Experimental hypervelocity impact into quartz sand: Distribution and shock metamorphism of ejecta

We report results of vertical impacts of 0.3-g cylindrical Lexan projectiles into noncohesive quartz sand in which vertical and horizontal reference strata were employed by using layers of colored sand. The impacts were performed at velocities of 5.9–6.9 km/s with the NASA Ames vertical gun ballistic range. The craters, 30–33 cm in diameter, reveal a radial decay of the ejecta mass per unit area with a power of –2.8–−3.5. Material displaced from the upper 15% of the crater depth d is represented within the whole ejecta blanket, material from deeper than 28% of d is deposited inside 2 crater radii, and no material from deeper than 33% of d was ejected beyond the crater rim. Shock-metamorphosed particles (glassy agglutinates, cataclastic breccias, and comminuted quartz) amount to some 4% of the total displaced mass and indicate progressive zones of decay of shock intensity from a peak pressure of 300 kbar. The mass distribution of these particles is distinctly different from that for the total displaced mass; 85% of the shock-metamorphosed mass was concentrated within the crater itself. The shock-metamorphosed particles and the shock-induced change in the grain size distribution of ejected samples have close analogies to the basic characteristics of the lunar regolith. Possible applications to regolith formation and to ejecta formations of large-scale impact craters are discussed.

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