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Last Updated: Thursday, 10 March, 2005, 09:57 GMT
Meteor Crater formation revisited
Meteor Crater (Nasa)
The crater is probably the most studied impact site on Earth
The iron mass that smashed into Arizona some 49,000 years ago to create Meteor Crater was just the crumbled remains of a far larger rock body.

What is more, this shower of debris was moving much slower than researchers had previously thought.

The re-assessment is the work of Drs Jay Melosh and Gareth Collins.

The scientists tell Nature magazine that a re-modelling of the impact has thrown up new ideas about the 1.2km-wide hole in the ground.

"It's astonished us because we thought there was little left to say about Meteor Crater," Dr Melosh, from the University of Arizona, Tucson, told the BBC News website.

The 170m-deep depression is probably the most famous, visible space impact site on the planet.

It is certainly the most studied, and was the first terrestrial crater identified as a meteorite impact scar.

The swarm

Most researchers agree that the iron projectile that collided with the top of the Earth's atmosphere was about 40m across, and past estimates of the speed at which the mass was travelling when it smacked the ground range from 9.4 to 20km/s - with the faster figures recently considered more likely.

But Melosh and Collins have now used a new, simple model to show the original body slowed up enormously on hitting the atmosphere, and almost certainly broke up into a pancake-shaped cloud of clustered fragments.

It fits with the observation that there are meteorites scattered all around Meteor Crater out to a distance of 10km
Dr Jay Melosh
"The model that we used is actually calibrated on something like half a dozen known meteorite entry events in which we could see the iron meteorite broke up," explained Dr Melosh.

"Evidently, the iron mass in space is fragmented, probably by previous impacts that occurred throughout the history of the body, so it is already partially fractured before it hits the atmosphere."

The pancake, Melosh and Collins say, would have spread out over about 200m. At its core would have been a 20m-wide dense "swarm" of material, perhaps even a single block.

Good fit

It was this smaller swarm or fragment that dug out the hole when it hit the ground at the relatively gentler pace of 11km/s.

Meteor Crater taken from Landsat spacecraft (Nasa)
This Landsat 4 satellite image shows the lighter material thrown out on impact
"By becoming a disc or pancake, [the original projectile] becomes a blunt body and because of the increased drag, that increases the forces decelerating the fragments, which makes them crumble even faster and the whole thing is a kind of snowball effect."

It still leads to a violent event, however - releasing the equivalent force of 2.5 megatonnes of TNT, or more than 150 Hiroshima atom bombs - but even more energy was dissipated in an atmospheric blast.

The scientists say their calculations are borne out by the recovery of small, un-melted iron fragments around the crater and by the surprisingly small amount of rock melted by the impact.

"It fits with the observation that there are meteorites scattered all around Meteor Crater out to a distance of 10km that were ripped off in the upper atmosphere and blew beyond the borders of the pancake," said Dr Melosh.

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