Asteroid 'destroyed life 250m years ago'

A crater left by an impact 500 centuries ago in Arizona

By BBC News Online science editor Dr David Whitehouse

Earth's biggest mass extinction 251 million years ago was triggered by a collision with a comet or asteroid, US scientists say.

They have reached this conclusion by looking at atoms from a star trapped inside molecular cages of carbon.

The impact of a body of this size releases an amount of energy that is about one million times the largest earthquake recorded during the last century

Dr Robert Poreda

The impact occurred when all the Earth's land was joined together into a supercontinent called Pangea. Researchers have no idea where on the planet's surface the space object came down, but they have identified traces of it.

In rock layers laid down at the time, there is a much higher concentration of complex carbon molecules called fullerenes that have different types, or isotopes, of helium and argon trapped inside them.

These molecules could only have been delivered from space, the researchers say.

Chemical clues

The fullerenes contain at least 60 carbon atoms in a structure resembling a soccer ball.

The researchers believe these particular fullerenes are extraterrestrial because the gases trapped inside have an unusual ratio of isotopes that indicate they were made in the atmosphere of a star that exploded before our Sun was born.

A fullerene or buckyball.(Image: University of Washington)

"These things form in carbon stars. That's what's exciting about finding fullerenes as a tracer," said Dr Luann Becker of the University of Washington, US.

The telltale fullerenes were extracted from sites in Japan, China and Hungary, where the sedimentary layer at the boundary between the Permian and Triassic periods had been exposed.

Dr Becker said the fullerenes were found at very low concentrations above and below the boundary layer, but they were found in unusually high concentrations at the time of the extinction.

Heavenly delivery

The research was made difficult because there are few 251-million-year-old rocks left on Earth. Most rocks of that age have been recycled through the planet's tectonic processes.

"It took us two years to do this research, to try to narrow it down enough so that we could see this fullerene signature," she said.

The fullerene concentration at the Permian-Triassic boundary means they were delivered to the Earth by a comet or asteroid.

Researchers estimate the comet or asteroid was six to 12 km (3.7 - 7.4 miles) across, or about the size of the asteroid believed responsible for the extinction of the dinosaurs 67 million years ago.

The scientists determined the size based on two factors:

• If the body was smaller than six km (3.7 miles), the effects would not have been global;
• if it was larger than 12 km (7.4 miles), there would have to be more gas-laden fullerenes distributed around the world.

"The impact of a body of this size releases an amount of energy that is basically about one million times the largest earthquake recorded during the last century," said Robert Poreda, associate professor of earth and environmental sciences at the University of Rochester, New York, US.

Two fronts

Researchers believe the impact and rapid extinction occurred simultaneously with some of the most extensive volcanic activity the world has ever seen - enough lava to cover the entire planet to a depth of three metres (10ft) oozed out of the ground in Siberia in less than one million years. These changes wiped out 90% of all marine species and 70% of land vertebrates.

If the species cannot adjust, they perish. It's a survival-of-the-fittest sort of thing

Dr Luann Becker

"If the species cannot adjust, they perish. It's a survival-of-the-fittest sort of thing," said Dr Becker. "To knock out 90% of organisms, you've got to attack them on more than one front."

The mass extinction of 251 million years ago was the greatest on record. Many fossils below the boundary, such as trilobites, which once numbered more than 15,000 species, are completely absent above it.

There is also strong evidence suggesting the extinction happened very rapidly, in as few as 8,000 to 100,000 years.

Dino 'bookends'

It was thought that any asteroid or comet collision would leave strong evidence of the element iridium, the signal found in the sedimentary layer from the time of the dinosaur extinction.

Lead researcher Dr Luann Becker. (Image: Univ of Washington)

The team believes the difference might be because the two space bodies that slammed into Earth had different compositions.

"These two extinctions are like bookends for the age of the dinosaurs," said Dr Poreda.

In 1996, Becker and Poreda discovered that fullerenes found in a huge impact crater in Canada came from space nearly two billion years ago.

Last year, they showed that even more complex carbon molecules, with as many as 200 atoms, had survived from the impact that wiped out the dinosaurs.

The findings are reported in the journal Science.