At Last, the Smoking Gun?

If a comet did in the dinosaurs, where is the giant crater left by its impact? The answer may lie on the coast of the Yucatan Peninsula.

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Hurtling through the atmosphere at nearly 70 km per sec. (150,000 m.p.h.), the giant comet struck with catastrophic force, punching a hole some 40 km (25 miles) deep through the earth's crust and into the mantle. The violence of the collision 65 million years ago completely vaporized the 8-km-wide (5 miles) comet and blasted out a tremendous crater. Huge rocks, hurled high into the + air, rained down for hundreds of kilometers. A great fireball rose above the atmosphere, carrying with it vast amounts of pulverized debris.

These finer particles remained suspended, drifting into a globe-enveloping shroud that blocked sunlight for months before blanketing the earth in a layer of dust. In the cold and dark, photosynthesis ceased, plants and animals died, and entire species, including the dinosaurs, perished.

This startling scenario, proposed in 1980 by the late Nobel laureate Luis Alvarez and his son Walter, ignited a scientific debate that still rages today. Opponents of the theory, notably paleontologists, blame the Great Extinction on climatic changes possibly brought on by volcanic activity. If the Alvarezes were correct, they ask, where is the smoking gun? Where is the crater?

Some 130 terrestrial impact craters had been identified, but none of them near the age of 65 million years was large enough to qualify as the Crater. Yet if a comet or asteroid massive enough to cause the extinction had struck the earth, it would have left a crater hundreds of kilometers wide. Some traces would still exist, despite the intervening millenniums of erosion, sedimentation and tectonic-plate movement.

Now, after a decade-long search, the attention of geologists is riveted on a circular basin some 180 km (112 miles) in diameter. It lies buried under 1,100 m (3,600 ft.) of limestone, centered beneath the town of Chicxulub, on the northern tip of Mexico's Yucatan Peninsula, and extending out under the Gulf of Mexico. The nature of the basin, its location and a preliminary estimate of its age suggest that it is the Crater, the one gouged into the earth by the comet or asteroid that killed the dinosaurs.

In the search for the Crater, the first clues were sifted out of clumps of gray clay. At dozens of sites around the world, that clay has been found in a thin boundary layer between the rock of the Tertiary period and the formations of the late Cretaceous period, which ended 65 million years ago. In the Cretaceous rock lie the fossil remains of giant dinosaurs and a profusion of other species. But in the Tertiary formations, just above the clay, no trace exists of the dinosaurs or many of the other Cretaceous species.

The Alvarezes analyzed this clay in the late 1970s and showed it had a far higher content of the rare element iridium than ordinarily found in the earth's crust. It was this discovery that led Luis Alvarez to his momentous - insight. Comets and asteroids have high iridium content, he reasoned, and the clay layer could have been formed by the worldwide fallout of the material vaporized when an errant asteroid or, as most scientists now suspect, a giant comet smacked into the earth.

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