocities of 2.5 to 5.3 km s^-1 and projectile diameters from 2.5 mm to 12 mm had impact energies of 874 to 81060 J. The PSDs of the bulk ejecta were quantified as Z)-values. No effect of impact energy or impact velocity on the PSDs was detected. A major increase in the D-values from dry compared to water-saturated sandstone was found. This may result from multiple effects of water saturation on ejecta fragmentation. A comparison of our results with literature data showed no correlation between the target lithology and the resulting D-value. A strong discrepancy between the quartzite and the sandstone experiment was detected. Disruption of the quartzite target may be the best explanation for this finding. Finally the energy spent for ejecta fragmentation was calculated and showed that the fraction of impact energy used for comminution is in the lower single-digit percentage, similar to other fragmentation processes.