[meteorite-list] Relic of Ancient Asteroid Found

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed May 10 16:07:42 2006
Message-ID: <200605101905.MAA12210_at_zagami.jpl.nasa.gov>

http://news.bbc.co.uk/1/hi/sci/tech/4757545.stm

Relic of ancient asteroid found
By Rebecca Morelle
BBC News
May 10, 2006

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.

"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."

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.
Received on Wed 10 May 2006 03:05:57 PM PDT