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).

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Contact Information

H. Jay Melosh 520-621-2806
Gareth Collins

Related Web sites
Impact Effects Calculator
http://www.lpl.arizona.edu/impacteffects

SIC Meteor Crater Web page
http://www.lpl.arizona.edu/SIC/impac...g/Enviropages/
Barringer/barringerstartpage.html
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"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