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       Extinction of the dinosaurs may have occurred 65 million years ago as result of single asteroid or comet impact, but it is also possible they died out as result of many comet impacts over one to three million years, group of scientists has theorized.

       Comet showers occurring over period of time may have caused the extinctions, said Dr. Paul Weissman of NASA's Jet Propulsion Laboratory in Pasadena, Calif., one of eight authors of paper recently published in Nature, the British scientific journal.

       Such showers are caused by close passage of neighboring stars through the Oort cloud of comets surrounding the solar system, the scientists said.

       In 1980, Drs. Luis and Walter Alvarez and others postulated the extinction of the dinosaurs, which had ruled the Earth for about 140 million years, by an asteroid impact and its secondary effects of darkening and cooling of the atmosphere.

       Their theory was substantiated by discovery at certain levels of the Earth's surface of iridium, an element rare on the Earth's surface, but known to be relatively abundant in asteroids and meteors.

       The iridium was found in the Earth's clay layers at levels corresponding to period 65 million years ago, called the Cretaceous-Tertiary boundary, the division between two separate geologic periods.

       "The paper points out that impacts of Earth- crossing asteroids take place randomly in time, but significant number of comets could arrive in discrete showers triggered by relatively close passage of star or interstellar gas cloud," Weissman said.

       "The showers would last about three million years, with the bulk of the comets arriving within one million years," he said.

       A study of the extinction events over the past 250 million years, reported in 1983, said extinctions were periodic every 26 million years, Weissman said, adding that this prompted several explanatory theories including that of companion star, or "death star," with 26-million year orbit.

       An alternative suggested extinctions could result from comet showers occurring as the solar system oscillated, moved up and down, through the galactic plane every 32 million years.

       But such an oscillation into the plane occurred one million years ago without significant extinction.

       "The periodic theories don't work out," he said. "There is no companion star, it's not the right solution."

       But the scientists agreed that random comet showers would be solution and they decided to study that one problem by approaching the question of what comet shower looked like, not what caused it.

       Their studies indicated that such an event would cause stepwise extinctions over period of time. They looked for multiple iridium layers in the Earth's clay and multiple craters.

       They found that there was evidence of three large craters occurring 38 million years ago, in the late Eocene period when mass extinctions of some life forms occurred.

       There also was evidence of an iridium abundance along with discovery of micro-tektites, or impact-caused glass particles.

       A major comet shower involving billion comets with diameters of 3 kilometers (1.8 miles), would result in about 20 comets striking the Earth over period ranging from one to three million years, the scientists said.

       In each of three known mass extinctions, dating from 95 million years ago to the most recent, 38 million years ago, it is believed extinctions occurred in stepwise manner over finite period of time, they said.

       The scientists said in their paper that it is too early to predict if the hypotheses of comet showers offers complete explanation for any or all mass extinctions, and suggested continued study into all aspects of the question.

       Other authors of the paper were Piet Hut, Institute for Advanced Study, Princeton; Walter Alvarez, University of California, Berkeley; William Elder and Erle Kauffman, University of Colorado, Boulder; Thor Hansen, Western Washington University, Bellingham; Gerta Keller, Princeton University; and Eugene Shoemaker, U.S. Geological Survey, Flagstaff, Ariz.