Transient Crater Growth in Low Density Targets

Introduction: Recent studies suggest that impacts into highly porous target materials require a new cratering regime controlled by compressional effects (1). Such research is important for interpreting the geologic history of small bodies and for the upcoming Deep Impact mission in 2005. An alternative view (2) suggests that even highly porous targets (density of 0.2g/cc) undergo gravity controlled growth and that previous studies underestimated the effects of an ambient atmospheres as previously documented (3,4). The present study reexamines these conflicting views Approach: Hypervelocity impact experiments were performed at the NASA Ames Vertical Gun range. Target materials included low-density perlite (and perlite mixes), microspheres, pumice, and sand. Impactor materials included aluminum and copper but primarily pyrex in order to ensure complete failure , as would be the case for much higher velocity impacts. High-speed imaging (500 and 6000 frames per second) captured the excavation process using both half-space and quarter-space target configurations. Quarter-space experiments previously documented atmospheric effects on crater excavation and demonstrated that the final crater measured was significantly different from the pre-collapse excavation crater (3,4). Results: Vertical impacts into half-space (normal) targets at 1-g using sieved perlite targets revealed significant reduction in the cratering efficiency consistent with previous reports (1). In contrast with these studies, however, significant material was ejected from the crater. High-speed imaging revealed two ejecta regime. The first is a long-lived, pillar-like plume above the crater containing high-angle ejecta (80-90°). The second is the classic outward advancing conical ejecta curtain with higher ejection angles (50-60°) than ejecta produced by impacts into less porous materials (pumice or sand). As the outward-moving curtain leaves the transient crater interior, the high-angle ejecta pillar returns to the crater and the crater collapses returning rim material to the interior. Quarter-space experiments reveal that three material displacement regimes occur at hypervelocity. A penetration tube characterizes the first regime at early-times as target material is compressed in front of the fragmenting projectile. This tube expands cylindrically similar to the mach tube creating during hypervelocity atmospheric entry. The second stage of displacement is represented by the high-angle ejecta. Such high angles develop as the result of an expanding cavity at depth analogous deeply buried explosive charge. A pillar-like plume characterizes the third stage and lasts throughout crater formation.