Abstract. -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.Introduction. -Granular materials often exhibit behavior intermediate between that of conventional solids and liquids. Probing the resulting combination of liquidand solid-like properties a number of recent studies investigated the impact of a large object into a bed of dry grains. These studies focused on issues such as the drag on the impacting object [1-6], crater formation [7-10] the corona-like splash formed immediately after the impactor hits the bed surface [10], and the subsequent jet of grains formed by the collapse of the cavity left by the impactor [11][12][13][14][15][16]. So far however, almost all work considered the limit of loosely packed, marginally stable beds that readily compact in response to perturbations. On the other hand, densely packed beds must dilate in order for grains to move out of the way of inserted objects. This implies different resistance not only for slow, quasi-static perturbations [17] but also suggests that there should be a significant change in the dynamics for faster impacts.An important feature of the impact dynamics in granular systems is the coupling between the interstitial gas, typically air, and the grain packing. For fine grained beds (grain diameters below ∼ 150 µm) this interaction can drastically change the impact dynamics. In particular, in

The interaction between fine grains and the surrounding interstitial gas in a granular bed can lead to qualitatively new phenomena not captured in a simple, single-fluid model of granular flows. This is demonstrated by the granular jet formed by the impact of a solid sphere into a bed of loose, fine sand. Unlike jets formed by impact in fluids, this jet is actually composed of two separate components, an initial thin jet formed by the collapse of the cavity left by the impacting object stacked on top of a second, thicker jet which depends strongly on the ambient gas pressure. This complex structure is the result of an interplay between ambient gas, bed particles, and impacting sphere. Here we present the results of systematic experiments that combine measurements of the jet above the surface varying the release height, sphere diameter, container size, and bed material with x-ray radiography below the surface to connect the changing response of the bed to the changing structure of the jet. We find that the interstitial gas trapped by the low permeability of a fine-grained bed plays two distinct roles in the formation of the jet. First, gas trapped and compressed between grains prevents compaction, causing the bed to flow like an incompressible fluid and allowing the impacting object to sink deep into the bed. Second, the jet is initiated by the gravity driven collapse of the cavity left by the impacting object. If the cavity is large enough, gas trapped and compressed by the collapsing cavity can amplify the jet by directly pushing bed material upwards and creating the thick jet. As a consequence of these two factors, when the ambient gas pressure is decreased, there is a crossover from a nearly incompressible, fluidlike response of the bed to a highly compressible, dissipative response. Compaction of the bed at reduced pressure reduces the final depth of the impacting object, resulting in a smaller cavity and in the demise of the thick jet.