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A projectile impact onto a granular target produces an ejecta curtain withthe heterogeneous material distribution. Understanding how the heterogeneouspattern forms is potentially important for understanding how crater rays form.Previous studies predicted that the pattern formation is induced by inelasticcollisions of ejecta particles in the early stages of crater formation and isterminated by the ejecta's expanding motion. In this study, we test thisprediction based on a hyper-velocity impact experiment together with N-bodysimulations where the trajectories of inelastically colliding granularparticles are calculated. Our laboratory experiment suggests that patternformation is already completed on a timescale comparable to the geometricalexpansion of the ejecta curtain, which is ~ 10 microseconds in our experiment.Our simulations confirm the previous prediction that the heterogeneous patterngrows through initial inelastic collisions of particle clusters and subsequentgeometric expansion with no further cluster collisions. Furthermore, to betterunderstand the two-stage evolution of the mesh pattern, we construct a simpleanalytical model that assumes perfect coalescence of particle clusters uponcollision. The model shows that the pattern formation is completed on thetimescale of the system's expansion independently of the initial conditions.The model also reproduces the final size of the clusters observed in oursimulations as a function of the initial conditions. It is known that particlesin the target are ejected at lower speeds with increased distance to the impactpoint. The difference in the ejection speed of the particles may result in theevolution of the mesh pattern into rays.

Modeling Early Clustering of Impact-induced Ejecta Particles Based on Laboratory and Numerical Experiments