TIME: Almanac 1993/TIME Almanac 1993.iso/time/070191/0701300.000

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                                                                                SCIENCE, Page 60

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.

By LEON JAROFF


    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."