Palaeozoic World

What appears to be a 480km-wide (300 miles) crater has been detected under the East Antarctic Ice Sheet.

The scientists behind the discovery say it could have been made by a massive meteorite strike 250 million years ago.

At that time, Antartica formed part of the Pangean supercontinent (the name Pangea is Greek, meaning 'all earth').

Pangea was the most recent of perhaps three supercontinents to have formed over the last billion years.

The feature at Wilkes Land was found by Nasa satellites that are mapping subtle differences in the Earth's gravity.

"This Wilkes Land impact is much bigger than the impact that killed the dinosaurs," said Prof Ralph von Frese, from Ohio State University, in the US.

If the crater really was formed at the time von Frese and colleagues believe, it will raise interest as a possible cause of the "great dying" - the biggest of all of Earth's mass extinctions when 95% of all marine life and 70% of all land species disappeared.

Some scientists have long suspected that the extinction at the boundary of the Permian and Triassic (PT) Periods could have occurred quite abruptly - the result of environmental changes brought on by the impact of a giant space rock.

It is a similar argument to the one put forward to explain the demise of the dinosaurs at the much later date of 65 million years ago.

A geological structure, known as the Bedout High, in the seabed off the northern coastal border of what is now Western Australia, has also been suggested as the possible crater remains from the PT impactor.

But the impact explanation for the great dying is an argument that has struggled to find favour.

The prevailing theory is that several factors - including supervolcanism (where Magma rises from the earth's mantle and accumulates in the crust. The heat of the liquid rocks causes more of the solid crust to melt, forming a magma chamber which often causes the pressure to rise more and more and can lead to eruptions on such a massive scale that an ice age can be triggered as a result), and extensive climate warming - combined over thousands of years to strangle the planet's biodiversity.

Earth may well have been hit by extraterrestrial objects, but it is unlikely there was some killer punch from space, these other researchers contend.

The Ohio-led team used gravity fluctuations measured by the US space agency's Grace satellites to peer beneath Antarctica's icy surface. Team members were drawn from the US, Russia and Korea.

The crater information was first presented at the recent American Geophysical Union Joint Assembly in Baltimore.

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