Seismic Images Show Dinosaur-Killing Meteor Made Bigger Splash (Forwarded)

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January 23, 2008

Seismic Images Show Dinosaur-Killing Meteor Made Bigger Splash

AUSTIN, Texas -- The most detailed three-dimensional seismic images yet of
the Chicxulub crater, a mostly submerged and buried impact crater on the
Mexico coast, may modify a theory explaining the extinction of 70 percent
of life on Earth 65 million years ago.

The Chicxulub crater was formed when an asteroid struck on the coast of
the Yucatan Peninsula. Most scientists agree the impact played a major
role in the "KT Extinction Event" that caused the extinction of most life
on Earth, including the dinosaurs.

According to Sean Gulick, a research scientist at the Institute for
Geophysics at The University of Texas at Austin's Jackson School of
Geosciences and principal investigator for the project, the new images
reveal the asteroid landed in deeper water than previously assumed and
therefore released about 6.5 times more water vapor into the atmosphere.

The impact site also contained sulfur-rich sediments called evaporites,
which would have reacted with water vapor to produce sulfate aerosols.
According to Gulick, an increase in the atmospheric concentration of the
compounds could have made the impact deadlier in two ways: by altering
climate (sulfate aerosols in the upper atmosphere can have a cooling
effect) and by generating acid rain (water vapor can help to flush the
lower atmosphere of sulfate aerosols, causing acid rain). Earlier studies
had suggested both effects might result from the impact, but to a lesser
degree.

"The greater amount of water vapor and consequent potential increase in
sulfate aerosols needs to be taken into account for models of extinction
mechanisms," says Gulick.

The results appear in the February 2008 print edition of the journal
Nature Geosciences.

An increase in acid rain might help explain why reef and surface dwelling
ocean creatures were affected along with large vertebrates on land and in
the sea. As it fell on the water, acid rain could have turned the oceans
more acidic. There is some evidence that marine organisms more resistant
to a range of pH survived while those more sensitive did not.

Gulick says the mass extinction event was probably not caused by just one
mechanism, but rather a combination of environmental changes acting on
different time scales, in different locations. For example, many large
land animals might have been baked to death within hours or days of the
impact as ejected material fell from the sky, heating the atmosphere and
setting off firestorms. More gradual changes in climate and acidity might
have had a larger impact in the oceans.

Gulick and collaborators originally set out to learn more about the
trajectory of the asteroid. They had hoped the crater's structure in the
subsurface would hold a tell-tale signature. Instead, the structure seemed
to be most strongly shaped by the pre-impact conditions of the target
site.

"We discovered that the shallow structure of the crater was determined
much more by what the impact site was like before impact than by the
trajectory of the impactor," says Gulick.

If scientists can determine the trajectory, it will tell them where to
look for the biggest environmental consequences of impact, because most of
the hazardous, shock-heated and fast-moving material would have been
thrown out of the crater downrange from the impact.

Researchers at Imperial College in London are already using computer
models to search for possible signatures in impact craters that could
indicate trajectory regardless of the initial surface conditions at the
impact site.

"As someone who simulates impact events using computers, this work
provides valuable new constraints on both the pre-impact target structure
and the final geometry of the cratered crust at Chicxulub," says Gareth
Collins, a research fellow at Imperial College.

Collaborators on the project included Gail Christeson of the Institute for
Geophysics, Penny Barton at the University of Cambridge, Joanna Morgan and
Mike Warner at Imperial College, and several graduate students.

Note: The paper "Importance of pre-impact crustal structure for the
asymmetry of the Chicxulub impact crater" can be downloaded online,
http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo103.html

Related Sites:

* Institute for Geophysics
http://www.ig.utexas.edu/
* Jackson School of Geosciences
http://www.jsg.utexas.edu/
* Research at The University of Texas at Austin
http://www.utexas.edu/research/


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