Thermodynamics of Impacts onto Icy Mixtures: Peak and Post-shock Temperature Measurements in an Ice- Sand Mixture

Introduction: Shock processes in heterogeneous materials is a challenging aspect of impact cratering studies. Understanding the equipartition of energy in shocked mixtures of widely varying impedence is necessary to predict a wide range of phenomena, including phase changes, post-shock temperatures, and chemical reactions. Here, we present the results from shock py-rometry experiments on a mixture of sand and H 2 O ice, which constitute a portion of an experimental and theoretical effort to investigate the thermodynamics of shock processes in heterogeneous mixtures. Due to multiple wave interactions between the components in the mixture, the thermodynamic loading path may not be along a single-shock Rayleigh line and the final state may not be on the Hugoniot of the mixture. The loading path may be along a quasi-isentrope; however, the thermodynamics will depend on the length scales in the mixture. Simple and elaborate procedures for estimating the Hugoniot equation of state for material mixtures have been postulated [1] but they are based upon mass or volume averaging. In general, the complexities of impacts on heterogeneous mixtures has not been well explained. In particular, for materials like quartz and ice that have a large number of phase transitions and a significant difference in compressibility, the models are not able to fully describe the experimental data. Experiments: Samples are made from a 57:43 volumetric mixture of degassed distilled H 2 O ice and high purity quartz sand (Mill Creek, OK, #1 Dry, U.S. Silica Co.). The constituent materials are sifted to a particle size of 125-250 μm, mechanically mixed, and cold pressed in a piston-die assembly (<>=1.65±0.02 g/cm 3 , 30.32.6 mm disc). Before pressing, the die is evacuated to approximately 10-2 Torr to reduce air in the sample. By analogous samples, the porosity is assumed to be 1.5±0.5%.