Journal Club

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Journal Club: Researchers may’ve finally solved mystery of crater ray formation

Ejecta rays surrounding impact craters, like these on the moon, are commonplace but the physics of their formation has been a mystery. Credit: NASA/Goddard Space Flight Center/Arizona State University

Ejecta rays surrounding impact craters, like these on the moon, are commonplace. But the physics of their formation has long been a mystery. Credit: NASA/Goddard Space Flight Center/Arizona State University

Look carefully at the full moon and you’ll see bright skinny streaks extending from large lunar craters. These are ejecta rays, spoke-like lines that seem to shoot from the circular impact site, a vivid testament to ancient explosions.

But scientists have never quite been able to figure out what aspect of an asteroid impact generates the rays. Now, a series of serendipitous events has led a research team to produce a new laboratory model for crater ray formation, concluding that it depends on two simple parameters–and potentially putting this long-standing mystery to rest. Their findings recently appeared in Physical Review Letters.

“I think the work that these guys did might have to be considered the breakthrough,” says geophysicist Jay Melosh of Purdue University in West Lafayette, Indiana, who was not involved in the research.

In 1647, Polish astronomer Johannes Hevelius published a lunar map showing thin lines extending from craters all over the moon, though no one at the time knew what they were. Craters themselves remained enigmatic for centuries, with leading scholars suggesting they could be remnant calderas of prehistoric volcanoes. It wasn’t until the 1960s that planetary scientist Eugene Shoemaker proved that craters were created when a fast-moving object slams into a planetary surface.

Specialized laboratory equipment, like the Vertical Gun Range at NASA’s Ames Research Center in Mountain View, California, has in the last five decades been used to shoot pea-sized pellets into powdery material at up to 24,000 kilometers per hour, helping to tease out nearly every detail of crater formation. But one aspect has stubbornly defied explanation—the rays. No matter what researchers have done in the last 50 years, their simulated impacts eject a perfectly circular blanket of material that fails to fall into straight, skinny lines.

Researchers were able to consistently generate ejecta rays by pressing a repeating hexagonal honeycomb pattern into a granular surface. The number of crater rays depended on only two factors—the size of the impactor and the spacing of the hexagonal bumps. Credit: Tapan Sabuwala.

Researchers were able to consistently generate ejecta rays by pressing a repeating hexagonal honeycomb pattern into a granular surface. The number of rays depended on only two factors: the size of the impactor and the spacing of the hexagonal bumps.    Credit: Tapan Sabuwala

Starting in 2014, paper coauthor and mechanical engineer Pinaki Chakraborty of the Okinawa Institute of Science and Technology in Japan and his colleagues ran relatively run-of-the-mill crater demonstrations, dropping 2.5-centimeter steel balls in a box of sand-like grains from a height of 1 meter. Though these experiments failed to capture the super-speeds of actual impacts, the team hoped to elucidate where the splash-like rays originated. Their inspiration came from a rather mundane source—Youtube videos where classroom students toss rocks or marbles into a landscape of flour consistently showed rays extending out from the tiny craters. “How on earth are they getting these rays when we, the so-called professionals, have been such failures?” Chakraborty wondered.

The team tried to vary every parameter they could imagine—changing the grains’ diameters, mixing grains of different sizes, increasing the speed of the balls—but the eventual solution came about by accident. One day, Chakraborty’s fatigued postdoc, Tapan Sabuwala, failed to follow normal protocols and smooth down the surface of their sandbox before dropping in their impactor. After the drop, voilà—a beautiful crater and its rays appeared.

The jagged landscape, which schoolroom students naturally produced when they haphazardly dumped out their bags of flour, seemed to be the key to creating crater rays. The team followed-up with 135 more experiments in which they pressed a repeating hexagonal honeycomb pattern into their granular surface and found that they could consistently make rays. Moreover, the number of crater rays depended on only two factors—the size of the impactor and the spacing of the hexagonal bumps.

The researchers next created computer simulations of their steel-ball-into-honeycomb trials, allowing them to zoom in and watch exactly which grains flew from the impact site into the crater rays. The models showed that particles directly beneath the asteroid strike were being flung up and out by the concave walls of the hexagons, which rose slightly above the surface. Without such concavity, the impact produced circular ejecta, like that seen in previous crater experiments.

Armed with this knowledge, the researchers turned to real-world results. The number of rays extending from two large craters on the moon, Tycho and Kepler, is already known. Though these ancient impacts have long since erased the landscape that was present during their creations, Chakraborty and his colleagues could guess at the primordial topography by looking at relatively untouched places on the modern-day moon. With this and the ray count, they could estimate the size of the asteroids that left behind the craters; their results lined up well with previous approximations that had relied on the physical parameters of rocks and impacts.

While praising the paper’s results, Melosh says the low-speed experiments are not perfectly analogous to actual crater formation. He’d like to see follow-up experiments using more sophisticated apparatuses that simulate high-velocity impactors. Chakraborty agrees, and would like to see other teams further test their results. But he’s happy to have solved at least some part of the crater ray puzzle. “This is one of those things that people have been trying to do for a long time,” he says. “And the reason turns out to be so superficial—literally it’s the surface.”

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