by Mark Peplow

June 2, 2006
from K8Science Website

Evidence of a cataclysmic meteorite impact has been unearthed in Antarctica, according to researchers who say the collision could possibly explain the greatest mass extinction ever seen on our planet. But scientists contacted by news@nature.com say they are skeptical, as no signs of such an enormous impact have been found in other, well-studied areas of Antarctica.

The first sign of this possible impact was spotted by NASA's GRACE satellites, a pair of orbiting probes that sense slight variations in the Earth's gravity field. They revealed a 320-kilometre-wide plug of dense mantle material more than 1.6 kilometers beneath the East Antarctic ice sheet in an area known as Wilkes Land.

This mass concentration, or 'mascon', can be caused by the upwelling of denser material from the Earth's mantle after a massive impact.

"If I saw this mascon signal on the Moon, I'd expect to see a crater around it," says Ralph von Frese, a geophysicist at Ohio State University, Columbus, who led the team that made the find.

When they looked at airborne radar images of the area, they found what they say looks like a crater — a circular ridge some 500 kilometers wide running around the mascon.

"It could be the biggest impact ever found on Earth," says von Frese.

Incoming!

"It's possible, but it's not the interpretation that would top your list," says Ian Dalziel, a geologist at the University of Texas at Austin.

The region of dense rock is certainly circular, he says, but it could easily be volcanic rock that had welled up during normal geological activity:

"You can find a lot of gravity anomalies like this."

The roughly circular feature thought to be the rim of the crater may just be part of the normal variation in terrain in the area, he adds.

If an incoming asteroid did gouge out the hole it must have been up to 50 kilometers across, says von Frese. That's four or five times larger than the object thought to have created the Chicxulub crater on the Yucatan peninsula in Mexico, which was probably responsible for wiping out the dinosaurs some 65 million years ago.

But that's peanuts compared with the Permian-Triassic extinction, which destroyed more than three-quarters of all species on Earth about 250 million years ago. The cause of this mass extinction is still hotly debated by scientists.

Most think that the extinction started when a vast volcanic eruption released a flood of lava to create the Siberian Traps — an area of basalt that covers an area larger than Europe.

"They represent the biggest volcanic event of all time, and coincide precisely with the extinction," says Paul Wignall, a palaeontologist at the University of Leeds, UK, who studies mass-extinction events.

Such an eruption would have belched huge amounts of gas, including sulphur dioxide and carbon dioxide, into the atmosphere, causing acid rain and greenhouse warming.

Other scientists have argued that a massive impact, like that at Chicxulub, could be responsible instead (see 'Comet impact theory faces repeat analysis').

"But nobody's been convinced of that," says Wignall.

Von Frese notes that the explanations aren't mutually exclusive: the shockwaves from a huge impact could have travelled through the planet to trigger the eruptions in Siberia, delivering a devastating combination of disasters.
 

Hot topic

Von Frese presented the discovery at an American Geophysical Union meeting in Baltimore, Maryland in late May. He admits that it was greeted with "a lot of skepticism", largely because there's no direct evidence that the feature is 250 million years old.

An impact of that size should also have melted and twisted nearby rock. Yet rocks in the Transantarctic Mountains of the same age show no evidence of the collision, says Jane Francis, a geologist also at the University of Leeds.

"That sequence has been worked on before, and no one has found evidence to support a massive impact like this," she says.

Wignall says that few scientists will be convinced by the hypothesis until the team can precisely date their crater directly, and find rocks there that have been altered by the searing heat of the explosion.

"Then we'll all sit up and take notice," he says.

Too much ice covers the putative crater for a drilling expedition.

But Von Frese hopes to make a research trip to Antarctica to look for rocks at the base of the ice sheet along the continent's coast that could attest to an impact.

BIG BANG IN ANTARCTICA

KILLER CRATER FOUND UNDER ICE

by Pam Frost Gorder

from ResearchNews Website

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.

Ralph von Frese

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.