Scientists discover that most of the asteroid that formed Meteor Crater was shock melted

July 1, 1999

Contact:

Elisabetta Pierazzo
520-626-5065
betty@lpl.arizona.edu

TUCSON, ARIZ. ? Most of the asteroid that blasted Meteor Crater out of
the Colorado Plateau melted, according to new evidence released today by
an international team of scientists. This new finding contradicts a
previously held theory that the Canyon Diablo meteor vaporized and gives
a glimpse of what happens when similar-sized meteors slam into Earth
every 6,000 years or so.

Meteor Crater, near Winslow, Ariz., the best-preserved impact crater in
the world, was formed 50,000 years ago -- just yesterday on the
geological time scale. Although modest by geological standards -- the
equivalent of a 20-to-40 megaton bomb -- it grabs our attention because
of its close proximity to our own time and for the story it tells about
what could happen again.

The bowl-shaped depression measures 1.2 kilometers (four-fifths of a
mile) wide and 180 meters (570 feet) deep and scientists say events like
this occur every 1,600 years, with a Canyon-Diablo-sized meteor slamming
into a land mass every 6,000 years.

In research published today (July 2) in Science, scientists conclude
that more than four-fifths of the Earth-crossing asteroid completely
melted and spread over the Four Corners Region where Colorado, Arizona,
New Mexico and Utah meet. Most of the iron asteroid, which was 30 meters
(100 feet) or more in diameter, spread as an enormous expansion plume
produced by gases released from Colorado Plateau limestone. A fraction
of the melted material survived to form sand-grain-sized particles
called "spheroids."

By using complex measurements of radioactive nickel 59 and computer
modeling, the researchers determined the probable depth within the
asteroid at which these spheroids were formed. Their experimental
measurements and modeling results indicate that Canyon Diablo was
travelling faster on impact that previously believed.

The scientists include faculty members from Rutgers University, The
University of Arizona in Tucson, Australian National University,
University of Rhode Island and University of California-Berkeley.

Keith Fifield of the Australian National University, led the team in
systematically measuring long-lived radioisotope nickel 59 in Canyon
Diablo meteorites and spheroids. Nickel 59 is a "cosmogenic nuclide"
produced in space when cosmic rays penetrate objects containing nickel
58. Nickel 58 changes to nickel 59 by absorbing an extra neutron from
cosmic radiation. Fifield used accelerator mass spectrometry to make the
measurements.

Canyon Diablo meteorites contain seven times more nickel 59 than do
recovered spheroids, meaning they had come from the surface or outer
shell of the asteroid, where exposure to cosmic radiation is greatest,
said Greg Herzog of Rutgers University.

Scientists find nickel 59 to be a far more useful cosmogenic nuclide for
such analysis than some more commonly used ones. That's because of the
mechanism by which it forms, its long half-life (76,000 years), its low
volatility and its resistance to weathering, team members add.

Elisabetta Pierazzo, a post-doctoral researcher at the UA Lunar and
Planetary Laboratory, used numerical models to simulate the impact. The
simulation, based on models developed at Sandia National Laboratories,
factored in the size and composition of Canyon Diablo and its target.
Pierazzo determined which parts of the Earth-smashing asteroid remained
solid and which melted and became spheroids. This was done by using
experimentally measured shock pressure values for melting iron/nickel
alloys. The composition of these alloys is close to that of meteorites.

The team concludes that the precursor material of the spheroids probably
came from depths of 1.3 to 1.6 meters (four to five feet) beneath the
surface of the meteor before it entered Earth's atmosphere.

Pierazzo says that only about 15 percent of the rear, outer part of the
asteroid remained solid after impact and that the other 85 percent of
the projectile melted. She bases this conclusion on combined
observational, experimental and theoretical evidence.

Impact velocities by Earth-crossing asteroids average around 15 to 20
kilometers per second. The 20 km/s velocity -- or 45,000 mph -- would
produce a melting profile that agrees with the experimental
measurements, she said. At lower velocities, a much larger fraction of
the projectile would have remained solid, leaving behind far more
meteorites.

"The model really makes sense when you match it with the hard evidence,"
Pierazzo said. "The modeling confirms the experimental results that say
the Canyon Diablo meteorites came from the outer part of the projectile,
and the spheroids from a depth of 1.5 to 2 meters below the surface.

"I feel confident that this impact was at higher velocity than many
people have believed it to be," she added. "This work gives no evidence
for vaporization. From what we know about shock pressure, melting and
vaporization of iron, the model indicates little or no vaporization of
the impact."

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