July 8, 2003	Tuesday	Jumadi-ul-Awwal 7,1424

STOCKHOLM, July 7: Evidence that meteorites rained down on Earth after a massive collision in the asteroid belt 500 million years ago has been uncovered by scientists.

Sandy remains of meteorites found in five Swedish limestone quarries could be traced to the break up of a huge asteroid. Analysis of the number and size of the grains revealed that the rate at which meteorites struck the Earth increased by up to 100 times about 480 million years ago.

The timing coincides with the destruction of the so-called “L- chondrite parent body” — a giant asteroid known to have been blasted apart in a collision beyond the orbit of Mars.

Today, meteorite activity is relatively uniform with an average of about one rock per year falling in every 7,700 square miles. About 20 per cent of meteorites landing on Earth today originate from the L- chondrite parent body.

Remnants of the parent asteroid can be identified by their mineral composition. US and Swedish scientists searching the 150,000 square mile quarry site looked for unique extraterrestrial forms of the mineral chromite found only in meteorites from the L-chondrite break up.

All the meteorite residues discovered contained the mineral. Matching concentrations of the grains were found in all five quarries, indicating that meteorite activity occurred at the same rate over the whole area.

Both intact meteorites and meteorite grains were sealed in the limestone when it formed from sea bottom sediments. The findings, reported in the journal “Science”, help explain why so many fossilised meteorites have been discovered in 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.—dpa