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Mentalcase

Giant Meteorite May Have Delayed Life

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WASHINGTON (Aug. 22) - A gigantic meteorite that slammed into the Earth 3.5 billion years ago may have caused such devastation that it affected the evolution of life, researchers reported Thursday.

Twice as big as the asteroid believed to have wiped out the dinosaurs 65 million years ago, the meteorite would have kicked up a thick layer of rock and dust that coated the whole planet and caused tidal waves that wiped clean the early continents, the researchers reported Friday in the journal Science.

The team, at Stanford University in California and Louisiana State University, pieced together evidence from ancient rock layers found in Australia and South Africa.

''We have no idea where the actual impact might have been,''  said Donald Lowe, a Stanford geology professor who helped write the study.

Louisiana State University geologist Gary Byerly said the meteorite they studied was one of several that would have struck around the same time, when the Earth was new and relatively hot, and populated only by bacteria.

''They are all probably objects in the size range of 20 to 50 kilometers in diameter. That's two to five times larger than the object that caused the dinosaur extinction 65 million years ago,'' Byerly said in a telephone interview.

''These impacts were very large. They really changed the course of the evolution of Earth,'' he said. The study did not suggest exactly what the changes might have been.

EMERGING BACTERIA

Lowe said it is not clear what effect the impacts would have had on the emerging bacteria. Unlike large animals such as dinosaurs, bacteria can exist in extreme conditions. Today they are found deep in the sea where no light reaches, buried miles under the Earth, in Antarctic ice and in hot sulfur springs.

''There isn't a big extinction event you can identify as cut-and-dried as the extinction of the dinosaurs,'' Lowe said.

The layers of sediment the geologists studied were in South Africa's Barberton greenstone belt and western Australia's Pilbara block. Both sites include rocks formed more than 3 billion years ago when the planet was just a billion years old.

These deposits contain zircons and rare metals, such as iridium, which are common in meteorites.

''Zircons are a geologist's best friend. They are very resistant to change and they contain information about the age of things all the way back to the beginning of the solar system,'' Byerly said.

Special U.S. Geological Survey equipment at Stanford dated the zircons to 3.47 billion years ago.

''We can study these very old events by going to these few places on Earth where these old rocks exist,'' Byerly said.

A DIFFERENT WORLD

The rock -- almost certainly from the asteroid belt that orbits between Mars and Jupiter -- would have hit an Earth very different from the planet of today, Byerly and Lowe said.

''There were probably no large continental blocks like there are today, although there may have been microcontinents -- very small pieces of continental-type crust,'' Lowe said. The ocean would have been fairly shallow --  about 2 miles deep.

''It would have taken only a second or two for a meteor that's 20 kilometers in diameter to pass through the ocean and impact the rock beneath,'' Lowe said.

''That would generate enormous waves kilometers high that would spread out from the impact site, sweep across the ocean and produce just incredible tsunamis -- causing a tremendous amount of erosion on the microcontinents and tearing up the bottom of the ocean.''

Byerly said some evidence of this widespread erosion has been found at the Australian site. The sediment they looked at would have come both from the meteorite itself and from the crater it would have blasted out.

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Stories like that fascinates me. I knew that the impact ending the reign of the dinosaurs was relatively small compared to the other of the 6 mass extinction sized impacts in earths history. But there is so little known about the first impacts. Thanks for the article MC.

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