  We examine the impact of a solid sphere into a fine-grained granular bed.
Using high-speed X-ray radiography we track both the motion of the sphere and
local changes in the bed packing fraction. Varying the initial packing density
as well as the ambient gas pressure, we find a complete reversal in the effect
of interstitial gas on the impact response of the bed: The dynamic coupling
between gas and grains allows for easier penetration in initially loose beds
but impedes penetration in more densely packed beds. High-speed imaging of the
local packing density shows that these seemingly incongruous effects have a
common origin in the resistance to bed packing changes caused by interstitial
air.
