Exploding star behind extinction, theory suggests

January 03, 1995|By Chicago Tribune

CHICAGO -- Scientists long have been puzzled by "mass extinctions" -- several devastating moments in the history of life on Earth when great numbers of species suddenly died off.

Although the riddle of the dinosaur's demise has been plausibly unraveled -- most scientists believe a giant meteor crashed into Earth, upsetting its climate -- other, even more dramatic mass extinctions have remained unexplained.

Now comes a new theory: A distant exploding star burned off Earth's protective ozone layer in a virtual flash-fire, allowing the unfiltered sun's light to flood the surface with deadly radiation.

Today, a University of Chicago astrophysicist and his colleague are publishing this new theory, which they believe takes a first step at providing an explanation for one or more perplexing mass extinctions.

One such event postulated by scientists could reign as one of Earth's greatest cataclysms: In a few ticks of the geologic clock, about 200 million years ago, 95 percent of Earth's species -- including giant, soaring insects and scuttling trilobites -- suddenly vanished.

The new theory, which synthesizes work from several fields, is the first to connect exploding stars -- called supernovae -- to the elimination of ozone, something that scientists only recently have learned could lead to catastrophe.

In fact, the theory borrows, in part, from work done in Antarctica showing the effects of low-level ozone depletions.

"What we've shown is that if a supernova blows up within about 30 light years of Earth it will have a much more dramatic effect on life than previously expected because it would eliminate the ozone layer," David Schramm said in an interview yesterday.

A paper written by Dr. Schramm and John Ellis, of the European Organization for Nuclear Research, will appear today in the Proceedings of the National Academy of Sciences.

Other scientists have explored the idea that supernovas might have been responsible for mass extinctions. But those theories suggested that the direct radiation from the supernova itself would kill off plants and animals. To do that, the exploding star would have to be as close as one or two light years away, which was highly improbable.

Drs. Schramm and Ellis, on the other hand, suggest that the supernova's deadly power would be indirect.

"The sun would be the thing that actually does the destruction of life on Earth, but it was the supernova that eliminated the filtering ozone," Dr. Schramm said.

That would allow the supernova to occur as far away as 30 light years, something the authors suggest could happen every 200 million years.

Scientists guess there have been at least five mass extinctions on Earth on an average of one every 120 million years or so.

The blast of energy from a supernova would deplete Earth's ozone layer through violent chemistry.

Radiation emitted by the explosion would shatter molecules of nitrogen in the upper atmosphere. The broken parts of the nitrogen molecules would then tear off atoms from neighboring ozone molecules and latch onto parts of them, leaving nitrogen oxide and oxygen where the ozone had been.

Unlike ozone, nitrogen oxide doesn't filter the sun's deadly ultraviolet radiation.

Drs. Schramm and Ellis say that could cause a die-off like the one that occurred more than 200 million years ago, when 95 percent of the species perished at the end of the Permian Period.

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