A Double Whammy?

By LEON JAROFF

Dinosaurs have been extinct for 65 million years, but the controversy over what killed them goes on, having apparently developed a life of its own. Last week the Great Dinosaur Debate was in the news again, as scientists sparred over two of the newer theories about the prehistoric doomsday.

While both proposals acknowledge that the demise of the fabled beasts -- and of many other species -- was triggered by the impact of a huge comet or asteroid, they present different views of the blast's calamitous aftermath. One holds that clouds of sulfuric acid, formed from the debris thrown skyward by the impact, played the major role in an earth-enveloping shroud that destroyed much of life below. The other suggests that the cosmic collision also caused intense volcanic activity on the opposite side of the earth, creating a double whammy that made the extinctions inevitable.

Most scientists now generally agree with the brilliant theory of Luis Alvarez, the late Nobel laureate who in 1980 blamed a giant celestial intruder for the dinosaurs' downfall. The clue that inspired Alvarez was found in a thin layer of clay that forms the so-called K-T boundary between the fossil- rich rock of the Cretaceous period, which ended 65 million years ago with the extinctions, and the overlying, younger and sparsely fossiled rock of the Tertiary period. When analysis of the clay revealed that it had a far higher content of the rare element iridium thatn ordinarily found in the earth's crust, Alvarez proposed that the element might be of extraterrestrial origin. Both comets and asteroids, he knew, are rich in iridium.

From that evidence, Alvarez constructed this scenario: some 65 million years ago, a comet or asteroid at least five miles wide struck the earth and blasted out a tremendous crater. The cosmic interloper was completely vaporized, and a great fireball rose into the stratosphere, carrying with it vast amounts of pulverized debris.

These finer partricles remained suspended and were circulated by air currents until they enshrouded the earth, blocking sunlight for months. In the ensuing cold and dark, plants and animals periished. When the dust shroud -- including the iridium-rich remnants of the comet or asteroid -- eventually settled back to earth, it formed the telltale worldiwde layer of clay found at the K-T boundary.

Many scientists, particularly paleontologists, scoffed at the Alvarez theory. They argued that gradual climatic change, perphaps brought on by heightened volcanic activity, had caused the worldwide extinctions. But the discovery in 1990 of a buried crater 112 miles in diameter, centered below the town of Chicxulub on the northern tip of Mexico's Yucatan peninsula, gave the doubters pause. And the subsequent confirmation fo the crater's age -- 65 million years -- has led most scientists to jump aboard the Alvarez bandwagon.

Still, problems remained. NASA scientists, for example, have suggested that most of the airbone dust from the impact explosion and soot from fires ignited in forests would have settled back to the ground within six months. In that short a time, they say, the earth could not have cooled enough to cause the extinctions.

But the NASA scientists have another explanation. In a newly published report they note that the Yucatan rock around Chicxulub contains abundant amounts of sulfur. The blast must have vaporized the sulfur, they say, and spewed more than 100 billion tons of it into the atmosphere, where it mixed with moisture to form tiny drops of sulfuric acid. These drops created a barrier that could have reflected enough sunlight back into space to drop temperatures to near freezing, and could have remained airborne for decades. "It could have been up to a century," says Kevin Baines, an atmostpheric scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "Most of us are betting on 20 to 40 years."

The NASA finding gives little comfort to those who still believe that volcanism alone did in the dinosaurs. But they are encouraged by a novel "double-whammy" theory suggesting that an impact on one side of the earth could produce massive volcanic activity at the antipode -- a point directly opposite on the far side -- and that the combined effect would cause disaster. Says Jon Hagstrum, the U.S. Geological Survey paleomagneticist who co-authored the theory: "This would be the best way to trigger worldwide mass extinctions because you have both hemispheres affected."

Hagstrum's proposal is based on the tracking of seismic waves generated by earthquakes and explsions, which indicates that such waves emanating from a huge impact would be focused by the earth, converging on the antipode and releasing their energy there. This concentration of energy might heat and distort the crust, eventually creating plumes through which magma could burst to the surface.

Is there evidence of ancient volcanic activity at the antipode of Chicxulub? Early speculation centered on the Deccan Traps, a basaltic plateau covering much of India that was formed over a few million years roughly around the time of the impact. But scientists have virtually eliminated that possibility. Taking continental drift into account, they estimate that what is now India was 1,000 miles or more away from where the Chicxulub antipode was 65 million years ago. And the location antipodal to the Deccan Traps at the time of their formation is now on the floor of the eastern Pacific Ocean. Because the ocean floor moves like a conveyor belt, rising to form ridges and diving back into the earth in a process called subduction, Hagstrum says, "half of the ocean floor has since disappeared" and evidence of an antipodal impact "would be on that half."

Hagstrum's proposal and some possible evidence of such double whammies on other planets and moons intrigued scientists at Sandia National Laboratories, in Albuquerque, New Mexico. Using a supercomputer, a Sandia team, headed by physicist Mark Boslough has been simulating impact effects, seeking to determine the size and velocity of an asteroid necessary to generate enough heat to cause volcanic outpourings at the antipode.

Boslough describes himself as "totally agnostic" on the existence of antipodal volcanism. J.P.L.'s Kevin Baines, however, isn't neutral when it comes to the NASA team's sulfuric acid theory. "If the asteroid had struck almost any other place on earth, it wouldn't have generated this tremendous amount of sulfur." he says. "Dinosaurs would still be roaming the earth."