Even schoolchildren know the story: A giant impact 65 million years ago kicked up a cloud of dust that blotted out the sun, triggering an ice age and famine that killed off the dinosaurs. But geologist Kevin Pope of the Geo Eco Arc Research center in Aquasco, Maryland, has cast doubt on a central tenet of this theory. He argues that even a six-mile-wide asteroid could not stir up enough dust to create a global shroud.
Pope grew skeptical while studying the layer of debris produced by the Chicxulub crater, located at the tip of Mexico's Yucatán peninsula, believed to be the dino-killer. According to scientific gospel, the impact must have generated very fine dust particles, less than a micron wide, that dispersed widely and remained aloft for many months. But Pope says field samples and computer simulations show that the debris from the impact contains mostly much larger dust particles and glass spherules. Moreover, there simply was not enough dust in the debris to cause a global photosynthesis-stopping blackout. "Most of the pulverized rock ejected by the crater landed within 600 miles," Pope says.
Still, he agrees that an asteroid impact had some connection to the great Cretaceous-Tertiary mass extinction; he merely suggests that dust didn't do it. Instead, Pope thinks the impact may have vaporized billions of tons of sulfate rock that was plentiful in the Yucatán region, liberating vast quantities of sulfur—in other words, the asteroid happened to hit a particularly damaging location. "Sulfuric acid clouds may have been thick enough to shut down photosynthesis for nearly a year, long enough to cause major global cooling," he says. If Pope is right, medium-size asteroids could be far less threatening than many scientists have claimed.