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As the evidence mounts, more researchers are convinced that the Chicxulub crater marks the impact point of the killer comet. Says Boynton: "This is nearly as close to a certainty as one can get in science." Some scientists disagree. David Archibald, a biologist at San Diego State University, believes the extinctions took place more gradually and in a complex pattern. "There is zero evidence that dinosaurs became extinct virtually overnight."
This week, at an astronomy conference in Flagstaff, Ariz., scientists will add an intriguing twist to the Alvarez scenario. Their interpretation is based on new evidence that the Cretaceous-clay boundary actually consists of two parts: a thin layer overlying a more substantial one. To Eugene Shoemaker, of the U.S. Geological Survey and a co-author of the report, two layers indicate not one but two impacts.
As Shoemaker and his colleagues see it, a giant comet broke apart as it whipped around the sun. Over time, chunks of the comet separated but remained strung out in the same orbit. Then 65 million years ago, as the earth passed through the comet's orbit, it collided with the largest chunk, causing the Great Extinction. Perhaps only a year or two later, as the earth again entered the trail of cometary debris, it met a second, smaller chunk. Where did the second impact occur? This time no search is necessary. Shoemaker points to a well-known crater, 35 km (22 miles) across, that lies partly buried near Manson, Iowa. Its age, established by radioactive dating: 65 million years. / Shoemaker believes the new findings will help persuade more scientists to "get off the fence" and side with the Alvarez theory. "Chicxulub is the smoking cannon," he says, "and Manson is the smoking pistol."