ATLANTA (AP) — The second-largest extinction in the
Earth’s history, the killing of two-thirds of all species,
may have been caused by ultraviolet radiation from the sun after
gamma rays destroyed the Earth’s ozone layer.

Astronomers are proposing that a supernova exploded within
10,000 light years of the Earth, destroying the chemistry of the
atmosphere and allowing the sun’s ultraviolet rays to cook
fragile, unprotected life forms.

All this happened some 440 million years ago and led to what is
known as the Ordovician extinction, the second most severe of the
planet’s five great periods of extinction.

“The prevailing theory for that extinction has been an ice
age,” said Adrian Melott, a University of Kansas astronomer.
“We think there is very good circumstantial evidence for a
gamma ray burst.”

Melott is the leader of a team, which includes some astronomers
from the National Aeronautics and Space Administration that
presented the theory yesterday at the national meeting of the
American Astronomical Society.

Fossil records for the Ordovician extinction show an abrupt
disappearance of two-thirds of all species on the planet. Those
records also show that an ice age that lasted more than a half
million years started during the same period.

Melott said a gamma ray burst would explain both phenomena.

He said a gamma ray beam striking the Earth would break up
molecules in the stratosphere, causing the formation of nitrous
oxide and other chemicals that would destroy the ozone layer and
shroud the planet in a brown smog.

“The sky would get brown, but there would be intense
ultraviolet radiation from the sun striking the surface.” he
said. The radiation would be at least 50 times above normal,
powerful enough to killed exposed life.

In a second effect, the brown smog would cause the Earth to
cool, triggering an ice age, Melott said.

The extinction “could have been a one-two punch,”
said Bruce Lieberman, a paleontologist at the University of Kansas
and a co-author of the theory. “Our theory builds on earlier
theories” that included an ice age.

Before the extinction, the Earth was unusually warm. Melott said
climate experts have been unable to find a model that would explain
the sudden onset of massive glaciers.

“They need something to jump start the ice age,” he
said. “The gamma ray burst could have done it.”

Jere Lipps, a paleobiologist at the University of California,
Berkeley, said gamma rays as a source of the Ordovician extinction
should be regarded as only one of several theories. “It is a
hypothesis that should be tested,” Lipps said.

He said the widely-accepted idea that the dinosaurs were wiped
out by an asteroid 65 million years ago started out as a
“wild idea,” but that it gained wide support after
other research.

Most of the life killed in the Ordovician extinction were
primitive sea creatures. Those that lived at or near the surface
would be greatest risk from the ultraviolet radiation. Melott the
species killed lived in shallow waters or reproduced with larvae
that spent part of their lives near the water surface. Animals
living in deep water were not harmed.

There were only primitive plants living on land, but they, too,
would have been affected, he said.

Melott said it is almost certain that Earth has been zapped by a
gamma ray several times in its 4.5 billion year history.

“You can expect a dangerous gamma ray burst every few
hundred million years,” he said. “It could happen
tomorrow or it could be millions of years.”

Supernovae, the source of gamma rays, usually leave behind
remnant clouds of dust, shock waves and black holes that can be
detected for millions of years. Melott said there is no known
evidence of such a nearby supernova, but that in 440 million years
the Milky Way would have rotated almost twice and traces of the
explosion could have been moved during that time.

The Ordovician was the first of five great extinction in
history.

The Devonian, 360 million years ago, killed 60 percent of all
species; the Permian-Triassic, 250 million years ago, killed 90
percent of all life; the late Triassic, 220 million years ago,
killed half of all species; and the Cretaceous-Tertiary event
destroyed the dinosaurs and half of all other species about 65
million years ago.