Gamma-Ray Burst Caused Mass Extinction?

Anne Minard
for National Geographic News
April 3, 2009
A brilliant burst of gamma rays may have caused a mass extinction event on Earth 440 million years ago—and a similar celestial catastrophe could happen again, according to a new study.

Most gamma-ray bursts are thought to be streams of high-energy radiation produced when the core of a very massive star collapses.

The new computer model shows that a gamma-ray burst aimed at Earth could deplete the ozone layer, cause acid rain, and initiate a round of global cooling from as far as 6,500 light-years away.

Such a disaster may have been responsible for the mass die-off of 70 percent of the marine creatures that thrived during the Ordovician period (488 to 443 million years ago), suggests study leader Brian Thomas, an astrophysicist at Washburn University in Kansas.

The simulation also shows that a significant gamma-ray burst is likely to go off within range of Earth every billion years or so, although the stream of radiation would have to be lined up just right to affect the planet.

Currently WR104, a massive star 8,000 light-years away in the constellation Sagittarius, is in position to be a potential threat, Thomas noted.

But the study, which has been submitted to the International Journal of Astrobiology, isn't necessarily sending other astrophysicists into a panic.

"There is certainly no harm in looking at what a gamma-ray burst might do if it were close enough to us, as this author has done. That's the way science works," said David Thompson, a NASA astrophysicist and deputy project director on the Fermi Gamma-ray Space Telescope.

But Thompson compares the risk to Earth from a future gamma-ray burst to "the danger I might face if I found a polar bear in my closet in Bowie, Maryland.

"It could happen, but it is so unlikely that it is not worth worrying about."

Lingering Damage

Study author Thomas' former graduate advisor, Adrian Melott, first proposed in 2004 that a gamma-ray burst near Earth wiped out Ordovician life. Since then, both researchers have been tackling pieces of the puzzle.

(Related: "Ancient Mass Extinctions Caused by Cosmic Radiation, Scientists Say.")

According to their newest models, gamma radiation from a nearby burst would quickly deplete much of Earth's protective ozone layer, allowing increased ultraviolet radiation (UV) from the sun to reach the surface.

In the longer term, chemical reactions in the atmosphere would produce dark, nitrogen-based gases that would block the sun's heat and trigger global cooling, even as the gamma rays continued to deplete ozone and let in UV rays, the authors suggest.

Some of the pollution would fall as damaging acid rain, which can severely disrupt ecosystems.

The atmosphere might be able to recover within a decade, and a rise in DNA damage caused by increased UV exposure might pass after a few months or years, the researchers note.

But other biological impacts—such as reduced ocean productivity—could linger for an unknown length of time, Thomas said.

The Trouble With Trilobites

Bruce Lieberman, a paleontologist at the University of Kansas, helped develop the initial theory about the Ordovician die-off but did not co-author the recent papers.

The prevailing idea is that an ice age caused the extinction event, he said, but he questions the completeness of that hypothesis.

"At other times there have been ice ages without mass extinctions," he said.

Furthermore, the ice age during the Ordovician was comparatively short, lasting only about 500,000 years before the climate cycled back to a warm spell—almost as if something unusual set the icy period in motion.

So far Thomas and Melott have uncovered a pattern of higher UV radiation during the Ordovician extinction that would match cosmic bombardment over the South Pole.

And Lieberman believes the disappearance of trilobites, extinct arthropods related to horseshoe crabs, could be tied to the Ordovician event.

Although most trilobites are mud-scurrying bottom dwellers, the juveniles of some species have a life stage that sends them floating in the shallow water column, making them vulnerable to higher UV radiation.

But like NASA's Thompson, Lieberman adds that worry over a future gamma-ray burst is "not the thing that's keeping me up at night."

Instead he appreciates the new work for pointing out that Earth is a vulnerable part of the cosmos.

"It gives us a new perspective on things like natural selection and adaptation."

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