  Non-cohesive granular media exhibit complex responses to sudden impact that
often differ from those of ordinary solids and liquids. We investigate how this
response is mediated by the presence of interstitial gas between the grains.
Using high-speed x-ray radiography we track the motion of a steel sphere
through the interior of a bed of fine, loose granular material. We find a
crossover from nearly incompressible, fluid-like behavior at atmospheric
pressure to a highly compressible, dissipative response once most of the gas is
evacuated. We discuss these results in light of recent proposals for the drag
force in granular media.
