How the Moon got its rays
Par Pinaki Chakraborty, Okinawa Institute of Science and Technology Graduate University, Japan
Mardi 11 Décembre, 15h15, Salle 103, Bâtiment A9
Ray systems, or set of radial streaks that encircle an impact crater, became known shortly after the advent of the telescope, perhaps in 1647, when Johannes Hevelius published what might have been the first map of the Moon to show them. Although they were recognized as settled ejecta, that is, deposits of debris thrown out when a meteorite impacts the surface of a planetary body, their origin proved to be the sort of question for which competing theories abound to date. Motivated by observations of ray systems in planetary cratering, we study an analog system: 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. Here we show that the uniform blanket of ejecta changes to a ray system 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 ∝D/λ, where D is the diameter of the ball and λ 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 on the Moon and other planetary bodies. This research was carried out in collaboration with Tapan Sabuwala, Christian Butcher, and Gustavo Gioia.