[meteorite-list] Rare Meteorites Rekindle Controversy Over Birth Of The Solar System

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:35 2004
Message-ID: <200103011656.IAA11416_at_zagami.jpl.nasa.gov>

CONTACT: Mark Shwartz, News Service (650) 723-9296;
         mshwartz_at_stanford.edu

COMMENT: Anders Meibom, Department of Geological and
         Environmental Sciences (650) 725-6536 or
         (650) 736-0752; meibom_at_pangea.stanford.edu

EDITORS: Anders Meibom`s study, ``A new astrophysical setting for
         chondrule formation,`` appears in the March 2 issue of Science
         magazine.

EMBARGOED until Thursday, March 1, at 2 p.m. U.S. Eastern Time

Rare meteorites rekindle controversy over birth of the solar system
By Mark Shwartz

A new meteorite study is rekindling a scientific debate over the creation of
our solar system.

The study, published in the March 2 issue of the journal Science, is based
on the microscopic analysis of two rare meteorites recently discovered in
Antarctica and Africa.

Most meteorites found on Earth are believed to be fragments of asteroids -
ancient rocks and that formed during the creation of the solar system about
4.56 billion years ago. Thousands of asteroids still orbit the Sun in the
asteroid belt between Mars and Jupiter, about 140 million miles from Earth.

"Asteroids and meteorites are solids that never got incorporated into the
planets. These objects have survived, unchanged, for 4.56 billion years,"
says physicist Anders Meibom, a postdoctoral fellow in the Stanford
Department of Geological and Environmental Sciences who co-authored the
Science study.

Chondrites and chondrules

Using electron microscopy and other laboratory techniques, Meibom and his
colleagues conducted a detailed chemical analysis of two chondrites -
primitive meteorites made up of thousands of tiny round particles called
chondrules.

"Chondrules are among the oldest objects in the solar system, dating back
to the birth of the Sun," says Meibom, "so when we look at chondrules,
we`re actually looking at the very first steps towards the creation of our
solar system."

Meibom points out that most chondrules are made of silicates and metals that
can only be produced at very high temperatures. Exactly how chondrules
formed in the early solar system is a hotly debated topic among scientists.

"The conventional view," notes Meibom, "is that chondrules started out as
dust balls in the asteroid belt region some 4.56 billion years ago. Today,
the asteroid belt is ultra-cold, but at that time, the temperature was just
below 700 degrees Fahrenheit. The dust balls melted after they were zapped
by quick bursts of lightning or shock waves, which briefly raised
temperatures to about 3000 degrees F."

According to this theory, as the melted particles cooled, they turned into
millimeter-size chondrules, which eventually clumped together to form larger
chondrites.

New theory

But in 1996, astronomer Frank Shu of the University of California proposed a
different theory based in part on dramatic images from the Hubble Space
Telescope, which - for the first time - allowed astronomers to witness the
actual birth of new stars elsewhere in the Milky Way.

The Hubble revealed that most young stars are created from enormous disks of
whirling gas and dust.

As the disk contracts, it rotates faster and faster, funneling tons of
interstellar dust toward the center, where temperatures reach 3000 degrees F
or more - hot enough to melt metal and vaporize most solids.

The rotating disk also produces enormous jets of gas capable of launching
debris far into space at speeds of hundreds of miles per second.

Using the Hubble images as a guide, Shu proposed that chondrules in our
solar system were created near the hot central disk of the newly emerging
Sun - not in the relatively cool asteroid belt hundreds of millions of miles
away.

According to Shu, dust particles were melted by the Sun, then launched into
space by powerful jets of gas and solar wind. While in flight, the molten
particles solidified into spherical chondrules, some of which landed in the
asteroid belt a few days later. Others ended up as the raw materials that
formed the Earth, Mars and the rest of the planets in our solar system.

According to Meibom, the March 2 chondrite study in Science magazine gives
Shu`s version of chondrule creation a tremendous boost.

"Our findings demonstrate that Frank Shu`s ideas are not just some
fantasy," he notes. "We now have actual rocks that provide hard numbers,
which fit very nicely into the general framework of Shu`s theory."

Rare meteorites

Meibom and his colleagues based their study on two rare meteorite specimens
- HH 237, a grapefruit-size chondrite recovered from the Hammadah al Hamra
region of north Africa; and QUE 94411, a walnut-size sample collected from
the Queen Alexander mountain range in Antarctica.

"Most chondrites are only seven to ten percent metal by volume, but these
two specimens are about 70 percent iron and nickel," says Meibom.

Microscopic analysis revealed that these iron-nickel compounds formed by
condensation from hot gas when the temperature was around 2500 degrees F.

"Because HH 237 and QUE 94411 contain pristine samples of condensed iron
and nickel, we were able to determine that these metal grains formed on a
time scale of a few days. Furthermore, the newly created metal grains must
have been transported out of their hot formation region very quickly.

"Shu`s model provides those kind of temperatures and time scales, and the
jets certainly provide a way to kick the grains out to much colder regions
of the solar nebula," adds Meibom.

"The scenario we are suggesting is that of a big blobs of hot gas rising
up through the disk - almost like bubbles in boiling spaghetti sauce. As the
gas bubbles rose and cooled, silicate and metal grains began to condense out
of the gas. When these grains got close enough to the surface of the disk,
they became trapped in the powerful jet streams. Days later, the particles
arrived in the asteroid belt, where the relatively cold temperatures
preserved them from destruction."

These chondrites allow us to look at the very frontier of the solar system,
concludes Meibom.

"For the first time, we`re really building a bridge between what we observe
in the meteorites and what astrophysicists like Shu are telling us."

Frank Shu agrees.

"In these two very special meteorites we finally have direct evidence that
certain portions of rock had to move from some place very hot to some place
very cold in a very short period of time," comments Shu. "This is a very
important study."

Meibom`s other collaborators in the Science study are Alexander N. Krot and
Klaus Keil of the University of Hawaii; Sara S. Russsell and Timothy E.
Jeffries of the Natural History Museum in London; and Conel M. O`D.
Alexander of the Carnegie Institution of Washington`s Department of
Terrestrial Magnetism.


Relevant Web URLs:
http://www.psrd.hawaii.edu/Sept00/primitiveFeNi.html
http://www.cwru.edu/affil/ansmet/
http://hubble.esa.int
Received on Thu 01 Mar 2001 11:56:18 AM PST