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University Relations

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April 6, 2005
Contact: Adrian Melott, physics and astronomy, (785) 864-3037; or Dan Lara, University Relations, (785) 864-8855.

KU researchers say explosions in space may have caused extinction on Earth

LAWRENCE -- Researchers at the University of Kansas and NASA say that a mass extinction on Earth hundreds of millions of years ago could have been triggered by a star explosion called a gamma-ray burst. Although the researchers do not have direct evidence that a gamma-ray burst activated the ancient extinction, their work is based on atmospheric modeling.

Adrian Melott, KU professor of physics and astronomy; Brian Thomas, a Ph.D. candidate whom Melott advises; and Daniel Hogan, Leawood senior in physics, joined Charles Jackman of the NASA Goddard Space Flight Laboratory, Greenbelt, Md., in the discovery. A scientific paper describing their findings appears in Astrophysical Journal Letters. Thomas is the lead author of the paper.

The researchers calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the earth for only 10 seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years. With the ozone layer damaged, ultraviolet radiation from the sun could kill much of the life on land and near the surface of oceans and lakes, and disrupt the food chain.

Gamma-ray bursts in our Milky Way galaxy are rare, but the researchers estimate that at least one nearby likely hit the earth in the past billion years. Life on Earth is thought to have appeared at least 3.5 billion years ago.

"A gamma-ray burst originating within 6,000 light years from Earth would have a devastating effect on life," Melott said. "We don't know exactly when one came, but we're rather sure it did come -- and left its mark. What's most surprising is that just a 10-second burst can cause years of devastating ozone damage."

Gamma-ray bursts are the most powerful explosions known. Most originate in distant galaxies, and a large percentage likely arises from explosions of stars more than 15 times more massive than our sun. A burst creates two oppositely directed beams of gamma rays that race off into space.

A gamma-ray burst may have caused the Ordovician extinction 443 million years ago, killing 60 percent of all marine invertebrates, Thomas said. Life was largely confined to the sea, although there is evidence of primitive land plants during this period.

This research, supported by a NASA astrobiology grant, represents a thorough analysis of the "mass extinction" hypothesis first announced by members of this science team in September 2003. In the new work, the team used detailed computer models to calculate the effects of a nearby gamma-ray burst on the atmosphere and the consequences for life.

Thomas and Jackman calculated the effect of a nearby gamma-ray burst on the earth's atmosphere. Gamma rays, a high-energy form of light, can break molecular nitrogen into nitrogen atoms, which react with molecular oxygen to form nitric oxide (NO). NO will destroy ozone and produce nitrogen dioxide (NO2). NO2 will then react with atomic oxygen to reform NO. More NO means more ozone destruction. Computer models show that up to half the ozone layer is destroyed within weeks. Five years on, at least 10 percent is still destroyed.

Next, Thomas and Hogan calculated the effect of ultraviolet radiation on life. Deep-sea creatures living several feet below the water's surface would be protected. Surface-dwelling plankton and other life near the surface, however, would not survive. Plankton are the foundation of the marine food chain.

Bruce Lieberman, KU associate professor of geology, originated the idea that a gamma-ray burst specifically could have caused the great Ordovician extinction, 200 million years before the dinosaurs. An ice age is thought to have caused this extinction. But a gamma-ray burst could have caused a fast die-out early on and could have triggered the significant drop in surface temperature on Earth.

" One unknown variable is the rate of local gamma-ray bursts," Thomas said. "The bursts we detect today originated far away billions of years ago, before the earth formed. Among the billions of stars in our galaxy, there's a good chance that a massive one relatively nearby exploded and sent gamma rays our way."

The Swift mission, launched in November 2004, will help determine recent burst rates. Other team members are Claude Laird, project coordinator for the KU Center for Research, and Richard Stolarski, John Cannizzo and Neil Gehrels of NASA Goddard.

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Daniel Hogan is a Leawood senior in physics. He is the son of Patrick and Jane Hogan and is a Blue Valley North High School graduate.


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