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Gamma Ray Burst Responsible For Ordovician Extinction
Some 440 million years ago, a nearby gamma-ray burst may have extinguished
much of life on Earth, say US astronomers1.
Adrian Melott, of the University of Kansas in Lawrence, and colleagues
reckon that the fossil record of the end of the Ordovician period fits
with how such a cosmic explosion a few thousand light years away could
have altered the environment. At that time, more than 100 families of
marine invertebrates died out; it was the second most devastating mass
extinction in our planet's history.
Researchers have also suggested that supernovae - explosions of old stars
- could flood our planet with deadly radiation if they happen within
around 100 light years of us (our galaxy is 150,000 light years across).
This has been put forward as the cause of the mass extinction two million
Water would protect marine organisms from the heat of a GRB, but not from
its other effects, argues Melott's team. Its gamma-rays would convert some
nitrogen and oxygen in the atmosphere into nitrogen dioxide, the brownish
gas present in urban smog.
Nitrogen dioxide would filter out sunlight, turning the skies dark. The
cooling effect could trigger an ice age - there is evidence of widespread
glaciation 440 million years ago. Nitrogen oxides also cause acid rain and
destroy the ozone layer, exposing Earth to more of the Sun's harmful
Ultraviolet radiation can penetrate tens of metres of water, so it could
harm marine organisms at these depths. Indeed shallow-dwelling species, or
those that spend their early lives in shallow water, seem to have suffered
more than deep species in the Ordovician extinction.
In short, a nearby GRB might first have showered harmful radiation onto
the exposed face of the planet, killing more or less indiscriminately, and
may then have exposed the other hemisphere to increased ultraviolet
radiation, damaging marine life decreasingly with increasing depth.