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University of Twente/De Jong, Zhao, and Van der Meer

This crater was formed by a raindrop—and one of the world’s tiniest avalanches

Craters left by bombs and asteroids get all the press, but what happens when a raindrop plows into sandy soil? To find out, scientists used syringes to create droplets of a consistent size, then released the drops over a bed of tiny glass beads meant to represent loose sand. The more tightly packed the beads, the shallower the crater, the researchers will report in a forthcoming issue of Physical Review E. That may not seem surprising, but the team did note one shocker: Craters left by high-speed drops started out with relatively steep sides but within seconds they avalanched to form slopes with a more stable configuration—a previously unstudied process that widened the crater and made it shallower (image above, where depth is exaggerated by a factor of three). Besides helping scientists better gauge how precipitation ultimately soaks into sandy soils, the research could help them better understand industrial processes, such as pharmaceutical manufacture, in which liquids are sprayed onto dry powdered materials—possibly improving quality control and consistency in the final products. The new results might even help earth scientists who seek to infer the thickness of Earth’s ancient atmosphere based on the size of raindrop craters left in now-hardened volcanic ash.