Earth is littered with cones from space, and it's our planet's own fault.
Most meteorites found on Earth are just randomly shaped blobs. But a surprisingly high number of them, about 25%, are cone-shaped when you fit all their pieces back together. Scientists call these conical space-stones "oriented meteorites." And now, thanks to a pair of experiments published online today (July 22) in the journal Proceedings of the National Academy of Sciences (PNAS), we know why: The atmosphere is carving the rocks into more aerodynamic shapes as they fall to Earth.
"These experiments tell an origin story for oriented meteorites," Leif Ristroph, a New York University (NYU) mathematical physicist who led the study, said in a statement. "The very aerodynamic forces that melt and reshape meteoroids in flight also stabilize [them] so that a cone shape can be carved and ultimately arrive on Earth." [The 10 Biggest Impact Craters on Earth]
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It's difficult to precisely replicate the environment meteoroids encounter on their way to our planet's surface. The space rocks slam into the atmosphere at high speeds, generating intense, sudden friction that heats, melts and deforms the objects as they freely tumble. Those conditions didn't exist in the NYU lab where the study occurred, but the researchers approximated those factors by using softer materials and water, and by breaking the experiment up into parts.
First, the researchers pinned balls of soft clay in the center of streams of rushing water, a rough approximation of a heavy rock hitting an atmosphere. The clay, the scientists found, tended to deform and erode into a cone shape.
But that experiment alone wouldn't explain much. The soft clay wasn't allowed to move in the water — a very different situation from a rock free to tumble loose through the upper atmosphere and somehow orient itself.
So, for the second step, the researchers dropped different sorts of cones into water to see how they fell. It turns out that cones that are too narrow or too fat tend to tumble, like rocks of any other shape would do. But there were "Goldilocks" cones, in between those two extremes, that flipped until their points aimed along their direction of travel, like an arrow, and then glided smoothly through the water.
These two experiments together seem to show that when certain conditions are met, space rocks will develop conical shapes under the extreme friction of an atmospheric entry. And sometimes those conical parts will help these tumbling rocks stabilize, pointing in a consistent direction as they fall. That stability, in turn, will make them more and more conical. Then, when these rocks strike the ground, meteorite hunters encounter the remains of "oriented," conical space rocks.
Originally published on Live Science.
