Asteroids destroyed earth life twice

A GIANT asteroid decimated life on Earth not once but twice, according to new geological evidence published yesterday.

Scientists say the findings suggest that a space rock smashing into the Earth accounted for the biggest mass extinction the world has ever known.

Over 90% of all marine life and about 70% of land animals and plants vanished at the boundary between the Permian and Triassic periods 251 million years ago. The catastrophe cleared the way for the rise of the dinosaurs and mammals. Scientists are fairly certain that an asteroid impact wiped out the dinosaurs at the Cretaceous-Tertiary (K-T) boundary 65 million years ago.

But the cause of the Permian-Triassic (P-T) extinction is much more controversial. Theories to explain the event have included sea-level changes, global cooling, volcanic eruptions, as well as an impact from space. Yesterday, a team headed by Asish Basu, from the University of Rochester, New York, claimed to have found new mineralogical evidence pointing to a P-T asteroid impact. It consists of dozens of unusual mineral grains from two Antarctic rock samples dating from this period.

The grains appear to be fragments from a "chondritic" meteorite a particular type of largely metallic asteroid.

The rock layer also contained microscopic bits of almost pure metallic iron, similar to those from Asian sites of a similar age.