[meteorite-list] new formation mechanism

From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Thu, 19 Apr 2007 19:45:57 -0700 (PDT)
Message-ID: <772698.30097.qm_at_web36912.mail.mud.yahoo.com>

Hi -

And just to think, only a few years ago I constantly
got reminded about McSween's Meteorites and Their
Parent Bodies whenever I brought the topic of a larger
parent body up.

Now we have the LPBE, that was 3.8 or 4.2 Gya, or
both?

good hunting all,
Ed

PS - is the Mohamed being held the Mohamed who was at
the party in Tucson, or someone else?


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
cooled.
?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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Received on Thu 19 Apr 2007 10:45:57 PM PDT