Implications of a hyperboloid transient cavity model for impact craters

English Abstract:

The hyperboloid model for the transient cavity stage of terrestrial hypervelocity impact craters is based on the position of shock features in two deep drill holes spaced 200m apart at the centre of the Brent crater. In the centre ~650m of breccia emplaced during late stage collapse overlies a 40m thick melt layer that is taken as the upper section of the ~100m thick lining of the residual transient cavity. Little melt is found 200m from the centre. In both holes shock levels diminish in the underlying 60m of breccia with a high apparent rate of attenuation of shock pressure versus depth from about -50 to -20, ending at the limit of total fragmentation where the shock level is about 5-10 GPa and the attenuation drops towards -2. The hyperbolic profile that satisfies these and other constraints has an eccentricity of about 1.6 and projects to the estimated original surface to give a diameter of 3.6km and a depth of 1.05km. This form resembles small fresh lunar craters and experimental craters formed by hypervelocity impacts into sand targets. The hyperboloid form can be applied to larger complex craters with little modification indicating no fundamental change in crater mechanic s and target properties at the simple to complex transition. This has important implications for the morphology and modification processes associated with complex craters. Compared to paraboloid models the rim is more subdued and near-surface motions are at lower inclinations, thereby placing more importance on radial displacement rather than ejection and implying buckling and slumping mechanisms for the formation of rings found outside the main rim, particularly in stratified targets. These results have direct implications for the modeling and understanding of the impact cratering process and highlight differences between explosion and impact craters.