- Scientists using satellites have mapped huge craters
under the Antarctic ice sheet caused by an asteroid as big as the one believed
to have wiped out the dinosaurs 65m years ago.
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- Professor Frans van der Hoeven, from Delft University
in the Netherlands, told the conference that the evidence showed that an
asteroid measuring between three and seven miles across had broken up in
the atmosphere and five large pieces had hit the Earth, creating multiple
craters over an area measuring 1,300 by 2,400 miles.
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- The effect would have been to melt all the ice in the
path of the pieces, as well as the crust underneath. The biggest single
strike caused a hole in the ice sheet roughly 200 by 200 miles, which would
have melted about 1% of the ice sheet, raising water levels worldwide by
60cm (2ft).
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- But the climatic conditions were different at the time
of the strike - about 780,000 years ago - from when the asteroid that is
believed to have wiped out the dinosaurs struck Yucatan in Mexico.
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- That impact created dust storms and fires that, by blocking
out the sun, cooled the Earth's atmosphere so much that the dinosaurs could
not survive. The Antarctica strike occurred during an ice age, so even
tidal waves would have been weakened to mere ripples by the calming effect
of icebergs on the ocean.
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- Prof Van der Hoeven first realised that there may have
been a giant asteroid strike in the Antarctic while on an expedition across
the continent in 1960 when he noticed severe anomalies in the gravity from
the rocks below, indicating a crater. By coincidence another scientist
had concluded that a giant event must have occurred around 780,000 years
ago somewhere in the southern hemisphere, probably Antarctica.
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- But it was not until this year, when two satellites operating
above Antarctica began to map the anomalies in the gravity, that the scale
of the crater emerged. The mapping showed that the holes in the rock created
by the strike had refilled with a mixture of ice, rock and other debris
far less dense. This material, called breccia, shows where and how deep
the craters are.
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- Prof Van der Hoeven said: "The extraordinary thing
about this meteor strike is that it appeared to do so little damage. Unlike
the dinosaur strike there is no telltale layer of dust that demonstrates
the history of the event. It may have damaged things and wiped out species
but there is no sign of it."
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- One thing that did happen at exactly the same time was
the reversing of the Earth's magnetic field. There is no other explanation
as to why this took place and Prof Van der Hoeven believes it was caused
by the impact.
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- Guardian Unlimited © Guardian Newspapers Limited
2004
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- http://www.guardian.co.uk/space/article/0,14493,1286033,00.html
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