[meteorite-list] Earth's Early Battering Revealed

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:10 2004
Message-ID: <200207242255.PAA03908_at_zagami.jpl.nasa.gov>

http://news.bbc.co.uk/1/hi/sci/tech/2149215.stm

Earth's early battering revealed
           
Today's Earth enjoys a more peaceful existence

By Dr David Whitehouse
BBC News
July 24, 2002

The first convincing evidence that the Earth was bombarded by a
devastating and prolonged storm of meteorites four billion years
ago has been found in the Earth's oldest rocks.

The British and Australian researchers say there is no other
conceivable explanation for new-found traces of a tungsten isotope
in rocks 3.7 thousand million years old from Greenland and Canada.

The finding had been expected as several lines of evidence from
Moon rocks and ancient craters on the Moon indicate it was
subjected to a so-called "Late Heavy Bombardment" (LHB).

Because the Earth is a larger target with a stronger pull of gravity it
was supposed that our planet was also subjected to the same
meteor storm as well.

Cataclysm

The LHB was one of the most violent events in the history of our
Solar System.

For 100 - 200 million years an unending rain of large meteorites
struck the rocky inner worlds of the inner Solar System.

Most of the craters in the southern hemisphere of Mars were
formed during this event.

The evidence for it on the Moon is everywhere. It was during the
LHB that most of the large lunar impact basins were formed. Later
they became filled with dark lava forming the now familiar lunar
"seas."

So should it have been for the Earth as well.

Computer estimates suggest that over 200 million years our planet
should have suffered over 22,000 craters larger than 20 km (12
miles), about 40 impact basins larger than 1000 km (624 miles) and
several massive 5000 km (3100 miles) basins.

There would have been a impact that affected global conditions
every 100 years or so.

But until now the evidence for that cataclysm on Earth had not
been found.

Sedimentary rocks from Greenland and Canada that have been
subsequently modified by heat and pressure are the oldest on Earth
- dating from the waning phases of the LHB.

Researchers from the University of Queensland and the University
of Oxford have detected the chemical fingerprints of the
meteorites, specifically various types of tungsten atoms that must be
extraterrestrial.

The research is published in the journal Nature.
Received on Wed 24 Jul 2002 06:55:13 PM PDT