[meteorite-list] Does a Giant Crater Lie Beneach the Antarctic Ice?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun Jun 4 22:24:23 2006
Message-ID: <200606050144.SAA02325_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2006/060529/full/060529-11.html

Does a giant crater lie beneath the Antarctic ice?
Signs of an ancient impact could help to explain a mass extinction.
Mark Peplow
nature.com
June 2, 2006

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_at_nature.com say they are sceptical, 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 kilometres 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 kilometres 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 kilometres 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 <http://www.nature.com/news/2004/041025/full/4311027a.html>').
"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 scepticism", 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.
Received on Sun 04 Jun 2006 09:44:33 PM PDT