Washington ? Researchers studying rocks from Antarctica have found 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 percent of all life.

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 was in the chemistry found in rocky fragments recovered on Graphite Peak in Antarctica. 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.

Scientists believe the six-mile-wide asteroid that killed the dinosaurs left a thin layer of a metal called iridium all over the globe. Basu said that element was not found in the fragments recovered from Antarctica, suggesting the earlier space rock had a different composition than the dinosaur-killer rock.

Basu said specimens recovered from Permian-Triassic rock formations in China, however, had a chemistry that matched that of the meteorite fragments found in Antarctica, a discovery that supports the impact theory. Also, shocked quartz — a telltale sign of an asteroid impact — has been found at both sites, he said.