[meteorite-list] Celestial Fender-Bender Left Asteroid To Cool Without Insulation

From: MexicoDoug <MexicoDoug_at_meteoritecentral.com>
Date: Wed, 18 Apr 2007 17:01:52 -0500
Message-ID: <005e01c78205$2d75f120$facf5ec8_at_0019110394>

Note: Asteroid #6 of course is 6 Hebe, the most frequently mentioned
possible H chondrite Parent Body. Hebe is easy to see in binoculars. Much
dimmer and metallic 16 Psyche just passed opposition when it was ideal for
viewing last month and still can be seen if you have a medium scope. -Doug

http://www.sciencedaily.com/releases/2007/04/070418141027.htm

Celestial Fender-Bender Left Asteroid To Cool Without Insulation

Science Daily ? A fender-bender between two celestial bodies that left a 200
mile-wide metallic chunk to cool in space was the likely source of a group
of meteorites known as the IVA iron meteorites, suggests new research by
University of Massachusetts Amherst scientists. Their findings, published in
the April 19 issue of the journal Nature, help explain conflicting meteorite
data that has long puzzled scientists, and sheds new light on how and when
asteroids form.

(Image)
Scientists analyze certain patterns in meteorites, such as those in this
Carlton Meteorite, to obtain the cooling rates of asteroids. (Credit: J
Goldstein UMass Amherst, and H. Yakowitz, NIST)
Jijin Yang and Joseph Goldstein of the UMass Amherst department of
mechanical and industrial engineering, and Edward Scott of the Hawaii
Institute of Geophysics and Planetology at the University of Hawaii at Manoa
collaborated on the research.

The standard model of asteroid formation says asteroidal bodies are just
leftover debris from the collisions and subsequent melting that happens when
planets form. Scientists find that these leftover chunks typically have a
dense iron core containing nickel, surrounded by an insulating layer of
silicate. Evidence has suggested that the iron-nickel core cools relatively
evenly, thanks to the insulating silicate mantle.

But when researchers have calculated cooling rates for the 60-odd meteorites
that are known as the IVA iron meteorites (believed to have come from a
single parent asteroid), they?ve gotten wildly different numbers, says
Goldstein.

?We find that these cooling rates of the IVA irons vary by a factor of more
than 50 and directly with the nickel content of the iron meteorite,? he
says. ?This means there?s something goofy happening.?

Given the insulating silicate mantle, the cooling rates of the IVA irons
ought to have been very similar, he says. So Goldstein and his colleagues
re-calculated cooling rates for 10 IVA irons and combined the data with
computer model simulations. They also examined the microstructure of several
of the irons using a transmission electron microscope.

The IVA meteorites must have cooled as one, big chunk, roughly 200
miles-wide and without an insulating mantle, the scientists conclude, not in
the form of a smaller insulated body as had previously been thought. If
correct, the parent asteroid would have been comparable in size to the
largest M class asteroid, 6 Psyche, says the research team.

?You can see the same phenomenon occurring when cooling steel,? explains
Goldstein. ?If you take a new piece of steel out of a huge blast furnace and
set it down, we know that the outside cools a lot faster than the inside
because there?s no insulation. The same would be true of the IVA irons in a
metallic asteroid.?

Roughly 60 meteorites retrieved from around the world have the chemical
makeup of the IVA irons, suggesting that they were all part of one metallic
asteroid that broke up about 450 million years ago and then fell to earth in
pieces.

Scientists have proposed several theories over the decades to rationalize
the varying cooling rates seen in IVA meteorites. One is that either the
data or the computer simulations are faulty. There?s the ?Rubble-Pile?
model, which posits that various pieces of the asteroid broke off at some
point and then were thrown back together by influences such as gravity and
centrifugal force. Another model, the so-called ?Raisin Bread? effect,
explains the various cooling rates by picturing various metal chunks spread
throughout the silicate mantel of the asteroid. None however, could explain
why the cooling rates vary directly with the nickel content of the
meteorites.

Now the researchers think the IVA irons? parent asteroid must have formed
after two protoplanets sideswiped each other, thus breaking off many
different pieces with varying amounts of silicate. The authors believe that
the metal containing the IVA irons was one of these pieces that contained
little or no silicate insulation.

?Our study was created to understand how the asteroid was formed almost one
million years after the formation of the solar system,? says Yang. ?Our
theory explains the different cooling rates as part of a comprehensive
description of the formation of the asteroid containing the IVA irons.?

Note: This story has been adapted from a news release issued by University
of Massachusetts Amherst.
Received on Wed 18 Apr 2007 06:01:52 PM PDT