Life At Risk When Star Exploded?
|SWIFT instrument, amidst an artist's concept of a burst
Scientists at NASA and the University of Kansas 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. The scientists do not have direct evidence that such a burst activated the ancient extinction. The strength of their work is their atmospheric modeling -- essentially a "what if" scenario.
The scientists calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten 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 indeed rare, but the scientists 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. 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.
"A gamma-ray burst originating within 6,000 light years from Earth would have a devastating effect on life," said Dr. Adrian Melott of the Department of Physics and Astronomy at the University of Kansas, Lawrence. "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," Melott added.
|The changing intensity of a gamma-ray burst. On the left is an image of the gamma-ray sky showing the burst becoming the brightest object. On the right is a plot of the changing brightness with time. The first gamma-ray burst was seen in the year 1967 (although it was not reported to the world until 1973) by satellite-borne detectors intended to look for violations of the Nuclear Test Ban Treaty. Credit: BATSE
A scientific paper describing this finding appears in Astrophysical Journal Letters. The lead author is Brian Thomas, a Ph.D. candidate at University of Kansas.
Gamma-ray bursts are the most powerful explosions known. Most originate in distant galaxies, and a large percentage likely arise from explosions of stars over 15 times more massive than our Sun. A burst creates two oppositely-directed beams of gamma rays that race off into space.
Thomas says that a gamma-ray burst may have caused the Ordovician extinction 450 million years ago, killing 60 percent of all marine invertebrates. Life was largely confined to the sea, although there is evidence of primitive land plants during this period
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, with Dr. Charles Jackman of NASA's Goddard Space Flight Center in Greenbelt, Md., 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 (N2) into nitrogen atoms, which react with molecular oxygen (O2) to form nitric oxide (NO). NO will destroy ozone (O3) 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,researchers calculated the effect of ultraviolet radiation on life. Deep-sea creatures living several feet below water would be protected. Surface-dwelling plankton and other life near the surface, however, would not survive. Plankton is the foundation of the marine food chain.
Dr. Bruce Lieberman, a paleontologist at the University of Kansas, 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. However, gamma-ray burst could have caused a fast die-out early on and also could have triggered the significant drop in surface temperature on Earth.
|"Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck." -David Grinspoon
Image Credit: NASA
"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," he added.
The Swift mission, launched in November 2004, will help determine recent burst rates. The prevailing explanation for this intense burst of radiation is that a collapsing star has burnt out its mass of fusionable energy, and thus announces its inevitable collapse into a black hole with a last, large explosion.
About 1,000 GRBs occur daily, but Earth-based telescopes only are able to see perhaps one a day Because SWIFT will be outside the Earth's atmosphere, it will see the GRB events even if the sky is cloudy or it's the middle of the day.
Related Web Pages
SWIFT: Goddard Spaceflight Center (NASA)
Allen Telescope Array Capabilities
HabStars: Speeding Up in the Zone
The Use of Gamma-ray Bursts as Direction and Time Markers in SETI Strategies
Great Impact: Part I
Great Impact: Part II
Great Impact: Part III
Dinosaur Era Ended Instantly
Piecing Together a Permian Impact
Rotten Sulfur Brew, The Great Dying?
Methane: the Great Dying?