Impact experiments on granular materials under low gravity: Effects of cohesive strength, internal friction, and porosity of particle layers on crater size

Impact craters on the surface of asteroids provide information about their surface properties and evolutionary processes. Investigating how crater size depends on these physical properties is important for establishing a crater-size scaling law. We conducted high-velocity impact experiments at 1 G and at simulated low gravity on dry particle layers with different physical properties. The crater diameter was larger for targets with smaller internal friction. Also, for similar internal friction, the crater diameter was larger for targets with lower porosity. The crater diameter for targets with particle diameters of several hundred micrometers was proportional to the gravitational acceleration to the power of-0.16 to-0.18. However, the crater diameters for targets with particle sizes of-40 & mu;m showed little or no difference between 1 G and low gravity. Although the results for targets with particle sizes of-40 & mu;m at 1 G showed a relationship in the gravity regime, the results under low gravity deviated from this tendency. We obtained the condition for the transition between the gravity-and strength dominated regimes: the ratio of the cohesion Ycoh to the term & rho;tgD, or the ratio of the effective strength YEFF to & rho;tgD, where & rho;t, g, and D are the target bulk density, gravitational acceleration, and crater diameter, respectively. The values of Ycoh/& rho;tgD and YEFF/& rho;tgD for targets with particle sizes of-40 & mu;m were 0.22-0.27 and 0.11-0.60, respectively. This study provides basic experimental data for comparison with numerical simulations.