March 13, 1998
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 impact of a comet or asteroid that collided with Earth approximately 214 million years ago.
The impacts may have contributed to a mass extinction that occurred at the end of the Triassic period -- one of the five greatest mass extinctions in history.
The work, by scientists at the University of Chicago, the University of New Brunswick (Canada) and The Open University (Milton Keynes, U.K.) is published in a paper in the Thursday, March 12, issue of the journal Nature.
"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," said David Rowley, University of Chicago Associate Professor in Geophysical Sciences. "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."
Rowley's colleagues, John Spray, a structural geologist from the University of New Brunswick, and Simon Kelley from The Open University, were interested in the relationship between impact craters of similar ages. Kelley had developed a technique to date such craters more precisely -- using laser argon/argon dating of the glass formed by localized heating of the rock. They asked Rowley to help figure out how the craters were aligned when the impacts occurred -- because of plate tectonics, the continents have moved extensively in the last 214 million years.
Rowley, a principal investigator for the University of Chicago's Paleogeographic Atlas Project, which is compiling an atlas of the paleogeography and paleoclimate of the world as it changed over the past 500 million years, had that kind of information at his fingertips.
"I get these kinds of requests all the time," said Rowley, "so at first I didn't think about it too much. But when they asked to me take a closer look at the data and I saw the alignment, I just said, 'wow!'"
Three of the five craters, Rochechouart in France, and Manicouagan and Saint Martin in Canada, were at the same latitude -- 22.8 degrees -- forming a nearly 5000-kilometer chain. The other two, Obolon' in Ukraine and Red Wing in Minnesota, lay on identical declination paths with Rochechouart and Saint Martin, respectively. All of the craters are previously known and well-studied, but the paleoalignment has never before been shown.
One possible explanation for the alignments of the five craters is a fragmented comet that crashed to Earth in three major groups over a period of time as short as four hours, in two groups of two and one solitary chunk. It is possible that the comet or asteroid actually broke into more than five pieces, but most of the Earth at that latitude was ocean 214 million years ago, and evidence of any ocean-bottom craters has long been obliterated. The impacts may have occurred over a period of several days, depending on how widely the fragments were dispersed.
Rowley said that the chance that these craters are randomly aligned is near zero.
Manicouagan, the largest of the five craters, is more than 100 kilometers in diameter, comparable to the 170-kilometer Chixulub crater in the Yucatan -- the impact that is believed to have caused the mass extinction at the end of the Cretaceous period 65 million years ago, killing the dinosaurs.
The Triassic extinction was equivalent in magnitude to the Cretacious/ Tertiary (K/T) extinction: about 80% of the species then living on the planet became extinct.
There are 150 known impact craters worldwide; the group is now studying others to see if there are other coincident crater chains.
The Paleogeographic Atlas Project at the University of Chicago is compiling an atlas of the world's changing geography and climate. The data are used for testing climate change models, finding probable sites for oil and minerals, and for providing a comprehensive look at the evolution of Earth's geographic features. The work is funded by a constortium of companies that has included Amoco, Exxon, Mobile, Total, Elf-Aquitaine and Shell, British Petroleum, Conoco and Marathon. More information can be found at http://plates.uchicago.edu.