[meteorite-list] Scientists Solve Mystery of Meteor Crater's Missing Melted Rocks

From: Gerald Flaherty <grf2_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:55 2005
Message-ID: <00a001c5250b$319d2f60$6401a8c0_at_Dell>

Thanks once more Ron. This List benifits soooo much from your participation.
It's like going to school and loving it(not just lunch and recess)!! Jerry
----- Original Message -----
From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Wednesday, March 09, 2005 1:32 PM
Subject: [meteorite-list] Scientists Solve Mystery of Meteor Crater's
Missing Melted Rocks


>
>
> SCIENTISTS SOLVE MYSTERY OF METEOR CRATER'S MISSING MELTED ROCKS
>>From Lori Stiles, UA News Services, 520-621-1877
> March 09, 2005
>
> Scientists have discovered why there isn't much impact-melted rock at
> Meteor
> Crater in northern Arizona.
>
> The iron meteorite that blasted out Meteor Crater almost 50,000 years ago
> was traveling much slower than has been assumed, University of Arizona
> Regents' Professor H. Jay Melosh and Gareth Collins of the Imperial
> College
> London report in the cover article of Nature (March 10).
>
> --------------------------------------------------------
> Contact Information
>
> H. Jay Melosh 520-621-2806 jmelosh_at_lpl.arizona.edu
> Gareth Collins g.collins_at_imperial.ac.uk
>
> Related Web sites
> Impact Effects Calculator
> http://www.lpl.arizona.edu/impacteffects
>
> SIC Meteor Crater Web page
> http://www.lpl.arizona.edu/SIC/impact_cratering/Enviropages/
> Barringer/barringerstartpage.html
> ----------------------------------------------------------
>
> "Meteor Crater was the first terrestrial crater identified as a meteorite
> impact scar, and it's probably the most studied impact crater on Earth,"
> Melosh said. "We were astonished to discover something entirely unexpected
> about how it formed."
>
> The meteorite smashed into the Colorado Plateau 40 miles east of where
> Flagstaff and 20 miles west of where Winslow have since been built,
> excavating a pit 570 feet deep and 4,100 feet across ? enough room for 20
> football fields.
>
> Previous research supposed that the meteorite hit the surface at a
> velocity
> between about 34,000 mph and 44,000 mph (15 km/sec and 20 km/sec).
>
> Melosh and Collins used their sophisticated mathematical models in
> analyzing how the meteorite would have broken up and decelerated as it
> plummeted down through the atmosphere.
>
> About half of the original 300,000 ton, 130-foot-diameter
> (40-meter-diameter) space rock would have fractured into pieces before it
> hit the ground, Melosh said. The other half would have remained intact and
> hit at about 26,800 mph (12 km/sec), he said.
>
> That velocity is almost four times faster than NASA's experimental X-43A
> scramjet -- the fastest aircraft flown -- and ten times faster than a
> bullet
> fired from the highest-velocity rifle, a 0.220 Swift cartridge rifle.
>
> But it's too slow to have melted much of the white Coconino formation in
> northern Arizona, solving a mystery that's stumped researchers for years.
>
> Scientists have tried to explain why there's not more melted rock at the
> crater by theorizing that water in the target rocks vaporized on impact,
> dispersing the melted rock into tiny droplets in the process. Or they've
> theorized that carbonates in the target rock exploded, vaporizing into
> carbon dioxide.
>
> "If the consequences of atmospheric entry are properly taken into account,
> there is no melt discrepancy at all," the authors wrote in Nature.
>
> "Earth's atmosphere is an effective but selective screen that prevents
> smaller meteoroids from hitting Earth's surface," Melosh said.
>
> When a meteorite hits the atmosphere, the pressure is like hitting a wall.
> Even strong iron meteorites, not just weaker stony meteorites, are
> affected.
>
> "Even though iron is very strong, the meteorite had probably been cracked
> from collisions in space," Melosh said. "The weakened pieces began to come
> apart and shower down from about eight-and-a-half miles (14 km) high. And
> as
> they came apart, atmospheric drag slowed them down, increasing the forces
> that crushed them so that they crumbled and slowed more."
>
> Melosh noted that mining engineer Daniel M. Barringer (1860-1929), for
> whom
> Meteor Crater is named, mapped chunks of the iron space rock weighing
> between a pound and a thousand pounds in a 6-mile-diameter circle around
> the
> crater. Those treasures have long since been hauled off and stashed in
> museums or private collections. But Melosh has a copy of the obscure paper
> and map that Barringer presented to the National Academy of Sciences in
> 1909.
>
> At about 3 miles (5 km) altitude, most of the mass of the meteorite was
> spread in a pancake shaped debris cloud roughly 650 feet (200 meters)
> across.
>
> The fragments released a total 6.5 megatons of energy between 9 miles (15
> km) altitude and the surface, Melosh said, most of it in an airblast near
> the surface, much like the tree-flattening airblast created by a meteorite
> at Tunguska, Siberia, in 1908.
>
> The intact half of the Meteor Crater meteorite exploded with at least 2.5
> megatons of energy on impact, or the equivalent of 2.5 tons of TNT.
>
> Elisabetta Pierazzo and Natasha Artemieva of the Planetary Science
> Institute in Tucson, Ariz., have independently modeled the Meteor Crater
> impact using Artemieva's Separated Fragment model. They find impact
> velocities similar to that which Melosh and Collins propose.
>
> Melosh and Collins began analyzing the Meteor Crater impact after running
> the numbers in their Web-based "impact effects" calculator, an online
> program they developed for the general public. The program tells users how
> an asteroid or comet collision will affect a particular location on Earth
> by
> calculating several environmental consequences of the impact. The program
> is
> online at http://www.lpl.arizona.edu/impacteffects
>
>
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Received on Wed 09 Mar 2005 07:50:44 PM PST