[meteorite-list] Tiny Meteorite Grains Help Settle An Astronomical Debate

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Sep 13 18:42:39 2004
Message-ID: <200409132242.PAA18395_at_zagami.jpl.nasa.gov>

http://www.carnegieinstitution.org/news_releases/News9-2-04.htm

Carnegie Institution of Washington
September 2, 2004

Contact-- Dr. Larry Nittler at Carnegie's Department of Terrestrial
Magnetism (DTM), 202-478-8460, email nittler_at_dtm.ciw.edu; Dr. Conel
Alexander at (DTM), 202-478- 8478, email Alexander_at_dtm.ciw.edu; or Dr.
Rhonda Stroud at the U.S. Naval Research Laboratory at 202-404-4143,
email stroud_at_nrl.navy.mil

For a copy of the paper contact the AAAS Office of Public Programs,
202-326-6440, or scipak_at_aaas.org

Tiny meteorite grains help settle an astronomical debate

Washington, D.C. "These tiny relics, a millionth of a meter small, could
point us to the first steps of dust formation in both old and young
stars," stated Dr. Larry Nittler of the Carnegie Institution's
Department of Terrestrial Magnetism. Nittler is co-author of a study
published in the September 3, 2004, issue of Science,* about the origin
of two presolar grains from the Tieschitz meteorite and the implications
they have for resolving observational and theoretical challenges of
dusty outflows surrounding asymptotic giant branch (AGB) stars - one of
the last evolutionary stages of low-mass stars like the Sun.

Both theoreticians and observational astronomers have long grappled with
the issue of whether aluminum oxide - which in its crystalline form is the
second hardest natural material - is the first solid to condense as hot,
gaseous winds from oxygen-rich AGB stars expand and cool. "Because AGB
stars are the most significant source of dust in the Milky Way galaxy,
determining how and in what form this dust condenses is important to
understanding how the chemical elements get cycled between stars and
interstellar space. Also, the first solids in cooling disks around new
stars form by analogous processes to those occurring around AGB stars,
so these grains give us a glimpse into the earliest stages of our own
solar system formation," said Nittler.

Observational astronomers have obtained telltale infrared spectra from
dusty AGB stars that have indicated the possible presence of two forms
of aluminum oxide - the crystalline form and an amorphous, or non
crystalline form. However, the data have not been precise enough to tell
if both forms are really present. "This study is really the first
definitive analysis that indicates that both forms are indeed produced
in AGB stars," said Professor Tom Bernatowicz of Washington University
in St. Louis.

The authors analyzed the ratios of oxygen and magnesium isotopes in the
grains in addition to their microstructures and chemical compositions.
Different isotopes of the same element are affected to differing degrees
by the nuclear reactions that power stars. The isotopic analysis
indicated that the grains originated in AGB stars and did not undergo
further processing as they made their way through time ultimately to
become part of the dusty cloud from which the solar system formed 4.6
billion years ago. However, their structures are very different, as are
their chemical compositions. One is a single-crystal of the most common
form of aluminum oxide - called corundum - while the other does not exhibit
a crystalline structure. The corundum grain has small, but measurable,
amounts of titanium impurities as well. The evidence clarifies
observations suggesting that the two different forms of aluminum oxide
are made in AGB outflows. It is also vital to the refinement of
condensation modeling and the understanding of how dust originates in
the universe.

---------------------

The authors of this paper are Rhonda Stroud of the Naval Research
Laboratory (NRL), Washington, DC; Larry Nittler and Conel M. O'D.
Alexander both of the Department of Terrestrial Magnetism, Carnegie
Institution of Washington, Washington, DC. The project received funding
from the National Aeronautics and Space Administration, and the Office
of Naval Research of the NRL.

The Carnegie Institution of Washington (www.CarnegieInstitution.org) has
been a pioneering force in basic scientific research since 1902. It is a
private, nonprofit organization with six research departments in the
U.S.: Embryology, in Baltimore, MD; the Department of Terrestrial
Magnetism and the Geophysical Laboratory in Washington, DC; The
Observatories in Pasadena, CA, and Chile; and Plant Biology and Global
Ecology in Stanford, CA.
 
Received on Mon 13 Sep 2004 06:42:36 PM PDT


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