[meteorite-list] Big Bang in Antarctica - Killer Crater Found Under Ice

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Jun 2 21:47:33 2006
Message-ID: <200606021637.JAA21014_at_zagami.jpl.nasa.gov>

http://researchnews.osu.edu/archive/erthboom.htm

BIG BANG IN ANTARCTICA -- KILLER CRATER FOUND UNDER ICE
Ohio State Research News
June 1, 2006

Ancient mega-catastrophe paved way for the dinosaurs, spawned Australian
continent

COLUMBUS, Ohio -- Planetary scientists have found evidence of a meteor
impact much larger and earlier than the one that killed the dinosaurs --
an impact that they believe caused the biggest mass extinction in
Earth's history.

The 300-mile-wide crater lies hidden more than a mile beneath the East
Antarctic Ice Sheet. And the gravity measurements that reveal its
existence suggest that it could date back about 250 million years -- the
time of the Permian-Triassic extinction, when almost all animal life on
Earth died out.

Its size and location -- in the Wilkes Land region of East Antarctica,
south of Australia -- also suggest that it could have begun the breakup
of the Gondwana supercontinent by creating the tectonic rift that pushed
Australia northward.

Scientists believe that the Permian-Triassic extinction paved the way
for the dinosaurs to rise to prominence. The Wilkes Land crater is more
than twice the size of the Chicxulub crater in the Yucatan
peninsula, which marks the impact that may have ultimately killed the
dinosaurs 65 million years ago. The Chicxulub meteor is thought to have
been 6 miles wide, while the Wilkes Land meteor could have been up to 30
miles wide -- four or five times wider.

"This Wilkes Land impact is much bigger than the impact that killed the
dinosaurs, and probably would have caused catastrophic damage at the
time," said Ralph von Frese, a professor of geological sciences at Ohio
State University.

He and Laramie Potts, a postdoctoral researcher in geological sciences,
led the team that discovered the crater. They collaborated with other
Ohio State and NASA scientists, as well as international partners from
Russia and Korea. They reported their preliminary results in a recent
poster session at the American Geophysical Union Joint Assembly meeting
in Baltimore.

The scientists used gravity fluctuations measured by NASA's GRACE
satellites to peer beneath Antarctica's icy surface, and found a
200-mile-wide plug of mantle material -- a mass concentration, or
"mascon" in geological parlance -- that had risen up into the Earth's
crust.

Mascons are the planetary equivalent of a bump on the head. They form
where large objects slam into a planet's surface. Upon impact, the
denser mantle layer bounces up into the overlying crust, which holds it
in place beneath the crater.

When the scientists overlaid their gravity image with airborne radar
images of the ground beneath the ice, they found the mascon perfectly
centered inside a circular ridge some 300 miles wide -- a crater easily
large enough to hold the state of Ohio.

Taken alone, the ridge structure wouldn't prove anything. But to von
Frese, the addition of the mascon means "impact." Years of studying
similar impacts on the moon have honed his ability to find them.

"If I saw this same mascon signal on the moon, I'd expect to see a
crater around it," he said. "And when we looked at the ice-probing
airborne radar, there it was."

"There are at least 20 impact craters this size or larger on the moon,
so it is not surprising to find one here," he continued. "The active
geology of the Earth likely scrubbed its surface clean of many more."

He and Potts admitted that such signals are open to interpretation. Even
with radar and gravity measurements, scientists are only just beginning
to understand what's happening inside the planet. Still, von Frese said
that the circumstances of the radar and mascon signals support their
interpretation.

"We compared two completely different data sets taken under different
conditions, and they matched up," he said.

To estimate when the impact took place, the scientists took a clue from
the fact that the mascon is still visible.

"On the moon, you can look at craters, and the mascons are still there,"
von Frese said. "But on Earth, it's unusual to find mascons, because the
planet is geologically active. The interior eventually recovers and the
mascon goes away." He cited the very large and much older Vredefort
crater in South Africa that must have once had a mascon, but no evidence
of it can be seen now.

"Based on what we know about the geologic history of the region, this
Wilkes Land mascon formed recently by geologic standards -- probably
about 250 million years ago," he said. "In another half a billion years,
the Wilkes Land mascon will probably disappear, too."

Approximately 100 million years ago, Australia split from the ancient
Gondwana supercontinent and began drifting north, pushed away by the
expansion of a rift valley into the eastern Indian Ocean. The rift cuts
directly through the crater, so the impact may have helped the rift to
form, von Frese said.

But the more immediate effects of the impact would have devastated life
on Earth.

"All the environmental changes that would have resulted from the impact
would have created a highly caustic environment that was really hard to
endure. So it makes sense that a lot of life went extinct at that time,"
he said.

He and Potts would like to go to Antarctica to confirm the finding. The
best evidence would come from the rocks within the crater. Since the
cost of drilling through more than a mile of ice to reach these rocks
directly is prohibitive, they want to hunt for them at the base of the
ice along the coast where the ice streams are pushing scoured rock into
the sea. Airborne gravity and magnetic surveys would also be very useful
for testing their interpretation of the satellite data, they said.

NSF funded this work. Collaborators included Stuart Wells and Orlando
Hernandez, graduate students in geological sciences at Ohio State;
Luis Gaya-Piqu?and Hyung Rae Kim, both of NASA's Goddard Space
Flight Center; Alexander Golynsky of the All-Russia Research Institute
for Geology and Mineral Resources of the World Ocean; and Jeong Woo Kim
and Jong Sun Hwang, both of Sejong University in Korea.

#

Contact: Ralph von Frese, (614) 292-5635; Von-frese.3_at_osu.edu

Laramie Potts, (614) 292-7365; Potts.3_at_osu.edu

Written by Pam Frost Gorder, (614) 292-9475; Gorder.1_at_osu.edu
Received on Fri 02 Jun 2006 12:37:33 PM PDT