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 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 NASA, that presented the theory Wednesday at the national meeting of the American Astronomical Society.
SEE FULL TEXT