July 1, 1999 NEW BRUNSWICK/PISCATAWAY, N.J. -- Researchers studying remains of the Canyon Diablo impactor have been able to describe the changing character of the meteoroid as it traversed Earth's atmosphere and hit its surface, ascertain how the remaining fragments were formed, and determine where within the body of the meteoroid the fragments originated.
Meteoroid refers to a natural object that moves through interplanetary space, as opposed to the term meteorite, which refers to such an object after it has fallen to Earth.
The team, which included Rutgers chemists Dr. Christoph Schnabel and Dr. Gregory Herzog together with colleagues in Arizona, California and Australia, used ultrasensitive measurements and computer modeling to gain insight into meteoroid dynamics.
The Canyon Diablo impactor was the object responsible for excavating Meteor Crater, the famous Arizona landmark. It struck the desert near Winslow, Ariz., some 50,000 years ago, producing a crater about 4,000 feet wide and 570 feet deep. This was the first crater on Earth to be identified as having been created by a meteoroid.
"The original meteoroid was thought to have been about 100 feet in diameter weighing approximately 60,000 tons, but little of it remains intact today," said Schnabel, a postdoctoral associate in the department of chemistry at Rutgers.
"Two types of material survive from the Canyon Diablo impactor – iron meteorites, which did not melt during the impact, and spheroids, which did," said Herzog, professor of chemistry with the Faculty of Arts and Sciences-New Brunswick. "Our challenge has been to determine the processes involved in the impact and the formation of the resulting products, specifically the spheroids -- millimeter-size fragments found in the soils around the crater."
In the July 2 issue of Science, the authors describe how they were able to deduce the original depth within the body of the meteoroid of the material that melted to form the spheroids. At the Australian National University, co-author Dr. L. Keith Fifield and his group employed accelerator mass spectrometry to analyze a rarely measured radioisotope of nickel (59Ni). Known as a cosmogenic nuclide, the 59Ni was produced by cosmic ray bombardment in the outer shell of the meteoroid while in space, and the relative concentration of this nuclide serves as a good indicator of depth of origin of the spheroid fragments.
The resulting depth figures were then compared with predictions from computer-modeled simulations of the impact that were carried out at the University of Arizona by another co-author, Dr. Elisabetta Pierazzo. Conclusions based on this comparison yielded new information about the dynamics of meteoroid strikes on Earth or other solid objects in the solar system, information that may be applied in general to medium-size meteoroids when they impact.
For example, the researchers were able to conclude that the trailing hemisphere of the meteoroid was the likely location for the molten material that gave rise to the spheroids. They further assert that material in the leading hemisphere of the meteoroid would more readily have mixed with and been lost in a large volume of rock at the impact site.
Four batches of spheroids had been analyzed with average masses in each group ranging from 1 to 10 mg. On average, the spheroids contained six to seven times less 59Ni than the meteorites. The 59Ni measurements yield evidence that the liquid material that formed the spheroids came from depths of 1.3 to 1.6 meters beneath the surface of the meteoroid.
The researchers also concluded that most of the spheroids did not form when atmospheric resistance to the incoming meteoroid melted surface material and blew molten droplets away, as had been previously held. Rather, computer numerical modeling of the meteoroid and its impact suggests explosive or shock melting of most of the object and dispersal of the spheroid fragments upon impact. They contend that little, if any, of the meteor vaporized. Moreover, the impact modeling suggests that the impact velocity of Canyon Diablo was higher than the velocity normally assumed for such an impact.
Full copies of the Science article, "Shock Melting of the Canyon Diablo Impactor: Constraints From Nickel-59 Contents and Numerical Modeling," can be obtained by contacting the American Association for the Advancement of Science at (202) 326-6440; fax (202) 789-0455; or e-mail firstname.lastname@example.org.
NOTE TO REPORTERS: Dr. Gregory Herzog can be reached at (732) 445-3955 for interviews Wednesday, June 30, through Friday, July 2, afternoons only.
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