Sebastian Sturm

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.

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.