[meteorite-list] Evidence Of Massive, Ancient Meteor Strike Published In Science

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:21 2004
Message-ID: <200208231747.KAA11555_at_zagami.jpl.nasa.gov>

Media Relations
Office of University Relations
Louisiana State University
Baton Rouge, Louisiana
Contact:
Ronald Brown, LSU Media Relations
225-578-3867, rbrown_at_lsu.edu

08/22/2002 03:10 PM

Evidence of massive, ancient meteor strike published in Science

A huge meteor, between 12 and 30 miles in diameter, smashed into
the Earth 3.5 billion years ago with the energy of 1 billion
atomic bombs, vaporizing the surface where it struck and creating
a tsunami more than half a mile high that raced around the world
at 500 miles per hour. 

This cataclysm is the earliest known meteor strike to hit the
Earth, and one of at least four that have been identified in a
geologically brief 300-million-year period. The strike is the
subject of an article published in the current issue of the
journal Science by LSU geologist Gary Byerly and others.

Byerly and Xiaogang Xie, also of LSU, and Donald Lowe and Joseph
Wooden of Stanford, identified traces of the event in some of
the oldest known rocks on Earth -- in South Africa and Northwest
Australia.

When the asteroid hit, it was vaporized by the extreme energy of
the impact. Condensation of this vapor produced droplets of melt,
called spherules, which dropped into the roiling sea over the
next few days and were deposited in layers on the sea floor. 

Byerly said it was not known where the meteor hit, but it was
probably some distance from where they found the spherules and
probably in water rather than on land. He deduced this because
the composition of the spherules lacked the mineral composition
that would have been expected from vaporization of the
continental crust, and because there was even more water
covering the surface of the Earth then than there is today.

Byerly illustrated with a slab of grayish rock about the size of
a large hand. In it could be seen layers of spherules interlaced
with layers of finer sand or silt. "It would take about 30 hours
from impact for the tsunami to travel all the way around the
world. Then, of course, it wouldn't stop, but bounce all the way
back till it met itself 30 hours later, then bounce the other
way again, setting up a harmonic." Several thin layers of mud
within the spherule layer represent periods of quiet
sedimentation between the arrival of tsunami waves. 

The water would likely have inundated everything but the
mountains, Byerly said, and drastically eroded the continental
land masses, changing their coastlines dramatically. The heat
of the impact would have evaporated the upper 30 to 300 feet of
water in the oceans. It would also have killed everything, or
almost everything, that was alive at that time on land or near
the ocean surface.         

"There was almost certainly life at this time. Primitive,
bacterial life, and if the impacts were made by a meteor 20 miles
in diameter, they would have killed everything on the surface of
the Earth," he said. First a hot steam of molten rock and water
would have withered most life, then the massively destructive
tsunamis would have destroyed even more. After that, years of
incredibly cold winters, caused by particles in the atmosphere
blocking out the sun, would have conspired to kill nearly
everything else.

"Anything that survived would have been in deep rocks or below
the surface of the Earth," he said.

Byerly first came upon evidence of these impacts by chance in
1984 while he was studying ancient volcanism in Australia and
South Africa. He published his first paper on them in 1986.
This year alone he and his team will have four papers published
on the subject. It is now generally accepted that the inner
solar system was battered twice by massive meteor impacts of
mysterious origin.

"It is assumed that the solar system was created by a cloud of
dust and rocks that condensed into the sun and planets, with
larger and larger chunks falling in near the end. This process
would have finished about 4.5 billion years ago.

"But about 3.8 billion years ago the inner solar system was torn
up by some cataclysmic event. The evidence is found in the crater
basins on the moon and Mars and Venus." The Earth was probably
heavily battered at that time too, but since the oldest known
surface rocks on Earth are about 3.5 billion years old, no
evidence of that exists, Byerly said.

A second battering took place 3.5 billion years ago, and it is
this event that left the record in the rocks Byerly is studying.
This was a smaller series of impacts with a gradual dropoff rate,
he said. That the 3.8 billion-year-old event occurred is accepted
by most scientists, and Byerly's work is substantiating the
existence of the second, which, up to now, has not been as well
accepted.

Byerly and his team have been able to date the event very
accurately using an instrument at Stanford that measures the
decay of uranium into lead. The uranium was found in zircons at
both the Australian and South African sites and was dated to
within 2 million years of 3.47 billion years ago. The fact that
zircons of identical ages were found in impact strata on two
continents shows the worldwide effects of the impact, Byerly
said.

The zircons were not created by the impact but probably by
volcanic action and deposited in the impact layers when the
tsunami washed over the land.

These massive, early impacts were similar to the one that killed
the dinosaurs 65 million years ago. But, they were hundreds to
thousands of times more powerful. Probabilities for a similar
impact today are predicted to be about one such strike every
100 million years.

"What that means is, eventually there will be another such event.
We know that large asteroids get disturbed by interactions with
Jupiter and fall into Earth's orbit. When that happens they will
strike the Earth. We can't say when it will happen but we can
say for certain that it will happen," Byerly said.

The evidence is on a small, grayish slab of rock on his desk.
Received on Fri 23 Aug 2002 01:47:00 PM PDT