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Global Impact: Space object may have spread debris worldwide

9:45am, August 21, 2002

Ancient rocky sediments in what are now western Australia and eastern South Africa contain remnants of what may have been an extraterrestrial-object impact large enough to disperse debris over the entire planet.

The sediments in question formed from particles deposited about 3.47 billion years ago in the normally quiet and probably shallow waters off the coasts of ancient continents, says Gary R. Byerly, a geologist at Louisiana State University in Baton Rouge. Some layers include BB-size and smaller spheres that typically form from airborne droplets of molten rock. Although the spherules could have come from volcanoes, the droplets' widespread distribution and mineral composition suggest they condensed from a cloud of rock vapor blasted into and above the atmosphere after a massive impact.

Chemical analyses of the South African sediments show that, on average, the layers that contain the spherules have more than 10 times the concentration of iridium that's normally fou

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