Cratering by impact

The impact of projectiles with a granular ground is common in nature and spans over a large range of scales. At small scales, it happens every time that cm-size drops and seeds fall on the ground, forming cm-size craters and involving low energies (of the order of 1E(-7) J). At large scales, it happens when km-size (or larger) asteroids collide with planets and moons, involving high energies (1E16 to 1E20 J, or even higher). Although high-energy impacts imply melting and evaporation of part of the material, the granular mechanisms for cratering remain valid across scales by assuring that the collisions happen within the same regime. We investigated cratering by performing DEM (discrete element method) computations, where we varied the projectile and grain properties for different available energies. We found that the solid friction and packing fraction affect the crater morphology, that the projectile rebounds by the end of its motion, and that the penetration depth increases considerably with the angular velocity of the projectile. Besides, we propose a scaling that can unify existing correlations for penetration. Those findings answer questions hitherto open, and shed new light on the mechanics of cratering.