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Sliver of Dinosaur-Killing Asteroid Is Believed Found

TIMES SCIENCE WRITER

A UCLA geochemist analyzing a rock sample drilled from the deep ocean muck has discovered what appears to be the first known piece of the massive meteorite that is widely believed to have wiped out the dinosaurs about 65 million years ago.

The tiny fossil meteorite--less than a 10th of an inch across--is the only surviving piece found so far of a six-mile wide cosmic fist that smashed into Mexico’s Yucatan Peninsula at the time the dinosaurs died off. It left a crater about 186 miles wide, suggestive traces of titanic tidal waves, and chemical evidence of a global blanket of smothering sulfuric acid clouds.

Unearthed from under 50 yards of Pacific Ocean sediment thousands of miles from the crater, the small fragment of oxidized iron and nickel was found in the crucial geologic layer that dates it to the impact. It is the strongest evidence yet that the catastrophe was triggered by a collision with an asteroid--perhaps the largest impact since life evolved--and not a comet, as many scientists have speculated.

“This was one of the worst days the Earth had in the last billion years,” said Frank T. Kyte at UCLA’s Institute of Geophysics and Planetary Physics, who found the meteorite while analyzing deep ocean cores taken northwest of Hawaii. “And it is important to understand what happened.”

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His research is published today in Nature.

In recent years, there has been growing scientific agreement that such a meteorite slammed into the ancient Yucatan, but many researchers still are at odds over whether the meteorite was an immense stone and iron asteroid or a smaller, faster-moving comet composed of loosely compacted ice and gravel.

A worldwide layer of clay dating from the end of the Cretaceous period, when the dinosaurs and many other species vanish from the fossil record, contains the unique chemical signature of an interplanetary impact but does not identify what kind of meteorite caused it.

“People have wondered ever since the [impact] theory first appeared in 1980 whether this meteorite was a comet or an asteroid,” said geologist James Powell, head of the Los Angeles County Museum of Natural History, who has written a book on the impact called “Night Comes to the Cretaceous.” “It has been difficult to know the answer because in either case the impacting object most likely would have blasted itself to pieces or vaporized.

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“Frank Kyte has come up with some very important physical evidence, which is quite valuable,” Powell said. “If it is confirmed, that would indicate it was an asteroid.”

For those astronomers trying to assess the risk of such cosmic impacts that Earth may face in the future, the discovery of an asteroid fragment was in a way reassuring.

Unlike the comets that lurk in the unfathomable darkness at the solar system’s edge, large asteroids usually can be detected far enough in advance for orbits to be calculated and any necessary protective measures planned.

Despite scholarly enthusiasm about the find, several experts wondered whether the fragment belonged to the object that caused such global devastation or was simply part of an unrelated meteorite that fell to Earth during the same period.

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“Certainly, the evidence he has found is very enticing,” said NASA planetary geologist Adriana Ocampo in Washington, D.C., who is an expert on the extinction event. But “the probabilities that any fragments of the impacting object would have survived are very low.”

Ocampo and her colleagues estimate that the ancient asteroid impact was between 10,000 and 50,000 times more powerful than the spectacular collision of the comet Shoemaker-Levy 9 and the planet Jupiter in 1994.

The ancient asteroid blew a hole in the Yucatan coast nine miles deep and kicked up 100 billion tons of sulfur and other debris--enough to cause monsoons of acid rain, blot out the sun for a decade and trigger a global freeze that killed off two-thirds of the species then living on Earth.

After 65 million years in the mud, however, the chip retained almost none of its original minerals. But its texture and shape allowed Kyte to conclude that the meteorite came from a rocky, carbonaceous chondrite--the kind of object typically found in the asteroid belt between Mars and Jupiter--rather than from the porous, fluffy type of interplanetary dust associated with comets.

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The chip contains high concentrations of a rare isotope of iridium, as is the impact layer itself, in addition to iron, nickel and chromium at levels peculiar to extraterrestrial objects. That--along with its resting place in the Cretaceous layer--was the critical clue to its origin, Kyte said.

“It is indisputably a meteorite,” he said. “The probability that this thing is from the asteroid is very, very high.”

Kyte’s findings bolster a study earlier this year by Alexander Shukolyukov of the Scripps Institution of Oceanography at UC San Diego, who found chemical traces of extraterrestrial chromium in rocks from the impact layer, which also is suggestive of an asteroid impact.

Galvanized by a false alarm this year of an impending collision with an asteroid, astronomers worldwide are more vigilant than ever for perils appearing in the night sky. Recently, NASA doubled the funding for the effort to detect threatening asteroids and comets that intersect Earth’s orbit.

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“The asteroids are the ones that are easy to identify and to chart their motions forward in time so we can tell if they pose any danger,” said comet expert Donald Yeomans, who is in charge of a new NASA office at the Jet Propulsion Laboratory to coordinate the search.

“The comets take us by surprise,” Yeomans said. “They are the wild cards.”


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