Meteorite wiped out life
2003-11-21 13:44
Washington - Researchers studying rocks from Antarctica have found chemical evidence that a huge meteorite smashed the earth 251 million years ago and caused the greatest extinction event in the planet's history, killing about 90% of all life.
Scientists believe the extinction, which scientists call the Permian-Triassic event, came some 185 million years before a similar meteorite collision with the planet killed off the dinosaurs.
"It appears to us that the two largest mass extinctions in earth history ... were both caused by catastrophic collisions" with meteoroids, the researchers say in their study appearing this week in the journal Science.
Asish R Basu, a professor of earth sciences at the University of Rochester, said proof of a massive impact 251 million years ago is in the chemistry found in rocky fragments recovered on Graphite Peak in Antarctica.
Unique to meteorites
He said the fragments were found at a geological horizon, or layer, that was laid down at the start of the Permian-Triassic extinction. Analysis shows the fragments have chemical ratios that are unique to meteorites.
"The only place you would find the chemical composition that we found in these fragments is in very primitive, 4.6-billion-year-old meteorites, as old as our earth," said Basu, the first author of the study.
That supports the theory that space rock the size of a mountain streaked in from outer space and smashed into the earth. The violence of the impact would have caused a huge fireball and sent billions of tons of dust into the atmosphere, enough to darken the sun for months. It also would have laid down a layer of dust bearing the same chemical composition as the meteorite.
At the time of the Permian-Triassic event, Africa, South America, India, Australia and Antarctica were believed to be joined in a giant continent called Pangea. Just where the asteroid hit in that land mass is uncertain, Basu said.
Sceptical
Life on earth at the time was far different from what it is now and what it was when dinosaurs lived.
"There were no large animals then, but there were lots of species living on the land and in the sea, and there were plants," said Basu.
Some experts are sceptical that Basu and his co-authors have found 251-million-year-old meteorite metals, although nobody questions that the material did come from outer space. The surprise is that the specimens survived the weathering on earth for so long.
"It's astonishing, it's incredible, it's unbelievable," Jeffrey Grossman of the US Geological Survey said in Science.
Birger Schmitz of the University of Goteborg in Sweden said in Science that it "would really be remarkable" to find unaltered meteoritic fragments such as those claimed by Basu and his colleagues.
"I get a gut feeling it's wrong," he said.
On the net:
www.sciencemag.org
- SAPA