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"Forensic" Evidence Of Dinosaur Killer



>From "The Guardian" December 3. URL:

http://go2.guardian.co.uk/science/881172751-dino.html


Fire from heaven

Tim Radford on a bullet hole and a smoking gun 


Scientists today offer forensic evidence for one of the great mass
murders of all time. They used seismic instruments to measure
ballistics more than a kilometre below the sediments of the Yucatan in
Mexico. They report in Nature today that whatever arrived from the
skies 65 million years ago to make the now-buried Chicxulub feature,
left behind a transient crater 100 kilometres across. 

The crater that survives is far bigger. But what interested Mike Warner
of Imperial College, London, and colleagues from Cambridge, Leicester,
Canada, Mexico and Texas, was the original hole in the ground. The one
that opened, deepened and then within minutes rebounded, hurling
superheated rock with such force that it left the planet altogether,
before falling back at unimaginable speed. It was the size before the
rebound that would answer questions about the projectile that did the
damage. 

The most famous disappearances from that date are the dinosaurs. It is
actually almost impossible to tie their disappearance to Chicxulub. It
is easier to connect the exit of plankton to the awful moment. "The
more common things are, the smaller they are, the easier it is to time
the extinction. If things are large and uncommon, like dinosaurs, then
it is very difficult to time accurately when they disappeared," says
Warner. 

It was an object of between 10 and 14 kilometres in diameter - the
height of the atmosphere. If it was an asteroid, it arrived at about
20km a second. If it was a comet, it would have hit the Earth at 65km a
second. Creatures in the sea would have been blotted out by clouds of
debris which screened the Sun, and stopped photosynthesis. Creatures on
land would have had a worse time. "You get a worldwide firestorm from
the fallout from the ejecta layer. That is equivalent to putting the
Earth?s surface under a grill or a broiler for several hours," says
Warner. "Little dust particles will do it. It?s not the heat of the
impact they take with them, it?s the heat of re-entry: they come in
cold and they hit the atmosphere. The whole sky is full of white-hot
shooting stars for several hours. It would cause even wet surface
vegetation to spontaneously ignite. 

"Had the dinosaurs still been alive - and that is open to debate - then
this impact would have killed them in most places on the Earth." 


03 December 1997