[meteorite-list] Unusual Meteorite Unlocks Treasure Trove of Solar System Secrets (Tagish Lake)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Sep 27 17:17:38 2005
Message-ID: <200509272116.j8RLGQ904704_at_zagami.jpl.nasa.gov>

http://www.newswise.com/articles/view/514887

Unusual Meteorite Unlocks Treasure Trove of Solar System Secrets
Florida State University
September 27, 2005

Description

An unusual meteorite that fell on a frozen lake in Canada five years ago
has led a Florida State University geochemist to a breakthrough in
understanding the origin of the chemical elements that make up our solar
system.


Newswise - An unusual meteorite that fell on a frozen lake in Canada
five years ago has led a Florida State University geochemist to a
breakthrough in understanding the origin of the chemical elements that
make up our solar system.

Professor Munir Humayun of the National High Magnetic Field Laboratory
and the geological sciences department at FSU and Alan Brandon of NASA
discovered an isotopic anomaly in the rare element osmium in primitive
meteorites. The anomalous osmium was derived from small stars with a
higher neutron density than that which formed our solar system. The
findings of the researchers, who also included colleagues from the
University of Maryland and Bern University in Switzerland, were recently
published in the journal Science.

"Our new data enabled us to catch a glimpse of the different star types
that contributed elements to the solar system, the parental stars of our
chemical matter," Humayun said. "It opens a treasure trove of prospects
for exploring the formation of the elements."

For about 50 years, scientists have known that all the elements beyond
iron in the periodic table were made in stars by up to three nuclear
processes. Osmium is mainly formed by two of those processes, the
so-called s-process in which neutrons are slowly added to nuclei over a
period of perhaps thousands of years in aging, medium-size stars and the
r-process that occurs in supernovae in which neutrons are pumped into
nuclei at a rate of hundreds of neutrons in a few seconds.

The new data gathered by Humayun's team not only shows the different
star types that contribute elements to the solar system, it also will be
used to test astrophysical models of production of the chemical elements
at a more sophisticated level than previously possible, he said.

Humayun and colleagues studied samples from an extremely fragile
meteorite that fell on Tagish Lake on Jan. 18, 2000. Unlike iron
meteorites, primitive meteorites like this one are not preserved long on
the Earth's surface because they disintegrate and form mud when exposed
to water. This one was retrieved within 48 hours of its fall in the dead
of an Arctic winter.

Most meteorites have a uniform osmium isotopic distribution, but
Humayun's team found that osmium extracted from the Tagish Lake
meteorite was deficient in s-process osmium. They are the first to
report an anomaly in the isotopic makeup of the element osmium from
meteorites.

Other researchers have found isotope anomalies in several other elements
in some primitive meteorites, but not in others. Because of the
disparity, scientists believed that the ashes of stars that preceded the
solar system must have been sprinkled in a non-uniform way into the
solar nebula, the disk of gas and dust that formed the sun, planets and
meteorites. Scientists had hypothesized that some of the dust could have
been created by an active nearby star.

Humayun's findings challenge that explanation. He believes that the
anomaly is an expression of presolar stardust that survived the
homogenization that affected nearly all other meteorites. Typically,
stardust accretes to form meteorites and is then heated by radioactivity
- a process that destroys the silicon carbide grains that are the
carriers of the anomaly. But in the case of the meteorites with osmium
isotopic anomalies, the heat was not significant enough to destroy the
silicon carbide.

"The previous interpretation of incomplete mixing of different sources
of dust at the scale of the solar nebula no longer seems tenable," he
said. "We now interpret those anomalies as incomplete dissolution of
silicon carbide grains that carried traces of molybdenum, ruthenium and
osmium. These anomalies reveal that the raw materials from which our
solar system was built are preserved in a few exceptional meteorites,
from which we can now recover the prehistory of our solar system."
Received on Tue 27 Sep 2005 05:16:26 PM PDT