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Blast survivors: fragments of asteroid found in ancient crater.

Pieces of an asteroid that blasted a 70-kilometer-wide crater in southern Africa millions of years ago have been found intact within the crater, a unique discovery that defies models of such collisions.

The Morokweng crater lies beneath a 150-to-200-meter-thick layer of sand in the Kalahari Desert. Scientists detected the ancient sear a decade ago after noting anomalies in the planet's magnetic and gravitational fields in that region, says Rodger J. Hart, a geochemist at University of the Witwatersrand in Johannesburg.

Analyses of samples that his group has drilled from the crater have revealed an 870-m-thick sheet of rock that melted from the heat of the asteroid's impact and then cooled in subsequent millennia. Radioactive dating of mineral crystals in that rock suggested that the collision occurred about 144 million years ago.

The scientists now report that at a depth of 766 m, a borehole pierced a beach ball-size rock that boars the chemical signature of an extraterrestrial object. The rock's inner portions seem so pristine that they must never have melted and mixed with the once-molten surrounding material. Smaller chunks of similar rock, many measuring less than a centimeter across, were found all along the borehole but were particularly common at depths between 345 and 400 m. Hart and his colleagues describe their find in the May 11 Nature.

Extraterrestrial objects that cross Earth's orbit do so at an average speed of about 20 km per second. Small, stony meteorites typically break up high in the atmosphere (SN: 7/19/03, p. 363. However, large objects such as asteroids slam into Earth at full speed, generating intense heat and pressure. Before the team's discovery, scientists had thought that any object large enough to create a crater more than 4 km across would completely melt or vaporize upon impact, says Hart. Obviously, he notes, the new find challenges that assumption.

Although fragments of rock purported to be pieces of crater-forming extraterrestrial objects have been found in debris thrown far from other impact sites around the world, no such fragment had ever been discovered within a crater.

"This is pretty interesting" says Frank T. Kyte, a geochemist at the University of California, Los Angeles. "This will make a lot of people rethink the impact process:'

The chemical signature of the object that blasted the Morokweng crater--low in sodium and potassium but high in uranium, thorium, and lanthanum--isn't typical of meteorites falling to Earth today, says Hart. That's a sign that the object originated in a different part of our solar system's asteroid belt than today's source of meteorites, he notes.
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Title Annotation:This Week
Author:Perkins, S.
Publication:Science News
Date:May 13, 2006
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