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Killer comet

发布时间:2019-03-07 06:05:02来源:未知点击:

By Jeff Hecht The traces of a devastating impact at the time of the world’s worst mass extinction, 251 million years ago, may have been detected. After a decade-long search, US geologists uncovered the suspicious “fingerprints” in the form of cage-like carbon molecules called buckminsterfullerenes. Analysis shows they contain the same blends of helium and argon isotopes found in meteorites. “For the first time, one wants to take an impact very seriously,” comments Harvard University palaeontologist Andrew Knoll. However, the lack of the telltale signs of other impacts, such as enriched iridium levels and shocked quartz crystals, leaves some doubts. The extinction at the end of the Permian period wiped out most plants, more than 90 percent of marine species, and 70 percent of land animals. Its cause has long been a mystery. Recent studies showed it took less than 100,000 years both on land and at sea, suggesting a catastrophic impact similar to the one that wiped out the dinosaurs at the end of the Cretaceous, 65 million years ago. However, a lack of impact evidence led many researchers to blame the Permian mass extinction on the massive volcanic eruptions occurring in Siberia at the time. Buckyballs had previously been found at the end of the Cretaceous, so Luann Becker of the University of Washington in Seattle and colleagues painstakingly analysed rocks from the end of the Permian. “We’re finding only parts per billion of fullerenes,” she told New Scientist. But the concentrations in samples from China and Japan were 50 times higher in the layer that formed at the very end of the period than in nearby layers. Rocks from also Hungary showed some enhancement though at lower concentrations. The abundance of helium and argon isotopes trapped inside the buckyballs matched the levels found in carbonaceous chondrite meteorites. “We think the fullerenes formed around a star outside our solar system,” says Becker, then were carried into the solar system and survived the impact to be scattered around the globe. She estimates the object was six to 12 kilometres across. The element iridium is relatively rich in meteorites and is found in rocks from the end of the Cretaceous, but not the end of the Permian. This could indicate the Permian object was a comet, composed mostly of ice. “What is perplexing,” says University of Washington palaeontologist Peter Ward, is that none of the rocks at the Permian boundary resemble the layers containing Cretaceous impact debris. “Nonetheless, this new methodology is a very powerful one,” he told New Scientist. More at: Science (vol 291,