Penguin Stumblings

Scientists say impact might have caused extinction 250 million years ago

By Robert Roy Britt
Senior science writer

An apparent crater as big as Ohio has been found in Antarctica. Scientists think it was carved by a space rock that caused the greatest mass extinction on Earth, 250 million years ago.

The crater, buried beneath a half-mile (1 kilometer) of ice and discovered by some serious airborne and satellite sleuthing, is more than twice as big as the one involved in the demise of the dinosaurs.

The crater's location, in the Wilkes Land region of East Antarctica, south of Australia, suggests it might have instigated the breakup of the so-called Gondwana supercontinent, which pushed Australia northward, the researchers said.

"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.

The crater is about 300 miles (500 kilometers) wide. It was found by looking at differences in density that show up in gravity measurements taken with NASA's GRACE satellites. Researchers spotted a mass concentration, which they call a mascon — dense stuff that welled up from the mantle, likely in an impact.

"If I saw this same mascon signal on the moon, I'd expect to see a crater around it," Frese said. (The moon, with no atmosphere, retains a record of ancient impacts in the visible craters there.)

So Frese and colleagues overlaid data from airborne radar images that showed a 300-mile-wide subsurface, circular ridge. The mascon fit neatly inside the circle.

"And when we looked at the ice-probing airborne radar, there it was," he said Thursday.

The Permian-Triassic extinction, as it is known, wiped out most life on land and in the oceans. Researchers have long suspected a space rock might have been involved. Some scientists have blamed volcanic activity or other culprits.

The die-off set up conditions that eventually allowed dinosaurs to rule the planet.

The newfound 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 space rock is thought to have been 6 miles (10 kilometers) wide, while the Wilkes Land meteor could have been up to 30 miles (50 kilometers) wide, the researchers said.

Postdoctoral researcher Laramie Potts assisted in the discovery.

The work was financed by NASA and the National Science Foundation. The discovery, announced Thursday, was initially presented in a poster paper at the recent American Geophysical Union Joint Assembly meeting in Baltimore.

The researchers say further work is needed to confirm the finding. One way to do that would be to go there and collect rock from the crater to see if its structure matches what would be expected from such a colossal impact.

Source: http://www.msnbc.msn.com/id/13089686/

By Paul Rincon
BBC News science reporter, in Houston, Texas

Evidence for what may be a large and relatively recent impact crater has been found off the coast of Antarctica.

Scientists say the evidence, if correct, points to a space rock some 5km across having crashed into the Ross Sea about three million years ago.

This could have generated a huge tsunami, according to a member of the team investigating the collision.

Details were reported at the Lunar and Planetary Science Conference in Houston, Texas.

Glass hints

Researchers from the Lamont-Doherty Earth Observatory in New York have been studying a 100km-wide depression, known as Bowers Crater, under the Ross Sea.

Team members examined cores drilled from around the area to look for evidence of an impact.

In the cores, they found microscopic glassy grains shaped like teardrops, spheres and dumbbells which are collectively known as tektites.

Some scientists believe these are created when rock fragments are hurled high up into the atmosphere by the impact of a large meteoroid or asteroid, and then partially re-melt as they fall back to the ground.

Other glasses were also found. These are thought to have been formed by cooling of the melted rock and sediment. Similar glasses can be formed through volcanism, but the Ross Sea specimens seem to have a distinct structure under the microscope.

Wave trace

The findings alone do not prove there was an impact in the area a few million years ago, but team member Dallas Abbott says she hopes to search the core material further for a mineral called shocked quartz.


This type of quartz can be distinguished from normal quartz by characteristic lines visible under the microscope which are thought to be formed by the intense pressure of an impact.

The presence of this mineral is considered most diagnostic of a space collision.

Dr Abbott told the BBC News website that an impact in the Ross Sea would have generated a "pretty big tsunami".

The waves could have crashed against the shores of South America; but, she added, the geological history of that continent made it unlikely that evidence of this event would be found.

Source: http://news.bbc.co.uk/1/hi/sci/tech/4816794.stm