Evidence outlines a vast disaster in Earth's past

WASHINGTON -- Scientists reported new evidence Thursday that might solve the world's
greatest case of mass murder: the extinction of most of the creatures living on our planet
long before the age of the dinosaurs.

The finger of blame now points to a giant comet or asteroid -- three to seven miles across --
from outside our solar system that smashed into Earth 251 million years ago.

The impact triggered a deadly chain reaction -- including an enormous volcanic eruption --
that wiped out 90 percent of marine species and 70 percent of higher land organisms.

Scientists previously had realized that the 251 million-year-old mass extinction was
connected to an colossal eruption of lava.

A flood of molten basalt (volcanic rock) -- enough to cover the entire globe 30 feet deep
-- poisoned the air, altered sea levels and rendered the world uninhabitable for most
creatures more complex than a bacterium.

Clues to the killer from outer space are molecules of two rare gases, helium and argon,
that were formed outside our solar system but were found on Earth, according to Luann Becker,
an Earth scientist at the University of Washington.

The scientist-detectives discovered molecules of the gases trapped inside little cages of pure carbon,
known as buckyballs, buried in ancient sediments in China, Japan and Hungary.
Buckyballs do not naturally occur on Earth but have been created by lab lasers.

At the time of the collision, when those ancient sediments were laid down, all of Earth's land
was gathered in a single supercontinent known as Pangea.

Robert Poreda, an earth scientist at the University of Rochester, said Pangean volcanoes
probably already were spitting out lava when the comet or asteroid struck.

``It is possible that you need both the impact trigger and the major eruption of flood basalts
to tip the Earth over the edge,'' said Poreda, a member of Becker's team.

The calamity is known as the P-T extinction, because it occurred at the boundary between
two geologic eras, the Permian and the Triassic.

A similar collision 65 million years ago is widely believed to have killed the dinosaurs.

Becker and her colleagues reported their findings Thursday at NASA headquarters and in Science magazine.