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News Services
University of Arizona

Elisabetta Pierazzo

Lori Stiles, News Services
Tel: 520-621-1877   FAX: 520-626-4121

July 1, 1999

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

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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