Experimental Study on Gravitational and Atmospheric Effects on Crater Size Formed by Low‐Velocity Impacts Into Granular Media

Abstract

Abstract Collisions are ubiquitous phenomena on the surface of solar system objects. When secondary craters form on the surface of small bodies, such as asteroids and comets, the impactors have very low velocities of several meters per second or less. These secondary collisions can affect the surface topography of small bodies. Therefore, it is important to investigate low‐velocity impacts on a target simulating regolith to understand the effects of secondary collisions on the surface of small bodies. We conducted impact experiments into granular materials with a velocity range of 1 to 4.6 m/s, under a gravity range of 0.20 to 1 G, to investigate the effects of gravity on crater diameter. The gravitational dependence obtained under the reduced ambient pressure almost agreed with those obtained for high‐velocity impacts in previous studies. Gravitational dependences obtained under the standard atmospheric condition roughly agreed with the dependence obtained under the reduced pressure condition when a term of nondimensional atmospheric pressure was added to the scaling relationship. It was found that the crater‐size scaling law obtained for high‐velocity impacts could be applied to low‐velocity impacts of several meters per second under the reduced ambient pressure when the density ratio of the projectile to the target was close to unity. Finally, we applied the experimental results to estimate the diameters of secondary craters on asteroid 25143 Itokawa and compared the diameters with the dimple topography on Itokawa. The results support the hypothesis that dimples are formed by low‐velocity impacts of meter‐sized boulders.