IMPACTS INTO POROUS AND NONPOROUS WATER ICE TARGETS

For many solar system bodies, impact cratering events occur in targets comprised primarily of water ice. The amount of energy spent in irreversible heating of these icy targets is of particular interest to workers looking to constrain the amount of melt and/or vaporization associated with such impacts. The prevalence of porous, low-density ice in small icy bodies (e.g., comets, Kuiper belt objects) presents an added challenge for accurate modeling of the impact process, as the crushing of pore space partitions a greater fraction of the initial impact energy into heat while simultaneously attenuating the shock wave. Recent progress in the development of water equations of state, coupled with increasingly efficient 3-D hydrocode calculations, has been used to construct careful numerical studies of melt and vapor generation for water ice targets. These studies generally rely on the pressure-entropy Hugoniot to determine the critical shock pressures for which phase changes will occur and do not include material strength effects. This approach assumes that dissipative heating is unaffected by the propagation of transverse waves through the target. However, a non-negligible role for the effects of shear heating has been supported in previous hypervelocity impact experiments. Here, we report on laboratory results for hypervelocity impacts (~5 km/s) into porous (ϕ ~ 0.5) and nonporous water ice. Experimental work was conducted at the NASA Ames Vertical Gun Range (AVGR) for a range of impact incidence angles. Time-resolved images of the expanding plume front allow the internal energy of the plume to be estimated; this derived energy provides a useful metric for detecting enhancements in target heating. In addition, the use of quarter-space target geometries with porous ice targets provides a detailed record of shock wave propagation through the target material. Results from these experiments will be directly compared with CTH hydrocode calculations for laboratory-scale impacts into porous and nonporous ice targets.