Hour long hailstorm may have caused 1,000-year freeze, say scientists

Telegraph.co.uk – April 2, 2010

A new theory put forward by according to Professor Bill Napier, from the Cardiff University Astrobiology Centre suggests it occurred when the Earth strayed into a dense trail of fragments shed by a large comet.

Thousands of chunks of material from the comet would have rained down on Earth, each one releasing the energy of a one megaton nuclear bomb.

The impacts would have triggered wildfires covering whole continents, filling the atmosphere with smoke and soot and blotting out the Sun.

What caused a sudden cooling of up to 8C just as the Earth was warming at the end of the last ice age has puzzled scientists.

The change caused glaciers to re-advance and coincided with the rapid extinction of 35 families of North American mammals.

Evidence exists of an extraterrestrial event, such as an asteroid or comet impact.

Experts have found a ”black mat” of soot a few centimetres thick thought to be left by continental-scale wildfires.

Microscopic ”nanodiamonds” produced by massive shocks and only found in meteorites or impact craters have also been discovered dating back to the disaster.

These findings led to suggestions that an object from space four kilometres across smashed into the Laurentide ice sheet, which at the time covered what is now Canada and the northern US.

But the likelihood of the Earth being struck by such a large object only 13,000 years ago are a thousand to one against.

Also, a single impact cannot explain the occurrence of such widespread fires.

The fragmenting comet theory is more plausible, according to Professor Bill Napier, from the Cardiff University Astrobiology Centre.

He believes there is ”compelling evidence” to indicate that such a comet entered the inner solar system between 20,000 and 30,000 years ago and has been breaking apart ever since.

The destruction of the comet has given rise to a number of closely related meteor streams known as the Taurid Complex.

”A large comet has been disintegrating in the near-Earth environment for the past 20,000 to 30,000 years and running into thousands of fragments from this comet is a much more likely event than a single collision,” said Prof Napier.

His model, published in the journal Monthly Notices of the Royal Astronomical Society, suggests that the ”hailstorm” would have only lasted about an hour.

It would have caused thousands of impacts, generating global fires and depositing nanodiamonds at the ”extinction boundary” marking the point in time when many species died out.

One recent impact that may have come from the comet is known as the Tagish Lake meteorite, said Prof Napier.

The object fell on Yukon Territory in Canada in January 2000. It contained the largest amount of nanodiamonds of any meteorite studied so far.