[meteorite-list] Meteor Crater formation revisited

From: kenoneill_at_kenoneill.com <kenoneill_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:55 2005
Message-ID: <000a01c52567$b71a9900$9600a8c0_at_VAIO>

Hi All,
An interesting article http://news.bbc.co.uk/1/hi/sci/tech/4333635.stm

Regards
Ken

Meteor Crater formation revisited
 
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 US 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 Arizona State University, 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.

"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.
 

"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.
Received on Thu 10 Mar 2005 06:52:48 AM PST