What Caused Argentina's Mystery Craters?

By Ben Harder
for National Geographic News
May 9, 2002

For more than a decade, planetary scientists have been puzzling over a mixed bag of meteorite evidence scarring Argentina's plains. They gradually pieced together clues to reconstruct what seemed to be a rough-hewn but generally accurate account of a prehistoric meteorite impact.

A mere 10,000 years ago, scientists deduced in the original theory, a sizable meteorite came hurtling through the atmosphere at a bizarrely low angle, smacked the ground with a glancing blow, and broke into numerous pieces that gouged separate, miles-long scars in the Argentine earth.

But now a fresh analysis has turned that theory on its head. The mysterious craters in Argentina may not have been caused by meteorites at all, but rather by the wind, sculpting the ground over a long time. Discovered in some of these crater-like trenches, ironically, were the remains of real meteorites that crashed into Earth over widely separated time periods. They struck at different angles and produced spectacularly different results—including, in the case of one, a widespread shower of molten glass.

The evolving interpretation of Argentina's mysterious craters, University of Arizona planetary scientist Jay Melosh writes in the May 10 issue of the journal Science, "is both less and much more than [its] discoverers originally believed."

Citing evidence presented by Philip A. Bland in the same issue of the journal, Melosh describes a newly emerging picture, in which a much older meteorite collision blasted tons of sandy, local soil into the air—melting it instantly and peppering a vast swath of country with glowing, glassy debris.

Bland's discovery of so many glass fragments over such a wide area adds a startling twist to a young but already storied saga in planetary science. One implication, the researcher from England's Open University says, is the possibility that a "well-preserved complex crater remains to be discovered beneath the Pampean Plain of Argentina."

Recent Impact

The buzz about the nature of Argentina's extraplanetary legacy began in 1991, when Brown University geologist Peter Schultz and his Argentine pilot, Ruben Lianza, took a flight over Rio Cuarto, a small city in a region of Argentina known as the Pampas or the Pampean Plains.

From the air, Schultz and Lianza observed ten elongated depressions in the ground that resembled the craters that might form if many different objects had impacted the ground at oblique, almost level angles. What's more, it seemed, the craters all ran parallel to each other, as though a low-flying herd of Dumbos had dragged their feet along the ground beneath them. The parallel orientations suggested that they were carved by multiple meteorite fragments coming from the same direction.

Back on terra firma, Schultz and Lianza explored some of the craters on foot. In one, they found a pair of meteorite fragments and pieces of glass that had been forged by the high temperatures of an impact.

Putting those and other clues together, they hypothesized that a large meteorite, perhaps 500 to 1,000 feet (150 to 300 meters) across and traveling at about 56,000 miles (90,000 kilometers) per hour grazed the ground at an angle of less than 7 degrees, falling like horizontal rain.

The powerful impact, they believed, occurred as recently as 10,000 years ago, when early Native Americans were already living in the region.

Continued on Next Page >>




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