[meteorite-list] Scientists Studying Two Big Craters On Earth Find Two Causes

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:07:04 2004
Message-ID: <200210281722.JAA24332_at_zagami.jpl.nasa.gov>

http://www.news.uiuc.edu/scitips/02/1025craters.html

News Bureau
University of Illinois at Urbana-Champaign
Champaign, Illinois

Contact:
Jim Kloeppel, Physical Sciences Editor
(217) 244-1073; kloeppel_at_uiuc.edu

October 25, 2002

Scientists studying two big craters on earth find two causes

CHAMPAIGN, Ill. -- Two of the three largest impact
craters on Earth have nearly the same size and structure,
researchers say, but one was caused by a comet while the
other was caused by an asteroid. These surprising results
could have implications for where scientists might look
for evidence of primitive life on Mars.

Susan Kieffer of the University of Illinois at Urbana-
Champaign, Kevin Pope of Geo Eco Arc Research and Doreen
Ames of Natural Resources Canada analyzed the structure
and stratigraphy of the 65 million-year-old Chicxulub
crater in Mexico and the 1.8 billion-year-old Sudbury
crater in Canada.

Chicxulub is well preserved, but buried, and can be studied
only by geophysical means, remote sensing and at a few
distant sites on land where some ejecta is preserved. In
contrast, Sudbury has experienced up to 4-6 kilometers of
erosion, and is well exposed and highly studied by mining
exploration companies because of its rich mineral resources.

By working back and forth with data from the two craters,
the researchers were able to re-create the structures and
then estimate the amount of melt in each structure. The
amount of melt is critical for determining if long-lived
hot-water circulation systems that might host life forms
could have been formed after the impacts.

In their field studies, the researchers found that both
craters were about 200 kilometers in diameter. In
addition, they identified five ring-shaped structures
with similar character and dimensions. A sixth ring --
the peak ring in the central basin -- was present at
Chicxulub, but had been eroded away at Sudbury.

"While the size and structure of the two craters were
similar, they differed greatly in the amount of impact
melt that was produced," said Kieffer, who presented the
team's findings at the annual meeting of the Geological
Society of America, held Oct. 27-30 in Denver.

"Through field studies, we determined that Chicxulub
has about 18,000 cubic kilometers of impact melt,
approximately four times the volume of water in Lake
Michigan," Pope said. "Sudbury has about 31,000 cubic
kilometers of impact melt, approximately six times the
volume of lakes Huron and Ontario combined, and nearly
70 percent more than the melt at Chicxulub. These
differences in volume have significant implications
about the amount of heat available to drive hot-water
circulation systems."

The researchers then used an analytical cratering model
to examine possible causes for the huge difference in
melt. According to the simulation results, the difference
in melt volume could be readily explained if Chicxulub --
the impact crater that doomed the dinosaurs -- was formed
by an asteroid and Sudbury was formed by a comet.

"Our calculation of 18,000 cubic kilometers of impact
melt at Chicxulub agreed well with model estimates for
an asteroid striking at a 45 degree angle," said Kieffer,
the Walgreen Professor of Geology at Illinois. "None of
the comet impact examples came close to agreeing."

In contrast, the Sudbury impact melt volume of 31,000
cubic kilometers fell between model estimates for a comet
striking at an angle of 30-45 degrees, Kieffer said.
"Similarly, none of the asteroid impact examples came
close to agreeing with the Sudbury melt volume."

Another clue to the craters' origins lies in the impact
melts themselves. The majority of the excess melt at
Sudbury is in the form of a melt-rich breccia -- called
suevite -- inside the crater. This material tends to
form in impacts where the crustal target rock contains
a lot of water. Sudbury has much more suevite in the
preserved crater than Chicxulub.

"The mystery was that there probably wasn't a lot of water
in the original rocks at Sudbury to account for the excess
suevite," Kieffer said. "But in a comet impact of this
size, somewhere around 1,400-2,000 cubic kilometers of
water from the comet gets mixed into the impact melt, and
that could play a major role in disrupting the melt and
creating the excess suevite."

There is other independent evidence for an asteroid impact
at Chicxulub, the team said, including the purported find
of an asteroid fragment in an oceanic drill core, the
amount of iridium spread around the world at the time of
impact, and a telltale chromium 53 isotopic signature.

By studying the origin and structure of large impact
craters on Earth, scientists might narrow the search for
life on Mars. At Sudbury, for example, "there is evidence
of a huge hydrothermal system that was driven by the heat
of the impact melt," Ames said. "As a result, there was
widespread hot spring activity on the crater floor
possibly capable of supporting life."

The researchers are interested in "extrapolating these
conclusions about comet and asteroid impacts to Martian
conditions and asking where we might go to look for
similar hydrothermal systems that could have hosted
primitive life forms on Mars," Kieffer said. "Our next
step is to model these hot-water circulation systems
that were set up by the impact melts with fluid flow
controlled by structures (fractures) inside the crater,
and then extrapolate the results to Martian conditions."

The National Aeronautics and Space Administration and the
Natural History Museum of Los Angeles County funded this
work.
Received on Mon 28 Oct 2002 12:22:50 PM PST