[meteorite-list] Interplanetary Dust Particles: Reproducing GEMS-Like Structure in the Laboratory

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Feb 14 11:40:04 2006
Message-ID: <200602141638.k1EGc0605907_at_zagami.jpl.nasa.gov>

http://www.physorg.com/news10849.html

Interplanetary dust particles: reproducing GEMS-like structure in the laboratory
physorg.com
February 14, 2006

In a coming issue, Astronomy & Astrophysics presents new laboratory
results that provide some important clues to the possible origins of
exotic mineral grains in interplanetary dust.

Studying interplanetary grains is currently a hot topic within the
framework of the NASA Stardust mission, which recently brought back some
samples of these grains. They are among the most primitive material ever
collected. Their study will lead to a better understanding of the
formation and evolution of our Solar System.

Through dedicated laboratory experiments aimed at simulating the
possible evolution of cosmic materials in space, C. Davoisne and her
colleagues explored the origin of the so-called GEMS (glass with
embedded metal and sulphides). GEMS is a major component of the
primitive interplanetary dust particles (IDPs). They are a few 100 nm in
size and are composed of a silicate glass that includes small, rounded
grains of iron/nickel and metal sulphide (Figure 1).

A small fraction of the GEMS (less than 5%) have presolar composition
and could therefore have an interstellar origin. The remainder have
solar composition and may have been formed or processed in the early
Solar System. The varied compositions of the GEMS make it difficult to
arrive at a consensus regarding their origin and formation process.

The team first postulates that the GEMS precursors originated in the
interstellar medium and were progressively heated in the protosolar
nebula. To test the validity of this hypothesis a joint experimental
project involving two French laboratories, the Laboratoire de Structure
et Propri?t?s de l?Etat Solide (LSPES) in Lille and the Institut
d?Astrophysique Spatiale (IAS) in Orsay, was set up. Z. Djouadi, at the
IAS, heated various amorphous samples of olivine ((Mg,Fe)2SiO4) under
high vacuum and at temperatures ranging from 500 to 750?C. After
heating, the samples show microstructures that closely resemble those of
the GEMS, with rounded iron nanograins that are seen to be embedded in a
silicate glass (Figure 2).

This is the first time that a GEMS-like structure has been reproduced by
laboratory experiments. There, they show that the iron oxide (FeO)
component of the amorphous silicates has undergone a chemical reaction
known as reduction, in which the iron gains electrons and releases its
oxygen, to precipitate in a metallic form. Since the GEMS component in
IDPs is usually closely associated with carbonaceous matter, the
reaction FeO + C --> Fe + CO will be at the source of the metallic iron
nanograins in these IDP?s. Such conditions may have been encountered in
the primitive solar nebula. This reaction has been known of for
centuries by metallurgists, but the originality of the LSPES/IAS
approach is the application of material science concepts to extreme
astrophysical environments.

In addition, the scientists found that, in the heated sample,
practically no iron remains in the silicate glass, since all the iron
has migrated into the metal grains. The team is thus able to explain why
the dust observed around evolved stars and in comets is mainly composed
of magnesium-rich silicates where iron is apparently lacking. Indeed,
iron in metallic spherules becomes totally undetectable by the usual
remote spectroscopic techniques. This work could therefore provide an
important and new insight into the composition of interstellar grains as
well.

The team shows that GEMS could form through a specific heating process
that would affect grains of various origins. The process may be very
common and could occur both in the Solar System and around other stars.
The GEMS could thus have diverse origins. Scientists now eagerly await
the analysis of grains collected by Stardust to find out for certain
that some GEMS truly come from the interstellar medium.

Source: Journal Astronomy and Astrophysics
Received on Tue 14 Feb 2006 11:37:58 AM PST