Ray Systems in Granular Cratering

In classical experiments of granular cratering, a ball dropped on an evened-out bed of grains ends up within a crater surrounded by a uniform blanket of ejecta. In this Letter, we show that the uniform blanket of ejecta changes to a ray system, or set of radial streaks of ejecta, where the surface of the granular bed includes undulations, a factor that has not been addressed to date. By carrying out numerous experiments and computational simulations thereof, we ascertain that the number of rays in a ray system proportional to D/lambda, where D is the diameter of the ball and lambda is the wavelength of the undulations. Further, we show that the ejecta in a ray system originates in a narrow annulus of diameter D with the center at the site of impact. Our findings may help shed light on the enigmatic ray systems that ring many impact craters on the Moon and other planetary bodies.