SCIENCE PALEONTOLOGY : Scientists Broaden Theory of How Impact Killed Off Dinosaurs
The comet or asteroid that killed off the dinosaurs may actually have been several extraterrestrial objects, or the remnants of a single one that shattered and sent pieces ricocheting through the atmosphere and back down to Earth, scientists supporting the impact theory of extinction said Tuesday.
Either way, recent finds of crater remnants leave little doubt that there is more than one impact 65 million years ago to look for, they said.
“A few years ago, our problem was that we didn’t have any craters to point our fingers at, whereas now we’ve got several,” said Walter Alvarez of UC Berkeley. “So now, maybe instead of a smoking gun we’ve got a smoking firing squad.”
Although Alvarez and others at the meeting contend there is little doubt that the dinosaurs were killed off by a large impact on the Earth, some scientists in other fields continue to favor volcanism or gradual climatic change as possibilities for the mass extinctions. Either volcanoes or a significant cooling of the atmosphere, they argue, could have blocked sunlight with ash and dust, causing plants and the animals that depend on them to die.
But it was the extinction theory that took center stage at the American Geophysical Union conference here. Alvarez was the keynote speaker during a day of scientific sessions on the idea that he and his late father, Luis, helped originate.
A succession of researchers at the meeting--none opposing the impact theory--detailed their findings:
* A rain of sand-size particles so hot that unprotected animals would have been broiled in their tracks pelted Earth. A computer study at the University of Arizona concluded that the cloud easily could have spread worldwide.
* Thick clouds of nitrogen oxides formed, resulting in an acid rain that killed land plants, plankton and the animals dependent on them. Recent studies of sediments from the time show abnormally high amounts not only of nitrogen oxides, but also of strontium and heavy metals, which are leached from rocks by acid rain, said Ronald G. Prinn, of Massachusetts Institute of Technology.
* Even small impacts could have had large planetary effects if an object hit the Earth at an oblique angle, sending pieces skipping across the surface and ricocheting into orbit and back again. Experiments shooting rocks from a special air gun at 2.7 miles per second showed there could be as many as 1,000 impacts from a single object in this way, said Peter H. Schultz of Brown University.
Only multiple impacts or perhaps a double asteroid would explain the sometimes contradictory evidence scientists are reading in rocks formed since then, Alvarez said.
He noted that Jet Propulsion Laboratory images taken last year of an asteroid that closely approached the Earth show it to be a double asteroid--two half-mile-wide objects that orbit each other, almost touching.
“It appears that multiple objects are not all that uncommon,” Alvarez said. “I think it’s really interesting that the first time anybody has taken a close-up photograph of an asteroid it turns out to be a double one.”
The need to investigate multiple-impact mechanisms comes about because recent studies have suggested strongly the existence of three separate candidates for the “smoking gun” crater: A crater in Iowa, a buried crater on a beach in the Yucatan Peninsula of Mexico and a suspected undersea crater off Colombia, he said.
Furthermore, the University of Arizona research team that identified the Yucatan and Colombia sites also found evidence in Haiti of an ocean impact. The sediments contained glass-like beads whose composition indicated they came from ocean crust--indicating an ocean impact by an extraterrestrial object.
However, no crater has been found, and locating one would be particularly difficult under water, said William Boynton, a member of the Arizona group.
An ocean impact might leave no evidence at all, said Brown University’s Schultz. His group’s airgun experiments found that, when the object was shot into water, it left very little cratering, he said.
The San Francisco presentations elaborated greatly on the simplistic impact picture first portrayed in the late 1970s--of an Earth made suddenly too dark for plants and the animals that ate them.
The picture can be expected to become even more complicated, Alvarez said, as the various environmental impacts are deduced. One particular puzzle remains.
“It used to be that people would say, ‘Well, an impact wouldn’t do much damage.’ And now we’re at a loss to see how some of these animals could have survived,” he said.
