Oblique Incidence Hypervelocity Impacts on Rocks

The behaviour of granite rock under normal incidence hypervelocity impact in the laboratory has been studied using impacts of 2-mm diameter stainless-steel spheres at between 1 and 6 km s(-1). It was found that, at normal incidence, there is a strong dependence of crater size with impact speed (v). Crater mass was found to depend on v((2.3+/-0.4)), i.e. compatible with scaling with impact energy. However, crater depth and diameter have different and dissimilar dependences on impact speed, such that the crater shape (depth/diameter) is not a constant, but decreases as impact speed increases, being 0.26 at 1 km s(-1) and falling linearly to 0.16 at 6 km s(-1). In addition, a second data set with the same projectile and target combination was taken at a mean impact speed of 5.4 +/- 0.2 km s(-1), with the angle of incidence of impact ranging from 0degrees (normal incidence) to 85degrees (glancing incidence). The oblique incidence data show that crater depth and excavated mass start to decrease immediately when non-normal incidence occurs. Crater length and width initially decrease at a slower rate, with little decrease apparent until the angle of impact exceeds 50degrees from the normal. At the highest angles of incidence (85degrees, glancing incidence), there is a sudden change in crater shape, with crater length no longer decreasing as impact angle increases. It was found that for oblique impacts, excavated crater mass scales linearly with cos theta and not with the square of costheta as indicated in earlier work. This is similar to the scaling observed in other brittle and ductile materials. Finally, when used to predict populations of non-circular craters on large Solar system bodies, the results yield estimates (3 per cent) close to the observed rate (5 per cent) on Venus, Mars and the Moon.