A Double Whammy?

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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."