An analysis by scientists in the USA in 2005
discovered that a huge cosmic explosion could have caused a
mass extinction on Earth 450 million years ago.
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 sixty percent of marine invertebrates at a time at
which 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 US scientists judged that a ten-second burst near Earth could
deplete up to half the planet's ozone layer.
With the ozone layer devastated by the cosmic blast,
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 Earth in the past one billion years. They also 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
of Earth would have a devastating effect on life, according to co-author
of the report, Dr Adrian Melott, an astronomer at the University of
It isn't known when such a burst occurred, but the
scientists knew that one did occur - and left its mark. What's most
surprising is that just a ten second burst can cause years of
devastating ozone damage.