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Evidence mounts of earlier asteroid annihilation Meteorite fragments, metal from Antarctica may boost theory By ALEXANDRA WITZE The Dallas Morning News December 14, 2003 SAN FRANCISCO - Scientists have come up with new evidence that a giant space rock slammed into Earth and killed off most of its species not once in history, but twice. Schoolchildren can relate the story of how the dinosaurs and many other creatures went extinct after an asteroid or comet hit the Earth 65 million years ago. But an even greater die-off occurred 250 million years ago in which as many as 90 percent of all species on Earth were snuffed out. In 2001, a group of researchers made headlines when they argued that this extinction, too, took place when a comet or asteroid smashed into the Earth. Now, the scientists have reported new proof of a cosmic collision - tiny fragments, unearthed in Antarctica, of what they say is the killer rock itself. If confirmed, the finding wouldn't be just the smoking gun linking the mass extinction to an extraterrestrial impact; it would be the bullet itself. The new work, reported last month in Science and elaborated upon last week at an American Geophysical Union meeting, hasn't met with universal acceptance. Other scientists say it's hard to imagine how pieces of a space rock could survive for 250 million years, rather than weathering away as meteorites often do. "It's mind-boggling to think that they could have survived 250 million years in those sediments," says Frank Kyte, a geochemist at the University of California, Los Angeles. In 1998, Dr. Kyte reported finding a piece of the meteorite that killed the dinosaurs in a core drilled from the bottom of the North Pacific Ocean. Over time, he has convinced most other specialists that the fragment is, indeed, a chunk of the doomsday rock. Something similar may have just been found in 250 million-year-old rocks from Graphite Peak, Antarctica. Gregory Retallack, a geologist at the University of Oregon, collected the rocks in the early 1990s, looking at the geologic record preserving the boundary between the Permian and Triassic geologic periods. Soon after, Dr. Retallack reported finding quartz grains in those rocks that had been marked by the shock of an extraterrestrial impact. He proposed that a collision had taken place 250 million years ago, but he never convinced many other researchers that he was right. Things changed in 2001, when a team led by Luann Becker, now at the University of California, Santa Barbara, reported finding molecules called buckyballs in 250-million-year-old rocks from Meishan, China. The soccer-ball-shaped buckyballs contained extraterrestrial gases, Dr. Becker's team argued, which suggested that an impact had occurred. Since then, Dr. Becker's team has gathered more evidence to support the impact theory. The recent Science paper describes the discovery of about 40 meteorite fragments from the same Antarctic rocks studied by Dr. Retallack. Chemical studies of the fragments show them to belong to the family of meteorites called chondrites, says Asish Basu, a geochemist at the University of Rochester and lead author on the paper. The scientists also say that blobs of metal found in the rock may have been formed by an extraterrestrial impact. Other researchers have argued that similar blobs found in the Chinese rocks may have condensed out of the vapor of an impact cloud. Finally, at the San Francisco meeting last week, his team introduced a third line of evidence - small glassy drops that also could have been produced during the searing heat of impact. But David Kring, an impact expert at the University of Arizona, says that all three lines of evidence can be explained without invoking an extraterrestrial impact. For instance, the meteorite fragments could have been a sprinkle of small meteorites like those that regularly bombard the planet, a possibility that Dr. Basu says he has ruled out. "Why would they be concentrated right where Retallack found shocked quartz?" he adds. Perhaps the biggest question is how the meteorite fragments managed to preserve so well over 250 million years.But the Antarctic rocks contain layers of clay that could have encapsulated and preserved the meteorite bits, argues Dr. Basu. "There are special geologic circumstances in which they could survive," he says. So far, the team has found no hint of iridium, a relatively rare element that appears in extraterrestrial rocks. Finding a worldwide layer of iridium helped clinch the impact hypothesis for the dinosaur extinction. But the space rock may have been low in iridium, or geologic forces may have churned up the Antarctic rocks enough to dilute the iridium signal, Dr. Basu says. The questions may be resolved relatively soon. Dr. Retallack is in Antarctica now, collecting more samples from Graphite Peak. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
