The effect of impact angle on the formation of meteorite impact craters: Insight from 3D numerical modeling

Most meteorite impact structures on Earth or any other planetary surface are circular in plan view. This may seem unintuitive given that most impacts occur at an oblique angle of incidence to the target, but it is simply a consequence of the hypervelocity nature of meteorite impacts and the fact that the resulting crater are much smaller than the projectile. Although the oblique angle in the vast majority of impacts does not have a major effect on the shape (for angles >10-15°) it is likely that crater size and other structural characteristics are affected and our understanding of oblique impact is poor. We use numerical models to quantify the effect of impact angle on the cratering process. We have developed a 3D extension to the well-known hydrocode iSALE-2D that is specifically optimized to investigate late-stage crater- formation processes. In contrast to most previous modeling studies, which investigated only the early-stage effects of impact angle or single specific oblique impact scenarios, we have produced a suite of impact models spanning a range of impactor parameters (velocity, angle, size) and target properties (gravity, strength). We demonstrate with our models that crater efficiency (the size of the crater relative to the size of the projectile) decreases with decreasing angle of impact (measured from horizontal). In scaling relations based on experimental studies and theoretical considerations for vertical impacts it is primarily the kinetic energy of the impactor, as opposed to the momentum, that controls crater-size. Our models show that even for impact-angles as low as 30° the crater efficiency is dominated by the kinetic energy of the impactor in strengthless material; however, in material with internal friction momentum becomes more important with decreasing angle of impact. We provide a modified scaling relation that includes the effect of impact-angle and is based on the assumption that the crater dimensions depend only on the vertical component of the impact velocity. Finally, we find structural (subsurface) evidence at the central peak in particular (in case of complex craters) that is characteristic of oblique impact. These observations might be used to determine the direction of impact at real crater structures on Earth.