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Scientists in the Netherlands have successfully recreated a small-scale meteoritic impact in the laboratory for the first time. The novel yet simple experiment, devised by Detlef Lohse and colleagues at the University of Twente, involves dropping a small steel ball onto the surface of a sand bed. The results could shed more light on the processes occurring during large-scale impacts on Earth and other planets in the solar system (arXiv.org/abs/cond-mat/0406368).
Lohse and colleagues first prepared a sand bed, around 25 cm thick, from fine sand grains measuring on average 50 microns across. The sand was "decompactified" by blowing air through it and then allowed to settle in an extremely loose-packed structure, so that it essentially behaved like a fluid. Next, the scientists dropped a steel ball, with a diameter of 2.5 cm, onto the sand from various heights and angles while taking images with a high-speed digital camera.
The Twente team observed a series of well-defined steps: on impact, sand is blown away in all directions to form a crown-shaped splash. The ball then penetrates the sand and creates a void, which then collapses under the influence of the hydrostatic-like pressure of the sand. This pressure subsequently ejects sand grains into the air to form jets (see figure). Using numerical simulations the scientists developed a theory to explain how the void collapsed.