Posted by on April 12, 2005 at 09:41:17:
A huge blast of radiation from an exploding star might have been behind one of the Earth's worst mass extinctions, some 450m years ago.
In the latest issue of the Astrophysical Journal Letters, scientists argue that a gamma ray burst, the most powerful explosion that occurs in the universe, was responsible for the Ordovican mass extinction in which 60% of all marine invertebrates died.
Gamma ray bursts are thought to be caused either when two neutron stars collide or when giant stars collapse into black holes at the end of their lives.
For around 10 seconds, intense pulses of energy are fired off, which can be detected right across the universe. All the bursts recorded by astronomers so far have come from distant galaxies and are therefore harmless to the Earth.
But if a burst occurred in our own galaxy, the effect would be devastating.
Dr Adrian Melott, of the University of Kansas and an author of the latest paper, said: "A gamma ray burst originating within 6,000 light years from Earth would have a devastating effect on life.
"We don't know exactly when one came, but we're rather sure it did come - and left its mark."
Such a burst would strip the Earth of its protective ozone layer, allowing deadly ultraviolet radiation to pour down from the sun.
Computer models showed that up to half the ozone layer could be destroyed within weeks. Five years later, at least 10% would still be missing.
Using computer models, the researchers calculated that plankton and other life in the first few feet of the oceans would have been destroyed.
The knock-on effect would have been huge: plankton are at the bottom of the marine food chain providing for animals which are then preyed upon by larger species.
Previously, scientists thought that an ice age caused the Ordovican extinction. A gamma ray burst would have had a similar effect, causing a fast die-off early on and triggering a significant drop in surface temperature.
Astronomers are planning to launch a robot spacecraft to study the mysterious gamma ray bursts further.
Because the bursts happen suddenly and are so short, scientists have been lucky to detect one a month with instruments on Earth.
Next month, Nasa will launch the £138m Swift probe, which will sweep up to one sixth of the sky at a time, looking for sudden bursts. If all goes well, the probe could catch two three explosions a week.
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