Cratering at the Icy Satellites: Experimental Insights

Impact cratering processes play a central role in shaping the evolution of icy satellites and in guiding interpretations of various geologic features at these bodies. Accurate reconstruction of icy satellite histories depends in large part upon observed impact crater size-frequency distributions. Determining the extent of impact-induced thermal processing and the retention rates for impact-delivered materials of interest, e.g. organics, at these outer solar system moons is of fundamental importance for assessing their habitability and explaining differing geophysical histories. Hence, knowledge of how the impact process operates in ices or ice-rich materials is critically important. 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. Complementary to this approach is experimental work to constrain the effects of differing ice target conditions, including porosity, rock mass fraction, and impact angle. Here we report on results from hypervelocity impact experiments (v~5.5 km/s) into water ice targets, performed at the NASA Ames Vertical Gun Range (AVGR). The setup at the AVGR allows for the use of particulate targets, which is useful for examining the effects of target porosity. Photometry and geophysical modeling both suggest that regolith porosity at the icy satellites is significant. We use a combination of half-space and quarter-space geometries, enabling analysis of the impact-generated vapor plume (half-space geometry), along with shock wave and transient crater growth tracking in a cross-sectional view (quarter-space geometry). Evaluating the impact-generated vapor from porous (φ = 0.5) and non-porous water ice targets provides an extension to previously published vapor production results for dolomite and CO₂ ice targets. For the case of a 90 degree impact into porous ice, we calculate that 0.6% of the initial kinetic energy of the impactor is partitioned into the internal energy of the vapor plume. This is slightly higher than values determined in prior studies for non-porous CO₂ ice (0.2%) [Schultz, 1996]. As CO₂ ice possesses a lower vaporization temperature than water ice, this effect strongly suggests a role for porosity in enhancing vaporization. This is expected, as the compaction of porous materials performs additional, irreversible PdV work on the target, causing enhanced partitioning of kinetic energy into internal energy. At oblique impact angles, plume morphology changes dramatically while vaporization is enhanced. Comparing shock wave velocity attenuation in porous materials, including mixes of materials (e.g., quartz sand and porous ice), to numerical results obtained from shock physics codes such as CTH, provides insight into how impacts into porous ice-rich materials can be most accurately numerically modeled.