Shock Wave Propagation in Porous Ice

We present data on shock wave propagation in porous ice under conditions applicable to the outer solar system. The equation of state of porous ice under low temperature and low pressure conditions agrees well with measurements under terrestrial conditions implying that data on terrestrial snow may be applicable to the outer solar system. We also observe rarefaction waves from small regions of increased porosity and calculate release wave velocities. INTRODUCTION EXPERIMENTS Porosity effects are significant in impact processes, affecting the strength, elastic moduli and shock attenuation. Interest in the effects of porosity on collisional processes in the solar system has increased with the discoveries of the Kuiper Belt and asteroid Mathilde. Main belt asteroid 253 Mathilde has a bulk density of only 1.3 g/cm3 (1) implying a bulk porosity greater than 50% (2). Mathilde’s surface has craters with unusual morphologies whose formation may be linked to the high porosity of the object (3). The Kuiper Belt is a disk of small icy bodies (Dc300km) similar to the Asteroid Belt but located outside the orbit of Neptune (4). At present, mutual collisions between Kuiper Belt Objects are erosional and destructive, producing dust and grinding the system into smaller bodies. The Kuiper Belt is the source of short period comets, and collisional evolution studies indicate that most comet-sized bodies are debris from collisions with larger bodies (4). The bulk porosity of comets has never been measured directly, but the Rosetta mission will measure both the bulk and surface porosity and other physical properties of a short period comet. The temperatures of interest in the outer solar system are between about 80-150 K. The velocities of mutual collisions between Kuiper Belt Objects are currently centered around 800 m/s (5), producing peak pressures in the high hundreds of MPa. The porosity of comets is expected to range from 30-80%. This is a large, and largely unexplored, range of states for shock studies on ice as most of the previous work on porous ice and natural snow have been conducted under low density (< 0.5 g/ cm3), much lower (< 200 MPa) or much higher (> 3 GPa) pressures, or at warmer temperatures (> -30°C). Targets were prepared in the Caltech cold laboratory at a temperature of -8OC. Commercial crushed ice was ground and sifted to grain sizes between 180-355 pm and hand pressed to the desired thickness and density. Discs were prepared ranging in thickness from 3-15 mm and densities between 0.3-0.6 g/cm3. We have studied catastrophic disruption of porous materials (6) and we are also conducting experiments on shock wave propagation in porous ice and ice-dust mixtures. Here we present data on shock propagation in porous ice under conditions applicable to the outer solar system. Shock wave profiles were measured with 0.5 mil copper film electromagnetic velocity gauges insulated with 1 mil Kapton on each side (Dynasen, Inc., Goleta, CA). For shock wave studies in ice, velocity gauges are preferable over pressure gauges because they do not have to be calibrated for low temperatures. Because of the high porosity of the target, the gauges were custom made to a length of 9 mm and width of 4 mm. Thus, the measurement is an average of the particle velocity over this area. The thin gauges equilibrate