Thursday, May 11, 2006

Relic of ancient asteroid found

By Rebecca Morelle
BBC News science reporter

Fragments were drilled from the crater (J.Hills/Science Museum)

A large fragment of an asteroid that punched 160km-wide (100 miles) hole in the Earth's surface has been found.

The beachball-sized fossil meteorite was dug out of the 145-million-year-old Morokweng crater in South Africa.

It is a unique discovery because large objects are widely believed to completely melt or vaporise as they collide with the planet.

Writing in the journal Nature, an international team says the find will further knowledge on asteroid impacts.

The Morokweng crater is one of the largest on Earth, and was formed at the boundary of the Jurassic and Cretaceous periods.

Created by an asteroid measuring about 5-10km (3-6 miles) in diameter, the impact bowl lies hidden beneath the sand of the Kalahari Desert.

'Fiery furnace'

Scientists discovered the meteorite fossil by drilling bore holes into the impact melt - the area where the asteroid fused with the Earth - in the centre of the crater.

The Morokweng crater is 145 million years old

"At about 770m (2,500ft) down, we came across some dark blocks - one was about the size of a beachball - but we couldn't figure out what it was," said Dr Marco Andreoli, an author on the Nature paper and a geologist at the South African Nuclear Energy Corporation and the University of Witwatersrand.

After chemical and mineral tests - which meant the material was cut up into smaller fragments - the scientists were astonished to find that the rock was a meteorite, a surviving relic from the collision.

When a large impactor strikes the Earth, a colossal amount of heat is produced; and the asteroid material is believed to vaporise or fuse with the surrounding rocks. A 10-km-diameter impactor is thought to generate temperatures of between 1,700-14,000C.

Consequently, scientists can only study these large impacts by looking at the chemical composition of material in the crater.

"What is amazing is that here we have these fragments - that may not have been attached to the asteroid, or maybe trailing behind it - that smashed into the Earth and survived the fiery furnace in the crater that formed; and then they got trapped," Dr Andreoli told the BBC News website.

"This is remarkable because this is something that people didn't think could happen."

New models

It meant, he said, that computer models of large impacts might now need to be revised, to take into account conditions where some of the asteroid material endures.

"Anything that helps scientists to model what happens when two bodies collide is good news."

Morokweng crater
The crater is hidden beneath the Kalahari Desert
Further investigation into the discovery has also revealed that the chemical composition of the space rock is slightly different to that of other meteorites that have been studied. It is a little more radioactive; there is more uranium, sodium, but less iron and nickel.

"All of our science of meteorites is based on meteorites that fell in the last few thousand years.

"But all of a sudden we can study a meteorite that fell 145 million years ago, and this opens the possibility that the nature of these impacting bodies has changed over the years," Dr Andreoli explained.

People in the UK can see fragments of the meteorite if they visit the Antenna Wing of London's Science Museum from Thursday.

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