Using fossil meteorites and ancient limestone unearthed throughout southern Sweden, marine geologists at Rice University have discovered that a colossal collision in the asteroid belt some 500 million years ago led to intense meteorite strikes over the Earth's surface.

The research, which appears in this week's issue of Science magazine, is based upon an analysis of fossil meteorites and limestone samples from five Swedish quarries located as much as 310 miles (500 km.) apart. The limestone formed from sea bottom sediments during a 2 million-year span about 480 million years ago, sealing the intact meteorites, as well as trace minerals from disintegrated meteorites, in a lithographic time capsule.

"What we are doing is astronomy, but instead of looking up at the stars, we are looking down into the Earth," said lead researcher Birger Schmitz, who conducted his analysis during his tenure as the Wiess Visiting Professor of Earth Science at Rice. Schmitz is professor of marine geology at Göteborg University in Sweden.

Meteorite activity on earth is relatively uniform today, with an average of about one meteorite per year falling every 7,700 square miles (12,500 sq. km). The new study found a 100-fold increase in meteorite activity during the period when the limestone was forming, a level of activity that was present over the entire 150,000-square-mile (250,000 sq. km.) search area.

Some 20 percent of the meteorites landing on Earth today are remnants of a very large asteroid that planetary scientists refer to as the "L-chondrite parent body." This asteroid broke apart around 500 million years ago in what scientists believe is the largest collision that occurred in late solar system history.

Schmitz and his colleagues looked for unique extraterrestrial forms of the mineral chromite that are found only in meteorites from the L-chondrite breakup. They found that all the intact fossil meteorites in the Swedish limestone came from the breakup. Moreover, they found matching concentrations of silt and sand-sized grains of extraterrestrial chromite in limestone from all five quarries, indicating that meteorite activity following the breakup was occurring at the same rate over the entire area.

The research helps explain why Schmitz and his colleagues at Göteborg have been able to collect so many fossilized meteorites from a single quarry near Kinnekulle, Sweden over the past decade. Fossil meteorites embedded in stratified rock are extremely rare. Only 55 have ever been recovered, and Schmitz's group found 50 of those.

"It is true that we are lucky to be looking in just the right place - a layer of lithified sediments that was forming on the sea floor immediately after this massive collision," said Schmitz. "But on the other hand, we would never have started looking there in the first place if the quarry workers hadn't been finding the meteorites on a regular, yet still rare, basis."

Until Schmitz's group started working with the quarry crew, the fossilized meteorites were discarded because they blemish the finished limestone. Schmitz believes it's possible that similar concentrations of fossilized meteorites and extraterrestrial chromite grains are present worldwide in limestone that formed during the period following the asteroid breakup. He recently got funding to look for evidence of this in China, and he said there are South American sites that are also favorable.

The research was sponsored by the National Geographic Society and the Swedish Research Council.