A huge cosmic explosion could have caused a mass extinction on
Earth 450 million years ago, according to an analysis by scientists
in the US.
A gamma ray burst could have caused the Ordovician extinction
(which occurred between 444-447 million years ago at the boundary
between the Ordovician Period and the subsequent Silurian Period),
killing 60% of marine invertebrates at a time when life was largely
confined to the sea.
This was the second largest of Earth's major extinction events
in terms of the percentage of life forms that became extinct.
Such cosmic blasts are the most powerful explosions in the
Universe, and the scientists think a 10-second burst near Earth
could deplete up to half of the planet's ozone layer.
With the ozone layer devastated, the Sun's ultraviolet radiation
could have killed off much of the life on land and near the surface
of oceans and lakes.
Gamma ray bursts are rare occurrences, but scientists estimate
that at least one must have occurred near the Earth in the past one
Scientists think that gamma-ray bursts are generated in two
principal scenarios. In one scenario, a star collapses in on itself,
giving birth to a black hole and releasing a high-energy jet of
material travelling at close to the speed of light.
The bursts could also be generated when two neutron stars
collide. "A gamma ray burst originating within 6,000 light-years
from Earth would have a devastating effect on life," said co-author
Dr Adrian Melott, an astronomer at the University of Kansas, US.
"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."