Heavy metals—some of those elements in the nether regions of the periodic table, like iridium—are rare on Earth’s surface, which is why most people have never heard of them. But tiny amounts of them fall at a more or less constant rate from the deep reaches of outer space as cosmic dust. The Alvarezes reasoned that if the clay layer had only a tiny peppering of iridium, then it had formed very quickly; if it had a larger amount, then it must have formed over a much longer time period. New instruments now allowed scientists to measure even very small concentrations of iridium, including one in
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