A EUROPEAN study has rekindled debate about one of Earth's oldest mysteries: What was it that smashed into the planet 35 million years ago and created the largest impact crater in the United States - at the mouth of the Chesapeake Bay?

Scientists agree that some kind of meteor hit, leaving a submerged formation in the bay, roughly the size of Rhode Island. Another meteor hit in Popigai, Russia, about the same time, digging a crater about the same size.


But there is little agreement about whether the meteors were asteroids or comets - or about the nature of the interstellar forces that sent the huge rocks hurling toward the planet.

"We may never know for sure," said Philippe Claeys, a geologist at the Free University in Brussels and a co-author of a study published Friday in Science.


Claeys and other European scientists say that tests on rocks collected at the Russian crater show both craters were carved out by asteroid strikes. The scientists say they were likely part of an asteroid shower, created by "a major collision" in the asteroid belt between Mars and Jupiter.

Their conclusions contradict a generally accepted theory that the Chesapeake crater and its Russian counterpart were formed by comets.

In 1998, scientists concluded that those comets fell from the sky during a 2 million-year stretch of comet showers that sprinkled interplanetary dust around the world.

The comet showers were probably triggered by a disturbance in the Oort cloud, a region where many comets orbit at the edge of the solar system. The researchers based their findings on analysis of sea floor sediments that are enriched with a helium isotope that's rare on Earth but common in space.

Authors of both studies concede that they can't be sure of their theories. Although both craters were probably formed by the same type of meteor, experts say that uncertainty comes with the territory when you're dealing with 35 million-year-old evidence.

"All of these things are speculative because we really don't have that much information," said S. Wyle Poag, a geologist for the U.S. Geological Survey, co-discoverer of the bay crater.

Poag and other scientists are evaluating rocks and sediment flushed out by drilling last spring over the center of the crater, at Cape Charles, Va. The project, part of a $250,000 federal study, is designed to resolve salinity problems in area ground water that were traced to the crater.

The test results, which will be available in several months, might also help settle the argument between comet and asteroid believers. But no one is betting on it.


"It could remain an open question," said Wright Horton, the U.S.G.S. geologist testing the bay crater debris.

Asteroids are small celestial bodies that orbit the sun, mostly in a doughnut-shaped belt between Mars and Jupiter. Comets generally travel much farther away and are less likely to strike Earth.

Claeys said the studies that pinpoint asteroid and comet strikes should help experts evaluate the risk of future strikes. "If we know the nature of the objects that hit in the past, we'll be better equipped to look out for other objects that could impact in the future," he said. "The universe is a violent place."

NASA pays astronomy labs $3 million a year to keep an eye out for asteroids and comets that might be approaching Earth. There have been about 165 known impacts found worldwide, most of them believed to be from asteroids.

Experts say crater studies also broaden our understanding of Earth history. A meteor strike near Australia 250 million years ago might have triggered the largest extinction in history, setting the stage for the appearance of the dinosaurs.

Another strike, 65 million years ago in Mexico's Yucatan Peninsula, is widely believed to have wiped out the dinosaurs, clearing the way for the evolution of mammals, including human beings.


"These strikes had major effects on the evolution of our planet. Understanding them is a key to understanding how the planet changed and how we evolved," Claeys said.

Claeys began his study in 1997 when he and colleagues embarked on a three-week expedition to the Russian crater, a remote site in Siberia 400 miles north of the Arctic Circle that is habitable for only a month each summer.

Collecting 32 samples of fist-sized rocks, the scientists used spectrometers and X-rays to map their chemical composition. The rocks were 15 times higher in levels of platinum-based elements than normal, a telltale sign of an asteroid.

Right or wrong, the findings came at a price: The team spent three weeks in tents in Siberia, where it rained every day and the July nighttime temperatures dipped to 20 degrees Fahrenheit.

"It was rough, and believe me, I'm no Indiana Jones," Claeys said. "It made me glad I work in an office."