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Until recently it was believed the iron projectile that caused Arizona�s 1.25 kilometre-wide Meteor Crater was travelling at 15 - 20 kilometres a second. An impact of that speed should have caused the iron to melt, but recovered iron fragments show no evidence of that.
"It's probably the most studied impact crater on Earth," said Jay Melosh of the University of Arizona, USA, and lead researcher for the study. "We were astonished to discover something entirely unexpected about how it formed."
The new findings, published in Nature, are the result of a computer simulation of the impact. "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 Melosh.
"Even though iron is very strong, the meteorite had probably been cracked from collisions in space. The weakened pieces began to come apart and shower down from about 8.5 miles (14 kilometres) high. And as they came apart, atmospheric drag slowed them down, increasing the forces that crushed them so that they crumbled and slowed more."
The cloud of fragments would have spread out over about 200 metres, with a 20 metre wide, dense �swarm� or block of material in the centre. The simulation shows that the object hit the ground at a much slower 11 kilometres a second. The 20-metre swarm, or block, would have made the Meteor Crater. Smaller, unmelted fragments would have produced little craters in the surrounding area.
Melosh added: �It fits with the observation that there are meteorites scattered all around Meteor Crater out to a distance of 10 kilometres�
More info: Nature
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