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Meteor showers such as November's Leonids usually provide a good celestial show as tiny bits of dust and rock debris burn up, creating flashes of light across the sky. A new analysis indicates that some 480 million years ago, the spectacle was even more dramatic. A report published today in the journal Science provides direct evidence that the influx of meteorites to the earth was 100 times what it is today.
Workers recovered 40 fossilized meteorites ranging from one to 20 centimeters in diameters from a limestone quarry in southern Sweden. Birger Schmitz, now at Rice University, and his colleagues at the Earth Sciences Center in Göteborg, Sweden, then searched ancient marine sediments preserved at the site for tiny grains from decomposed meteorites. "What we are doing is astronomy, but instead of looking up at the stars, we are looking down into the earth," Schmitz says. Specifically, the researchers looked for traces of the mineral chromite, which is a signature of an extraterrestrial body known as the L-chondrite parent body that broke up around 500 million years ago. The size and number of the grains recovered indicate that the asteroid explosion that spawned the meteorite shower was "one of the largest bodies that disrupted the asteroid belt during its late history."
Signs of a heightened bombardment by meteors were consistent over the entire 250,000-square-kilometer area that the scientists studied. The team will next head to China to search for asteroid remains dating from the same time period, and Schmitz notes that several sites in South America hold promise for yielding well-preserved chromite evidence as well.
