Planet Earth

Raindrops Are like Tiny Asteroid Strikes

InkfishBy Elizabeth PrestonJan 2, 2015 2:32 PM

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Rainshowers are a lot more dramatic if you imagine every drop is a tiny asteroid imperiling miniature dinosaurs or sending little astronaut Ben Afflecks into space. It turns out your fantasy wouldn't be that far off, aside from that last part. Researchers have found startling similarities between asteroid craters and the fleeting indentations left by raindrops on sand. At the University of Minnesota, physicist Xiang Cheng and three undergraduate students scrutinized what happens when a drop of water hits a granular surface. Because of all the variables in this kind of collision—the viscosity of the liquid, how much the drop squishes when it hits the surface, how the liquid and the grains interact—the researchers call it "notoriously complicated." Cheng and his students used high-speed photography and laser measurements to observe falling water drops and the miniature craters they made. They used sand-like grains of glass instead of actual sand. Onto this unromantic beach, they dripped water drops in a range of sizes mimicking natural raindrops. To create the smallest impacts, they dropped water from less than 2 millimeters from the surface. Other drops started as high as 12 meters. The resulting slow-motion movies revealed weird, amazing phenomena at the moments the drops splashed down. Raindrops are sculptors, leaving new sand formations behind them. What exactly they create depends on how hard they hit. Drops with low energy, for example, spread out and grab sand grains, then spring back to create a "liquid marble." This sandy sphere may even jump off the ground in the raindrop's wake:

A bouncing liquid marble of sand and water. If the raindrop hits with a little more energy, the liquid spreads for an instant into sandy "fingers":

A puddle with fingers. And at even higher energy levels, the drop makes a deep crater with a "water crown" that throws off new droplets from its points:

A bejeweled water crown. After the drops melt into the sand, they may leave behind sculptures in bizarre shapes, ranging from Cheerios to lumpy marbles to flowers.

For all the fascinating sand art they found, though, Cheng and his students were struck by how ultimately familiar these collisions were. They used their measurements to create a general model of raindrops hitting sand. The mathematical relationship between the energy of a raindrop collision and the diameter of its crater, they discovered, is surprisingly similar to an asteroid strike. Another similarity to asteroids showed up in the relationship between impact craters' diameter and their depth. This ratio in raindrop craters is the same as in craters on the moon, Mars, and Mercury. The difference between the energy in an asteroid strike and the plop of a raindrop is abut 18 orders of magnitude. In other words, the asteroid hits the ground roughly 1,000,000,000,000,000,000 times harder. Yet a raindrop is a better model for a high-energy asteroid strike than even a solid sphere is, the authors write. Maybe little Ben Affleck should head for his rocket after all. Video and images: Zhao et al.

Runchen Zhao, Qianyun Zhang, Hendro Tjugito, & Xiang Cheng (2014). Granular impact cratering by liquid drops: Understanding raindrop imprints through an analogy to asteroid strikes Proceedings of the National Academy of Sciences arXiv: 1407.7420v2

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