First Silicate Stardust Found In A Meteorite

Contact: Tony Fitzpatrick

314-935-5272
Washington University in St. Louis

First silicate stardust found in a meteorite
March 4, 2004

Ann Nguyen chose a risky project for her graduate studies at Washington
University in St. Louis. A university team had already sifted through
100,000 grains from a meteorite to look for a particular type of stardust --
without success.

In 2000, Nguyen decided to try again. About 59,000 grains later, her gutsy
decision paid off. In the March 5 issue of Science, Ann Nguyen of Washington
University in St. Louis and her advisor, Ernst K. Zinner, Ph.D., research
professor of physics and of earth and planetary sciences, both in Arts &
Sciences, describe nine specks of silicate stardust -- presolar silicate
grains -- from one of the most primitive meteorites known.

"Finding presolar silicates in a meteorite tells us that the solar system
formed from gas and dust, some of which never got very hot, rather than from
a hot solar nebula," Zinner says. "Analyzing such grains provides
information about their stellar sources, nuclear processes in stars, and the
physical and chemical compositions of stellar atmospheres."

In 1987, Zinner and colleagues at Washington University and a group of
scientists at the University of Chicago found the first stardust in a
meteorite. Those presolar grains were specks of diamond and silicon carbide.
Although other types have since been discovered in meteorites, none were
made of silicate, a compound of silicon, oxygen and other elements such as
magnesium and iron.

"This was quite a mystery because we know, from astronomical spectra, that
silicate grains appear to be the most abundant type of oxygen-rich grain
made in stars," Nguyen says. "But until now, presolar silicate grains have
been isolated only from samples of interplanetary dust particles from
comets."

Our solar system formed from a cloud of gas and dust that were spewed into
space by exploding red giants and supernovae. Some of this dust formed
asteroids, and meteorites are fragments knocked off asteroids. Most of the
particles in meteorites resemble each other because dust from different
stars became homogenized in the inferno that shaped the solar system. Pure
samples of a few stars became trapped deep inside some meteorites, however.
Those grains that are oxygen-rich can be recognized by their unusual ratios
of oxygen isotopes.

Nguyen, a graduate student in earth and planetary sciences, analyzed about
59,000 grains from Acfer 094, a meteorite that was found in the Sahara in
1990. She separated the grains in water instead of with harsh chemicals,
which can destroy silicates. She also used a new type of ion probe called
the NanoSIMS (Secondary Ion Mass Spectrometer), which can resolve objects
smaller than a micrometer (one millionth of a meter).

Zinner and Frank Stadermann, Ph.D., senior research scientist in the
Laboratory for Space Sciences at the university, helped design and test the
NanoSIMS, which is made by CAMECA in Paris. At a cost of $2 million,
Washington University acquired the first instrument in the world in 2001.

Ion probes direct a beam of ions onto one spot on a sample. The beam
dislodges some of the sample's own atoms, some of which become ionized. This
secondary beam of ions enters a mass spectrometer that is set to detect a
particular isotope. Thus, ion probes can identify grains that have an
unusually high or low proportion of that isotope.

Unlike other ion probes, however, the NanoSIMS can detect five different
isotopes simultaneously. The beam can also travel automatically from spot to
spot so that many hundreds or thousands of grains can be analyzed in one
experimental setup. "The NanoSIMS was essential for this discovery," Zinner
says. "These presolar silicate grains are very small -- only a fraction of a
micrometer. The instrument's high spatial resolution and high sensitivity
made these measurements possible."

Using a primary beam of cesium ions, Nguyen painstakingly measured the
amounts of three oxygen isotopes -- 16O, 17O and 18O -- in each of the many
grains she studied. Nine grains, with diameters from 0.1 to 0.5 micrometers,
had unusual oxygen isotope ratios and were highly enriched in silicon. These
presolar silicate grains fell into four groups. Five grains were enriched in
17O and slightly depleted in 18O, suggesting that deep mixing in red giant
or asymptotic giant branch stars was responsible for their oxygen isotopic
compositions.

One grain was very depleted in 18O and therefore was likely produced in a
low-mass star when surface material descended into areas hot enough to
support nuclear reactions. Another was enriched in 16O, which is typical of
grains from stars that contain fewer elements heavier than helium than does
our sun. The final two grains were enriched in both 17O and 18O and so could
have come from supernovae or stars that are more enriched in elements
heavier than helium compared with our sun.

By obtaining energy dispersive x-ray spectra, Nguyen determined the likely
chemical composition of six of the presolar grains. There appear to be two
olivines and two pyroxenes, which contain mostly oxygen, magnesium, iron and
silicon but in differing ratios. The fifth is an aluminum-rich silicate, and
the sixth is enriched in oxygen and iron and could be glass with embedded
metal and sulfides.

The preponderance of iron-rich grains is surprising, Nguyen says, because
astronomical spectra have detected more magnesium-rich grains than iron-rich
grains in the atmospheres around stars. "It could be that iron was
incorporated into these grains when the solar system was being formed," she
explains.

This detailed information about stardust proves that space science can be
done in the laboratory, Zinner says. "Analyzing these small specks can give
us information, such as detailed isotopic ratios, that cannot be obtained by
the traditional techniques of astronomy," he adds.

Nguyen now plans to look at the ratios of silicon and magnesium isotopes in
the nine grains. She also wants to analyze other types of meteorites. "Acfer
094 is one of the most primitive meteorites that has been found," she says.
"So we would expect it to have the greatest abundance of presolar grains. By
looking at meteorites that have undergone more processing, we can learn more
about the events that can destroy those grains."

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