Slow Impacts on Surfaces of Small Bodies: Coefficient of Restitution and Penetration Depth into the Regolith

Introduction: The dusty regolith covering the surfaces of asteroids and planetary satellites is subject to environmental conditions very different from those found on Earth. This regolith evolves at low ambient pressures and gravity levels. Its response to slow impacts, such as those occurring upon secondary impacts on small bodies or in planetary ring environments, may be completely different than what is intuitively expected. We carried out a series of impact experiments into simulated planetary regolith in zero- and reduced-gravity conditions using a range of microgravity platforms. Coefficient of Restitution: For speeds >30 cm/s, impacts systematically produced an ejecta blanket and coefficients of restitution of the projectile were of the order of 10^-2. Below 20 cm/s, only rebounds without ejecta were observed, with coefficients of restitution rising up to 0.3. Our results showed a similar power law index than the value of -1/4 found in 1g. This is twice as high as expected from the theory of an elastic sphere impacting a plane surface and indicates that energy absorption in a bed of granular material is not entirely captured by the mechanics of elastic surfaces. Penetration Depth: We determined the maximum penetration depth of the projectile into the target (z_max). No correlation between z_max and the equivalent total drop distance (H) was observed. This is in contrast with the relation z_max H^-1/3 observed in 1g. Instead, we showed that the penetration depth scales with the quantity µ^-1(ρ_p/ρ_g)^0.1D_p^2/3 H^-1/3, with µ the coefficient of friction of the target material, ρ the density of the target (g) and projectile (p), and D_p the projectile diameter. Conclusion: Our experiments show projectile rebound off the target in microgravity (<10^-4g). None of the impacts in 1g and reduced gravity (0.05g) displayed a similar behavior. The projectile penetration depth into the target in microgravity was observed to depend more on the projectile size and energy and less on its density difference with the target than in 1g. These experiments show that the ambient gravity has a significant influence on the behavior of granular material on small bodies of the Solar System.