The Smoking Crater, Carl Zimmer

Discover, January 31, 1992

Over the past decade geologists have been marshaling scattered evidence that the dinosaurs were done in by an asteroid hitting Earth some 65 million years ago. But in 1991 hey really got down to specifics. The fatal asteroid, they said, hit not far off the coast of the Yucatán peninsula in Mexico–in June.

The impact theory got its start 11 years ago when a team led by Luis and Walter Alvarez examined the thin layer of clay that appears at the geologic dividing line marking the transition from the Cretaceous Period to the Tertiary Period–a point known as the K-T boundary. Inside the clay were high levels of the metal iridium, uncommon in Earth’s crust but plentiful in extraterrestrial objects.

The Alvarezes suggested that a giant asteroid hit the planet, sending up an iridium-rich spray of dust that obscured sunlight and created a nuclear-winter-like climate for months. When the dust settled, the dinosaurs were gone.

In the early 1980s several scientists came up with a countertheory based on volcanoes. Dinosaurs and other animals, they pointed out, had actually been in decline for millions of years before the iridium layer formed. Moreover, they said, asteroids were not the only source of iridium; the metal could also have entered the atmosphere through volcanoes, which have roots that reach below Earth’s crust to the iridium-rich mantle.

This past year geologists finally found evidence fingering a killer from outer space: glass. When an asteroid or meteorite hits the planet, it kicks up not only dust but also molten rock. After the rock cools, it condenses into balls of glass, known as tektites, that fall back to the ground. Volcanoes can also produce glass, but with a different structure: the slow cooker of volcanic magma produces telltale crystals; the sudden searing explosion of an impact doesn’t.

In February Haraldur Sigurdsson, a geochemist at the University of Rhode Island, and his colleagues reported the discovery of tektites in Haiti. Sigurdsson found the glass was crystal-free. “This stuff has all the earmarks of an impact,” Walter Alvarez says.

Sigurdsson found he could duplicate the mix of elements in the glass by melting andesite, a crustal rock, together with evaporite, a rock formed as salty, shallow water evaporates. That led him to suggest that Cretaceous ground zero could have been near a coastline.

And a coastline was where other researchers finally discovered signs of a crater: not far from Haiti, on the Caribbean coast of Mexico’s Yucatán peninsula. Ten years ago Glen Penfield and Antonio Camargo, geophysicists then working for Mexico’s national oil company, were surveying part of the Yucatán when they discovered a mysterious ring-shaped rock formation a mile underground. This ring of rock measured 115 miles wide and appeared less dense than the surrounding material. The geologists suggested it might be an impact site, but when their drilling samples were lost in a warehouse fire, the research slipped into obscurity.

Alan Hildebrand, a University of Arizona planetary geologist, heard about the Yucatán site last year and talked Penfield into hunting down some surviving fragments of the samples that were in private hands. In those samples they found pieces of quartz that had been blasted by intense pressure–the type created during a cosmic crash.

“We finally have an excellent candidate for the impact site,” says Walter Alvarez. “It ties everything together.”

One paleobotanist even claims that he can now pin down the month the asteroid fell (although the precise year still can’t be specified). Jack Wolfe, who works for the U.S. Geological Survey, found plant fossils, relatives of modern water lilies and lotus plants, in a K-T boundary layer formation in Wyoming. Seen under a microscope, the waxy covering of the leaves was creased with many irregular ridges.

“I had a hunch these ridges had come from freezing,” Wolfe says, and frigid months are supposed to have followed the impact. “But the only way to test that hunch was to try freezing modern plants.” When he did so, he found that the same patterns showed up. Wolfe thinks the ridges form when ice builds up under the waxy covering. He also noticed that the lotus plants hadn’t fully matured, having bloomed but not having produced fruit. In modern lotuses this happens in June. And that, claims Wolfe, is when the dinosaurs died.

Some observers think the impact advocates are getting carried away with this wealth of detail. Jean-Jacques Jaeger, a paleontologist at the University of Marseilles, points out that even if there was an impact, it may not have been crucial to extinctions. “The Americans spend a lot of money and time trying to pinpoint something that may have been a small thing, and that is a pity,” Jaeger says. “I think we are just at the beginning of this story, not the end.”