Laboratory experiments of crater formation on ice–silicate mixture targets Original Research Article

Water ices exist mostly in a variety of forms of mixtures with silicate or some other components (e.g., ammonia) in the solar system. Thus, it is of importance to understand the impact process on ice–silicate mixtures. We performed impact experiments on ice–silicate mixture target using a gas gun and a two-stage light-gas gun in a cold room at 263 K in the Institute of Low Temperature, Hokkaido University. Silicate content of the target was varied from 0 to 50 wt%. Targets were 10 or 30 cm in diameter, and 5 cm in height. Projectile velocity was varied between 299 and 657 m/s and projectile mass was 1.6 g for the gas gun shot, and 1480 and 3684 m/s and 7 mg for the two-stage light-gas gun. Crater morphology changes with silicate content and impact velocity: craters formed on targets of 50 wt% silicate content had less spallation than those with the other silicate content and crater profiles at higher impact velocity became sharper than those at lower impact velocity. Although diameter-to-depth ratio is not dependent on silicate content of the target, this ratio became large at low peak pressure for the gas gun shots. This is probably because of accretion of pure ice projectile onto the target surface. The decrease of crater volume with increasing silicate content was visible for the results from the two-stage light-gas gun. We estimated the target tensile strength based on a hydrodynamic ejection model of [Melosh, H.J. Impact ejection, spallation, and the origin of meteorites. Icarus, 59, 234–260, 1984]. The ejection model predicts a relation between target tensile strength, spall fragment thickness and ejection point. If a constant shock attenuation rate is assumed for all the ice–silicate mixture targets, the tensile strength derived for the mixture with 50 wt% rock content turns out to be higher than the pure ice by a factor of 2 or more.