SAN FRANCISCO — Scientists who support a controversial theory that a meteor crash coincided with the largest mass extinction in Earth's history now assert that they have narrowed the location of the impact — somewhere on land in the tropics.
The extinction occurred 250 million years ago at the boundary between the Permian and Triassic geological periods and killed 90 percent of the living species.
Scientists generally agree that the most recent, and perhaps most well-known mass extinction, which killed off the dinosaurs 65 million years ago, was caused when a meteor struck earth near the Yucatan Peninsula in Mexico.
They disagree on the causes of earlier extinctions.
Evidence at the Permian-Triassic extinction is sketchier, but most scientists theorize that the most likely cause was a series of giant volcanic eruptions in Siberia that spewed 600,000 cubic miles of lava and might have induced catastrophic ecological changes.
Last month, a team of scientists headed by Dr. Asish R. Basu, a professor at the University of Rochester, reported in the journal Science that it had found shards of the meteorite in rocks from Antarctica. Other researchers agreed that the fragments looked extraterrestrial but questioned whether they were as old as claimed.
On Friday, at a meeting of the American Geophysical Union here, the same group of scientists reported finding tiny glass spheres about one-thousandth of an inch in diameter in the same layer of rock in Antarctica.
The spheres, the scientists said, are pieces of the Earth's crust melted by the meteor impact and then cooled.
Such spheres can also form in volcanic eruptions. But the composition of glass does not resemble lava. Instead, the spheres contain a mix of elements typical of old continental soil in the tropics and relatively high in metals like titanium, aluminum, silicon and iron, which are left behind.
"This seems to really convince people," said Dr. Stein B. Jacobsen, a member of the research team. "There are no volcanic rocks of that composition."