At Last, the Smoking Gun?

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.

As the quest for the telltale crater intensified in the middle 1980s, William Boynton, a professor of planetary science, and graduate student Alan Hildebrand, both of the University of Arizona, wondered if the boundary clay might also help reveal the site of the impact. Measuring the content of rare earth elements in samples of the clay, they determined that it contained both the basaltic rock found in the ocean floor and a lesser amount of continental rock. Their conclusion: the comet had hit on the edge of an ocean basin.

So great an impact in water must have produced monstrous seismic waves, perhaps as great as 5 km (3 miles) high, that raced across the waters, tearing up the bottom sediments and sweeping rocky debris inland. Searching through scientific literature, they uncovered reports of chaotic mixes of large rocks at the 65-million-year boundary level in Texas, Mexico, Cuba and northern South America, but none anywhere else. This suggests, says Hildebrand, "that the comet hit somewhere between North and South America."

Scientists also reasoned that the thickest layers of ejecta -- rocks that fell back to earth after the impact -- would be found closer to the Crater. Investigating one suspected ejecta layer in Haiti early in 1990, Hildebrand and another Arizona colleague, David Kring, found tektites, teardrop-shape pieces of glass formed when molten rock is splashed high into the atmosphere and solidifies on its way back down. To the Arizona scientists, the tektites suggested that the impact had occurred no more than 1,000 km (622 miles) away.

A few months later, Hildebrand learned of a report made a dozen years earlier by Glen Penfield, a geophysicist who had surveyed the Yucatan Peninsula for Pemex, the Mexican national oil company. Studying both magnetic and gravity measurements, Penfield and his Pemex supervisor, Antonio Camargo, had discerned a huge circular basin buried under the peninsula and suspected it might be an impact crater. Their report was largely ignored.

Seeking out Penfield, Hildebrand teamed up with him in a search for samples of material brought up in old oil-drilling operations in the vicinity of the basin. Analyzing a few core samples, Kring discovered compelling evidence that the basin is an impact crater. Most convincing are crystals of quartz with striations that could only have been caused by powerful shock waves stemming from a great impact, as opposed to, say, from volcanic action. Finally, the dating of nearby fossil evidence has narrowed the crater's age to within 5 million years of the Great Extinction.

Unexpected confirmation of the crater site has come from a team of scientists led by Charles Duller at NASA's Ames Research Center. While examining satellite photographs of the Yucatan in the mid-1980s, the NASA scientists were intrigued by a strange semicircle of sinkhole lakes on the northern tip of the peninsula. The Chicxulub discovery could provide an explanation. Reporting in Nature magazine, the NASA team proposes that the lake pattern developed as the buried crater rim gradually collapsed, producing depressions in the overlying limestone that were filled in by groundwater.

As the evidence mounts, more researchers are convinced that the Chicxulub crater marks the impact point of the killer comet. Says Boynton: "This is nearly as close to a certainty as one can get in science." Some scientists disagree. David Archibald, a biologist at San Diego State University, believes the extinctions took place more gradually and in a complex pattern. "There is zero evidence that dinosaurs became extinct virtually overnight."

This week, at an astronomy conference in Flagstaff, Ariz., scientists will add an intriguing twist to the Alvarez scenario. Their interpretation is based on new evidence that the Cretaceous-clay boundary actually consists of two parts: a thin layer overlying a more substantial one. To Eugene Shoemaker, of the U.S. Geological Survey and a co-author of the report, two layers indicate not one but two impacts.

As Shoemaker and his colleagues see it, a giant comet broke apart as it whipped around the sun. Over time, chunks of the comet separated but remained strung out in the same orbit. Then 65 million years ago, as the earth passed through the comet's orbit, it collided with the largest chunk, causing the Great Extinction. Perhaps only a year or two later, as the earth again entered the trail of cometary debris, it met a second, smaller chunk. Where did the second impact occur? This time no search is necessary. Shoemaker points to a well-known crater, 35 km (22 miles) across, that lies partly buried near Manson, Iowa. Its age, established by radioactive dating: 65 million years. / Shoemaker believes the new findings will help persuade more scientists to "get off the fence" and side with the Alvarez theory. "Chicxulub is the smoking cannon," he says, "and Manson is the smoking pistol."