[meteorite-list] What Caused Argentina's Mystery Craters?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:48 2004
Message-ID: <200205120443.VAA26709_at_zagami.jpl.nasa.gov>

http://news.nationalgeographic.com/news/2002/05/0509_020509_glassmeteorite.html

What Caused Argentina's Mystery Craters?
By Ben Harder
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.

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.
Received on Sun 12 May 2002 12:43:30 AM PDT