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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 and NASA funded this work. Collaborators included
Stuart Wells and Orlando Hernandez, graduate students
in geological sciences at Ohio State; Luis Gaya-Piqué
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.
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Ohio
State Research -
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