Hypervelocity impact: Ejecta velocity, angle, and composition

The low projectile component in tektites in contrast to the high projectile component in the Cretaceous-Tertiary (K-T) boundary clay has prompted a study of hypervelocity target-projectile mixing processes. Results from a 6.4-km/sec impact of a Fe-Ni-PGE alloy projectile (90% Fe) into a Mo target indicate that high-angle (55° to 75°), high-velocity (< 6 km/sec) melted ejecta is relatively projectile-rich, whereas low-angle (10° to 40°), low-velocity ejecta (< 1 km/sec) contains less projectile material and is more enriched in the target component. These results support theoretical predictions. Not predicted by theoretical calculation, but observed here, is a break in the compositional trend such that at angles of ejection between 50° and 70°, the projectile/target ratio in the melted ejecta decreases suddenly with increasing angle, only to rise to very high values at higher angles. It appears that for large-body terrestrial impacts, the composition of the high-angle, high-speed ejecta which reaches stratospheric heights will be critical to sudden changes in global climate and the induced environmental stresses. Application of these results to large impacts such as the K-T boundary event, are expected to provide new data pertinent to physical theories of extinction mechanisms.