- An asteroid impact probably caused the largest mass extinction
in the history of Earth, according to new evidence published today.
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- An asteroid impact is already thought to have wiped out
the dinosaurs around 65 million years ago.
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- Now there is mounting evidence that an earlier impact,
around 250 million years ago, caused the most severe crisis for life in
the history of the planet.
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- Scientists had blamed a range of causes, from gradual
sea fall to climate change, dead oceans and massive volcanic eruptions
for this earlier mass extinction, at the end of what geologists call the
Permian period, which wiped out 90 per cent of all marine species and 70
per cent of land vertebrates.
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- Then, two years ago, Japanese geologists working in southern
China uncovered evidence that an asteroid or a comet smacked into our planet.
They found unusual isotope readings and iron particles that probably condensed
from the vapour of an impact cloud.
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- A team of American scientists discovered another extra-terrestrial
calling card in Meishan, China: football shaped carbon molecules (Buckyballs)
that contained gases with an unusual isotopic makeup.
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- Today, in the journal Science, another mineralogical
"signature" of an asteroid impact - dozens of unusual mineral
grains from two Antarctic rock samples - is described by an American group,
including two members of the team that found the Buckyballs.
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- Dr Asish Basu of the University of Rochester, New York
State, and colleagues found 40 mineral grains resembling meteorite fragments
in Antarctica that date to the end of the Permian period.
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- The rock layer also contains metallic pieces similar
to those from Asian sites of similar age.
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- According to Dr Basu, this now provides "clear evidence"
that the largest mass extinction in Earth's history was caused by a catastrophic
impact.
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- © Copyright of Telegraph Group Limited 2003.
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- http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2003/11/21/
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