ASU microscopes help solve decades-old asteroid-impact deposit mystery
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Axel Wittmann had always had “a fondness for exotic rocks,” as he puts it, his favorite being suevite, formed from intense meteorite collisions. But in 2009, when he met fellow geologist Philippe Lambert on an excursion to the Rochechouart impact structure in southern France, Wittmann learned of a new, mysterious kind of rock, the formation of which would hold his attention — and evade explanation — for the next 16 years.
Discovered by Lambert in 1972 on the first day of his PhD fieldwork, impactoclastite, as he named it, was found to be unique to the Rochechouart impact structure and thought to be made of debris that fell back from the asteroid’s giant impact plume. However, unlike similar deposits from other impact sites around the world that vanished over time, the strange, ash-like impactoclastite had managed to extend down into the suevite rock layers in veins that were at least 27 meters deep and occurring in many orientations — and thereby had survived for millions of years.
How this happened remained a mystery until Wittmann took a sample of impactoclastite he had picked up on his excursion and put it under some high-resolution microscopes at Arizona State University’s Eyring Materials Center, part of the university's Core Research Facilities.
Now, in an article published in Earth and Planetary Science Letters, Wittmann, an associate research scientist at ASU, and Lambert, founding director of the Center for International Research and Restitution on Impacts and Rochechouart, are putting forth a new theory for the phenomenon called “debris inhalation.”
In their article, the scientists suggest that after the Rochechouart asteroid hit, a hot plume of vapor and molten droplets rose into the sky. In the aftermath of the impact, the central peak of the crater rose and collapsed in a matter of minutes, creating a “cave several square kilometers wide and tens of meters high” under the existing stacked rock slab. Then, anywhere from an hour to one day after the initial impact, the slab collapsed into this cave — like a soufflé falling — which created cracks in the partly cooled suevite. As the plume rained ash and molten droplets back onto the crater, a temporary vacuum formed as the collapsed slab displaced the giant cavern, sucking the falling debris into the cracks — like the ground itself taking a heaving, gasping breath.
“It only took me 16 years to properly analyze it, interpret the observations and craft a narrative for publication,” Wittmann said.
Using the Eyring Materials Center’s JEOL JXA-8530F electron microprobe — a high-precision instrument capable of detecting trace elements in tiny particles — Wittmann found compositional signatures (chemical fingerprints) in the impactoclastite that were known to form from the admixture of asteroid metals at extreme temperatures.
That allowed the researchers to assert that the impactoclastite was indeed made of debris from the vapor plume, rather than having been created some other way, like phreatic explosions (caused by hot impact melt interacting with groundwater), an oceanic resurge (tsunami) at the time of the impact or later erosion.
Understanding how impacts behave helps scientists make better sense of impact craters, identify asteroid materials and learn more about ancient environments, Wittman and Lambert said. It also improves planetary defense science by helping scientists model the atmospheric consequences, hazard zones and effects of future asteroid impacts.
“Communicating this science to the public is part of a broader global effort to better understand and safeguard our planet,” Lambert said.
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