Melt production in oblique impacts

Hydrocode modeling is a fundamental tool for the study of melt production i n planetary impact events. Until recently, however, numerical modeling of i mpacts for melt production studies has been limited to vertical impacts. We present the first results of the investigation of melt production in obliq ue impacts. Simulations were carried out using Sandia's three-dimensional h ydrocode CTH, coupled to the SESAME equation of state. While keeping other impact parameters constant, the calculations span impact angles (measured f rom the surface) from 90 degrees (vertical impact) to 15 degrees. The results show that impact angle affects the strength and distribution of the shock wave generated in the impact. As a result, both the isobaric cor e and the regions of melting in the target appear asymmetric and concentrat ed in the downrange, shallower portion of the target. The use of a pressure -decay power law (which describes pressure as function of linear distance f rom the impact point) to reconstruct the region of melting and vaporization is therefore complicated by the asymmetry of the shock wave. As an analog to the pressure decay versus distance from the impact point, we used a "vol umetric pressure decay," where the pressure decay is modeled as a function of volume of target material shocked at or above the given shock pressure. We find that the volumetric pressure decay exponent is almost constant for impact angles from 90 degrees to 30 degrees, dropping by about a factor of two for a 15 degrees impact. In the range of shock pressures at which most materials of geologic interes t melt or begin to vaporize, we find that the volume of impact melt decreas es by at most 20% for impacts from 90 degrees down to 45 degrees. Below 45 degrees, however, the amount of melt in the target decreases rapidly with i mpact angle. Compared to the vertical case, the reduction in volume of melt is about 50% for impacts at 30 degrees and more than 90% for a 15 degrees impact. These estimates do not include possible melting due to shear heatin g, which can contribute to the amount of melt production especially in very oblique impacts. Studies of melt production in vertical impacts suggest an energy scaling la w in agreement with the point source limit. An energy scaling law, however, does not seem to hold for oblique impacts, even when the impact velocity i s substituted by its vertical component. However, we find that for impact a ngles between about 30 degrees and 90 degrees (a range that includes 75% of impact events on planetary surfaces) the volume of melt is directly propor tional to the volume of the transient crater generated by the impact. (C) 2 000 Academic Press.