W hen a heavy sphere is dropped onto a bed of loose, fine sand, a remarkable phenomenon occurs: a large, focused jet of sand shoots upwards 1-4 . Although similar looking jets are observed on impact in fluid systems 5-7 , they are held together by surface tension. Surprisingly, the granular jet exists in the absence of both surface tension and cohesion, thus fluid jet models are of limited use. Previous work 1,2 proposed that the jet is created solely by the gravity-driven collapse of a void left by the sphere's descent through the pack. Here we present experimental evidence that granular jets are instead driven by a more complex process involving the interaction between the sand and interstitial air. Using high-speed X-ray radiography, and high-speed digital video, we observe the formation of the jet both inside and above the bed. We find that what previously was thought of as a single jet in fact consists of two components: a wispy, thin jet that varies little with pressure followed by a thick air-pressure-driven jet. This is further evidence that qualitatively new phenomena in granular systems can emerge as a function of air pressure 8-11 . Our results highlight the importance of the dynamic coupling between gas and granule motion.As discovered in ref.

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.