Thomas Kenkmann

The Ries impact, a double-layer rampart crater on Earth

The ejecta blankets of impact craters formed on a planetary body that is free of significant quantities of volatiles show substantial differences from those formed on a volatile-rich planetary body. Craters in volatile-rich environments often have layered ejecta blankets with lobe-like ramparts and long runout flows, as seen for Martian impact craters. Under volatile-free conditions, present on the Moon and Mercury, radial textures and patterns, and a gradational decrease in ejecta thickness with distance, can be observed. The Ries crater in Germany is one of the rare impacts on Earth with a preserved ejecta blanket. This crater was previously regarded as an analogue for impact formation on the Moon. Here we demonstrate for the first time that the recent Ries ejecta blanket contains a massive and continuous rampart structure at 1.45–2.12 crater radii from the crater center. Ejecta distribution and thickness, as well as the ejecta fabric, indicate the presence of fluids during the emplacement process. Although Mars differs in atmospheric pressure and water distribution from Earth, the Ries crater shows striking similarities to Martian craters; in particular, those with double-layered ejecta. Consequently, terrestrial impact craters can be better used as analogues for understanding impact formation on Mars than for planetary bodies with volatile-free conditions as seen on the Moon and Mercury.

Impact-generated pseudotachylitic breccia in drill core BH-5 Hättberg, Siljan impact structure, Sweden

Pseudotachylitic breccia (PTB) in the form of cm-wide melt breccia veinlets locally occurs on the exposed central uplift of the 380 Ma Siljan impact structure. The host rock to the PTBs is the so-called Järna granite of quartz monzonitic to syenodioritic composition. The nearly 603 m long BH-5 drill core from Hättberg, near the centre of the Siljan central uplift, contains numerous veins and pods of PTB. In particular, two major zones of 60 m combined width contain extensive PTB network breccias (30% actual melt breccia component), with individual melt breccia occurrences up to >1 m in length. Core logging and petrographic and geochemical analysis of the core have been performed, and the data are interpreted to suggest the following. (1) The impact event caused low to moderate (at essentially < 20 GPa) shock deformation in the host rock and in clasts of this lithology within the PTB. (2) Macroscopic deformation of the basement mainly comprises fracturing, with only localised cataclasis. (3) No evidence for shock melting (i.e. compression/decompression melting early in the cratering process) could be observed. (4) Optical and scanning electron microscopy showed that dark PTB contains a definite melt component. (5) Shearing has significantly affected this part of the central uplift, but its effects are limited to very short displacements and likely did not result in extensive melting. (6) A frictional heating component upon melt generation can, however, not be excluded, as many PTB samples contain clasts of a mafic (gabbroic) component, although only in one place along the entire core, a 1.2 cm-wide section through such material in direct contact to host rock was observed. Consequently, we suggest that, upon uplift in the central part of the impact structure, considerable melt volumes were generated locally, especially in areas that had been affected by extensive cataclasis and where grain size comminution favoured melt formation. Rapid decompression related to central uplift formation is the preferred process for the generation of the PTB melt breccias.

Ejecta thickness and structural rim uplift measurements of Martian impact craters: Implications for the rim formation of complex impact craters

The elevated rim in simple craters results from the structural uplift of preimpact target rocks and the deposition of a coherent proximal ejecta blanket at the outer edge of the transient cavity. Given the considerable, widening of the transient cavity during crater modification and ejecta thickness distributions, the cause of elevated crater rims in complex craters is less obvious. The thick, proximal ejecta in complex impact craters is deposited well inside the final crater rim and target thickening should rapidly diminish with increasing distance from the transient cavity rim. Our study of 10 complex Martian impact craters ranging from 8.2 to 53.0 km in diameter demonstrates that the mean structural rim uplift at the final crater rim makes 81% of the total rim elevation, while the mean ejecta thickness contributes 19%. Thus, the structural rim uplift seems to be the dominant factor to build up the total amount of the raised crater rim of complex craters. To measure the widening of the transient cavity during modification and the distance between the rim of the final crater and that of the transient cavity, we constructed balanced cross section restorations to estimate the transient cavity of nine complex Martian impact craters. The final crater radii are ~1.38–1.87 times the transient cavity radii. We propose that target uplift at the position of the final crater rim was established during the excavation stage.

In situ measurements of impact-induced pressure waves in sandstone targets

In the present study we introduce an innovative method for the measurement of impact-induced pressure waves within geological materials. Impact experiments on dry and water-saturated sandstone targets were conducted at a velocity of 4600 m/s using 12 mm steel projectiles to investigate amplitudes, decay behavior, and speed of the waves propagating through the target material. For this purpose a special kind of piezoresistive sensor capable of recording transient stress pulses within solid brittle materials was developed and calibrated using a Split-Hopkinson pressure bar. Experimental impact parameters (projectile size and speed) were kept constant and yielded reproducible signal curves in terms of rise time and peak amplitudes. Pressure amplitudes decreased by 3 orders of magnitude within the first 250 mm (i.e., 42 projectile radii). The attenuation for water-saturated sandstone is higher compared to dry sandstone which is attributed to dissipation effects caused by relative motion between bulk material and interstitial water. The proportion of the impact energy radiated as seismic energy (seismic efficiency) is in the order of 10−3. The present study shows the feasibility of real-time measurements of waves caused by hypervelocity impacts on geological materials. Experiments of this kind lead to a better understanding of the processes in the crater subsurface during a hypervelocity impact.

Evidence for a large Paleozoic Impact Crater Strewn Field in the Rocky Mountains

The Earth is constantly bombarded by meteoroids of various sizes. During hypervelocity collisions a large amount of energy is coupled to the Earth’s atmosphere leading to disruption of decimeter to hundred meter-sized meteoroids. Smaller meteoroids may form meteorite strewn fields while larger initial bodies and high-strength iron meteoroids may form impact crater strewn fields. Impact crater strewn fields are ephemeral and none documented to date are older than about 63,500 years. Here we report on a newly discovered impact crater strewn field, about 280 Myr old, in tilted strata of the Rocky Mountains near Douglas, Wyoming. It is the oldest and among the largest of impact crater strewn fields discovered to date, extending for a minimum of 7.5 km along a SE-NW trajectory. The apparent width of the strewn field is 1.5 km, but the full extent of the crater strewn field is not yet constrained owing to restricted exposure. We probably see only a small section of the entire crater strewn field. The cascade of impacts occurred in an environment that preserved the craters from destruction. Shock lithification aided this process.