[meteorite-list] New theory for formation of iron meteorites

From: tracy latimer <daistiho_at_meteoritecentral.com>
Date: Thu, 19 Apr 2007 05:30:31 +0000
Message-ID: <BAY115-W23781DAA9C34B3DDDC0744CA570_at_phx.gbl>

http://www.hawaii.edu/cgi-bin/uhnews?20070418141659A new origin for iron meteorites discovered


University of Hawaii at Manoa
Contact: Tara Hicks Johnson, (808) 956-3151School of Ocean and Earth Sciences and Technology
Web: http://www.psrd.hawaii.edu/April07/irons.html
Posted: April 18, 2007


HONOLULU - An article in the prestigious science
journal Nature, published this week, goes a long way toward resolving a
controversy about asteroids and meteorites that has raged in the
scientific community since the 1960s. What do key data about the
cooling rates of material inside these heavenly bodies indicate about
their makeup and formation, and the origins of planets?
Studies of Moon rocks show us that the rocky planets were formed by
giant collisions between planetary embryos 4.5 billion years ago that
lasted for 50 million years and ended with a mighty impact on the Earth
that created the Moon. Now a research team from the University of
Massachusetts Amherst (UMA) and the University of Hawaii (UH) has
discovered evidence in iron meteorites that shows that planetary
embryos, approximately 1000 km is size, formed less than one million
years after the birth of the solar system, and quickly began colliding.

Iron meteorites are widely thought to be pieces of once molten cores of
about 100 asteroids 5-200 kilometers in diameter that took millions of
years to cool and billions of years to collide and break up. But
according to researchers, Jijin Yang and Joseph Goldstein (UMA) and Ed
Scott (UH), this view is wrong. The molten cores, they argue, formed in
planetary embryos up to 1000 km or more in size that broke apart in
only a few million years -- long before they cooled.
Scientists have proposed several theories over the decades to
rationalize the diverse cooling rates of iron meteorites thought to
come from a single core. One is that either the data, or the computer
simulations using those data, are faulty. A second theory is that a
major impact fragmented the core and scrambled up the pieces. A third
idea is that the asteroid was never hot enough for a core to form so
that the metal chunks were spread throughout the silicate mantle.
Yang and his coauthors developed a new theory by figuring out how long
iron meteorites took to cool down. A set of iron meteorites from a
single molten asteroidal core enclosed by a rocky mantle would all cool
at the same rate because iron metal conducts heat much more rapidly
than rock. But two independent techniques showed that the set of irons
cooled at rates that differed by as much as a factor of 50. ?We
realized? said Scott, ?that the accepted explanation for the origin of
the iron meteorites must be wrong.?
After trying many different ideas, the researchers discovered that if a
metal body 300 km in diameter were to cool without any silicate mantle,
samples a few km from the surface would cool 50 times faster than those
near the center. But how could a metallic body of this size have formed
in the asteroid belt?
Until last year, there was no plausible answer to this question. But a
paper published by researchers at the University of California at Santa
Cruz showed that Moon-to-Mars sized planetary embryos didn?t just stick
together when they collided, as previously inferred. In half the
impacts, the smaller bodies would have been torn apart by glancing
collisions. Yang, Goldstein, and Scott deduced that a glancing
collision by a planetary embryo 1000 km or more across with a molten
core could have formed the 300 km metallic body in which the meteorites
?We used to think that iron meteorites come from asteroids that formed
and melted long after the unmelted meteorites called chondrites had
formed? said Scott. ?But the latest dating techniques show just the
reverse. Iron meteorites come from the first generation of bodies that
formed less than one million years after the solar system was created,
and the chondrites formed over the next few million years. Our work
therefore shows that planetary embryos 1000 km across formed in less
than one million years and that debris from these bodies survived in
the form of meteorites and asteroids.?
Another UH meteorite researcher at the Hawaii Institute of Geophysics
and Planetology, Sasha Krot, recently proposed that a group of
chondrites once thought to be the oldest meteorites may have formed in
a giant collision between planetary embryos over 4 million years after
the birth of the solar system. ?These are very exciting discoveries?
says Klaus Keil who heads the cosmochemistry research group at the
University of Hawaii. ?They have dramatically changed our understanding
of the first few million years of Solar System history.?

This work was funded by NASA.

Article Information: Yang, J., J. I. Goldstein, and E. R. D. Scott
(2007) Iron Meteorite Evidence for Early Formation and Catastrophic
Disruption of Protoplanets. Nature, v. 446, p. 888-891.
For more information, please also see the Planetary Science Research
Discoveries article http://www.psrd.hawaii.edu/April07/irons.html.

For interviews, please contact: Edward R.D. Scott,
Planetary Scientist, Hawai`i Institute of Geophysics and Planetology,
University of Hawai`i at Manoa. Email: escott at higp.hawaii.edu, Phone
Number: (808) 956-3955
University of Massachusetts, Amherst Contact: Joseph Goldstein,
Mechanical and Industrial Engineering Department, College of
Engineering (413) 545-2165, jigo at ecs.umass.edu

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