What Caused Argentina's Mystery Craters?

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That original story, published in 1992, faced immediate challenges from skeptical scientists. The low angle and large size of the meteorite were improbable, they argued. On the moon, which preserves a thorough record of its past meteorite impacts, only one example of such an oblique impact is known to exist.

But, comments Melosh, "mere improbability is not proof that such an event did not occur."

A Shower of Glass

That's where Bland enters the picture. He and his team of ten researchers from six different countries analyzed satellite images of thousands of square miles of Argentine Pampas, including the region that Schultz and Lianza studied. What the scientists found was a more complex picture than they had imagined.

Elongated depressions like those previously described around Rio Cuarto exist across the entire region. In any given locale, all depressions run parallel to one another. But from the perspective of the satellite it became clear that the "craters" weren't craters at all—like sand dunes, they had been produced by the action of wind on soil and vary in their orientation according to the direction of local prevailing winds.

The meteorites that Schultz found inside the bogus craters, Bland's team further concludes, are once-buried remnants of ancient impacts that became exposed when the wind carved out the depressions. That notion was bolstered by their discovery of two additional meteorites—of different types and ages, and therefore not from the same impact—in nearby depressions.

Nevertheless, one piece of evidence suggests a highly unusual impact. Bland and his researchers found numerous pieces of glassy rock that could only have been formed under the intense natural heat and pressure produced at the instant of a meteorite impact.

These bits of glass, called tektites, appear to be scattered across the entire region that the researchers examined, suggesting that much of the Pampean Plain is a vast tektite field.

Tektite fields are few and far between. Only four such fields are known to exist on Earth, one each in North America, Africa, and Europe, and one—gouged from a crater that has not yet been identified—that stretches for more than 6,000 miles (10,000 kilometers) across Australia and Southeast Asia. The Argentine tektite field, if that's what it is, would be the fifth ever discovered.

The age of the tektites is greater than that of the formerly proposed oblique impact. Whatever cataclysmic collision forged the glassy rocks occurred about half a million years ago, Bland says. But Bland and his researchers have yet to identify a crater produced by that impact, and they don't even know the full scale of the tektite field, since they haven't yet found its edge.

Nevertheless, the researchers now know enough to imagine the fallout from a stunning event, which must have melted and ejected silty material from the ground, showering molten glass for hundreds of miles around.

"As terrifying as the original picture of an oblique impact that scarred the Pampas…was, the present view of a shower of hot glass over a region as large as Texas suggests a far more lethal event," says Melosh.

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