sun combo blamed for mass extinction
Thursday, January 8, 2004 Posted: 12:48 PM EST (1748 GMT)
-- 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 L. 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 on Wednesday at the national meeting of the American Astronomical
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 kill exposed
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
S. 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 H. 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. 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 gamma
rays 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
The Ordovician was the first of five great extinctions 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 Cretacious-Tertiary event destroyed the dinosaurs and half of
all other species about 65 million years ago.