Crater chain points to impact of fragmented comet

Geophysicist David Rowley, working with colleagues in Canada and the United Kingdom, has discovered that five craters in 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 discovery was reported in the 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 gravitational field is too weak to break a comet into pieces," said Rowley, 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 in Canada, and Simon Kelley, from The Open University in Milton Keynes, United Kingdom -- were interested in the relationship between impact craters of similar ages. Kelley had developed a technique to precisely date these craters, using laser argon/argon dating of the glass formed by localized heating of the rock. They asked for Rowley's help in determining how the craters were aligned when the impacts occurred -- because of plate tectonics, the continents have moved extensively in the last 214 million years.

As a principal investigator for the University's Paleogeographic Atlas Project, Rowley had that kind of information at his fingertips. The project is compiling an atlas of the paleogeography and paleoclimate of the world as it changed over the past 500 million years.

"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 5,000-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 were previously known and well-studied, but the paleoalignment had 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 one solitary chunk and in two groups of two. It is possible that the comet or asteroid actually broke into more than five pieces, but 214 million years ago most of Earth at that latitude was ocean, and evidence of any ocean-bottom craters has long been obliterated. The impacts also 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 so aligned is near zero.

Manicouagan, the largest of the five craters, is more than 100 kilometers in diameter, comparable to the 170 kilometer Chicxulub crater in Yucatan -- the impact that is believed to have caused the mass extinction that killed the dinosaurs at the end of the Cretaceous period 65 million years ago.

The Triassic extinction was equivalent in magnitude to the Cretaceous/Tertiary (K/T) extinction: about 80 percent of the species then living on the planet became extinct.

There are 150 known impact craters worldwide; the scientists are now studying others to see if there are other chains of co-latitudinal craters. Based on current estimates of the age of the 150 craters, no other comparable chains have been found.

The Paleogeographic Atlas Project 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 providing a comprehensive look at the evolution of Earth's geographic features. More information can be found at plates.uchicago.edu.