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In this fluid dynamics video, we present an experimental investigation of theshape of impact craters in granular materials. Complex crater shapes, includingpolygons, have been observed in many terrestrial planets as well as moons andasteroids. We release spherical projectiles from different heights above agranular bed (sand). The experiments demonstrate two different techniques tocreate non-circular impact craters, which we measure by digitizing the finalcrater topography. In the first method, we create trenches in the sand to mimicfault lines or valleys on a planetary target. During impact, ejecta move fasterin the direction of the trenches, creating nearly elliptical craters with themajor axis running parallel to the trench. Larger trenches lead to more oblongcraters. In the second method, a hose beneath the surface of the sand injectsnitrogen gas. The pressure of the gas counters the hydrostatic pressure of thesand, greatly reducing static friction between grains above the injectionpoint, without disturbing the surface. The affected sand has lower resistanceto impact, creating a knob in the otherwise circular crater rim.

Dropping the Ball: The effect of anisotropic granular materials on ejecta and impact crater shape