Subsurface structure of oblique impact craters
Final Report Abstract
The majority of impacts on planetary bodies occur at an oblique impact angle to the target surface, where the incidence angle follows a Gaussian probability distribution with a mean value of 45°. Although oblique impacts are prevalent occurrences, the crater shape remains circular for impact angles above 10–15° from the target surface and thus can rarely give implications for the impact direction or angle. The distribution of the ejecta blanket, on the other hand, is a distinctive indicator for oblique impacts. It loses its radial symmetry at angles below 45-35° and at lower angles forms ‘‘forbidden’’ zones and ‘‘butterfly’’ patterns. Unfortunately, on Earth most ejecta blankets are eroded and cannot be used to determine the impact direction for terrestrial craters. Hence, other potential indicators for an oblique impact are needed if the impact trajectory is to be determined in terrestrial craters. The internal structure of central uplifts has been interpreted to be influenced by an oblique impact angle. The subsurface structures of eroded central uplifts of some terrestrial impact craters show a preferential, non-radial orientation of folding, faulting and stacking of layered (and originally subhorizontal) bedrock that implicates a preferred transport direction during the crater formation process, as seen in Upheaval Dome (Utah, USA), Spider (Australia), Gosses Bluff (Australia), Matt Wilson (Australia) and Jebel Waqf as Suwwan (Jordan). The results of the prior project show that this imbrication is caused by remnant horizontal momentum transferred from the impacting projectile to the target during an oblique impact. Three-dimensional numerical simulations support these results and show that the excavation flow field of oblique impacts is asymmetric, with stronger excavation downrange. This in turn influences the formation of the central uplift, which in these models initially develops at an uprange position and migrates downrange during the modification process. To date, the observed structural trends could be directly connected to an impact direction in the case of Matt Wilson crater due to its elliptical crater shape (prior project), with upturned bedding striking perpendicular to the long axis of the crater ellipse, which is parallel to the impact trajectory. In order to verify the connection between an oblique impact trajectory and the observed preferential, nonradial deformation in central uplifts, further ‘‘ground truth’’ examples were needed in which an independent indicator of the impact direction, i.e., the asymmetric ejecta blanket, is preserved. Martian impact craters now offer an ideal opportunity to verify both terrestrial observations and numerical models of the central uplift. The increasing quality, resolution and availability of remote sensing data of the Martian surface permit the application of structural geology methods to Martian craters with a high measure of precision. Therefore, in this project three Martian complex impact craters were selected that exhibit an oblique ejecta blanket and layered bedrock in the central uplift. The central uplifts were structurally mapped using High Resolution Imaging Science Experiment (HiRISE) imagery and additional high-resolution digital terrain models (DTMs). Structural data were evaluated and show a correlation between the orientation of at least two structural criteria and the direction of impact: (1) The strike of upturned bedrock layers is on average perpendicular to the impact trajectory and (2) the majority of faults show a preferred trend parallel to the impact trajectory. A fourth Martian crater was examined as a counterexample, which had a radial ejecta blanket and thus no indication of an oblique trajectory. Strike in this crater showed no preferred orientation. These results will help to solidify the suggested correlation between non-radial structural features in impact craters and the horizontal component of momentum transferred from an obliquely impacting projectile to the target. The presented structural features can thus be used to unravel formerly unknown impact trajectories. The formation of preferred orientations of strata strike in central uplifts is at least as sensitive as the asymmetry of ejecta blankets. These findings are in agreement with observations from terrestrial impact craters and will help to classify already confirmed impact craters and possible future discoveries as oblique impact craters.
Publications
- (2011). Indicators for an Impact Direction in the Central Pit of an unnamed Martian Crater. 74th Annual Meeting of the Meteoritical Society, Vol 46, A255, #5292
Wulf G., Poelchau M. H., Kenkmann T.
- (2011). Structural Deformations in the Central Pit of a Martian Crater as an Indicator for Impact Direction. European Planetary Science Congress 2011, Vol. 6, EPSC2011-697
Wulf G., Poelchau M. H., Kenkmann T.
- (2011). Structural Trends in the Central Uplift of an unnamed Martian Crater as an Indicator for Impact Direction. 42th Lunar and Planetary Science Conference, #1440
Wulf G., Poelchau M. H., Kenkmann T.
- (2012). Structural Asymmetry in Martian Impact Craters as an Indicator for an Impact Trajectory. Icarus 220, 194-204
Wulf G., Poelchau M. H., Kenkmann T.
(See online at https://doi.org/10.1016/j.icarus.2012.04.025) - 2013. Structural analysis of Martian impact craters using several remote sensing data. DGPF Tagungsband 22/2013 – Dreiländertagung DGPF, OVG, SGPF, Freiburg, p. 263-270
Wulf G., Poelchau M. H., Kenkmann T.
