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Last Updated: Friday, 9 May, 2003, 12:29 GMT 13:29 UK
Quarries record bombardment

The Earth experienced a massive bombardment from meteoroids 480 million years ago after the break-up of a giant space rock.

Meteor, BBC
Material constantly rains down on Earth

The amount of material that rained down on our planet was 100 times that experienced today, say researchers.

The evidence comes from fossil meteorite fragments found in limestone deposits spread over a wide area of Sweden.

These all contain a particular mineral signature that links them to the same mighty asteroid that came apart in a collision with another huge rock.

The research team, from Göteborg University and Rice University, US, reports its findings in the journal Science.

Mighty collision

Today, meteorites impact the Earth at a rate of about one per year every 20,000 square kilometres (7,700 square miles).

But the new study found a 100-fold increase in this activity when carbonate rocks were being deposited in what is now Sweden a little under half a billion years ago.

The researchers investigated more than 40 meteorite fossils caught in these limestones at quarries spread over a 390,000-sq-km (150,000-square-mile) search area.

The fossils ranged in size from one to 20 centimetres.

All the meteorites recovered contained unique extraterrestrial forms of the mineral chromite, which links them to the so-called L-chondrite parent body.

Scientists think this huge asteroid was involved in one of the last great collisions in the Solar System. Debris from this mighty rock still accounts for about 20% of all meteorites hitting the surface of the Earth today.

China check

"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.

The team says the Swedish limestones were laid down in a two-million-year period.

Concentrations of silt and sand-sized grains of extraterrestrial chromite were found at all five quarries visited, indicating that meteorite activity following the break-up was occurring at the same rate over the entire area.

The team believes similar concentrations of fossilised meteorites and extraterrestrial chromite grains are present worldwide in limestone that formed at the same time.

The scientists plan to check this in China.

Asteroid threat reassessed
20 Nov 02  |  Science/Nature
Impact 'showered debris over Britain'
15 Nov 02  |  Science/Nature

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