Chain of craters provides clues.

A team of scientists working on two continents has discovered that a series of five craters on Europe and North America form a chain, indicating the breakup and subsequent impacts of a comet or asteroid that collided with Earth approximately 214,000,000 years ago. The impacts may have contributed to a mass extinction that occurred at the end of the Triassic period---one of the live greatest such occurrences in history.

"When scientists observed the impacts of the pieces of Comet Shoemaker-Levy 9 on Jupiter in July, 1994, they said that the impact of a fragmented comet could never happen here on Earth because the Earth's gravitational field is too weak to break a comet into pieces," notes David Rowley, associate professor in geophysical sciences, University of Chicago (Ill.). "But our studies of these five craters provide compelling evidence that this happened at least once, and there's no reason it couldn't have happened more than that." He worked with John Spray, a structural geologist from Canada's University of New Brunswick, and Simon Kelley from The Open University, Great Britain, on the research.

Three of the five craters--Rochechouart in France and Manicouagan and Saint Martin in Canada--were at the same latitude, forming a nearly 5,000-kilometer (about 3,000-mile) chain. The other two--Obolon' in Ukraine and Red Wing in Minnesota--lay on identical declination paths with...