Melt droplet formation in energetic impacts

Impacts between rocky bodies at velocities exceeding about 15 km/sec are capable of melting or vaporizing both the impacting object and a portion of the target. Geological materials initially shocked to high pressure approach the liquid-vapor phase boundary from the liquid side as they decompress, breaking up into an expanding spray of liquid droplets. A simple theory is presented for estimating the sizes of these droplets as a function of impactor size and velocity. It is shown that these sizes are consistent with observations of microtektites and spherules found in the Cretaceous-Tertiary boundary layer, the Acraman impact structure, Archean beds in South Africa and lunar regolith. The model may also apply to the formation of chondrules.