Impact ejection, spallation, and the origin of meteorites

A model for the ejection of material from an impact crater which links ejection velocity, fragment size, and shock pressure through a simplified stress-wave propagation and reflection scheme is presented. It is shown that a small amount of material (0.01 to 0.05 projectile mass) may be ejected at high velocity without suffering petrologically detectable shock pressures. The largest fragments ejected at any velocity are spalls that originate from the target planet's surface. The spall size is proportional to the radius of the primary impactor and the target tensile strength and inversely proportional to ejection velocity. The shock level in the spalls is low, typically half of the dynamic crushing strength of the rock. The model also predicts the aspect ratio of the spalled fragments, the angle of ejection, and the sizes and shock level of other fragments originating deeper in the target. Comparison with observational and experimental data shows generally good agreement.