Interplanetary dust particles: reproducing GEMS-like structure in the laboratory (Forwarded)

Journal Astronomy & Astrophysics
Paris, France

Contact persons:

Science:

Dr. Louis d'Hendecourt
"Astrochimie Expérimentale" - Institut d'Astrophysique Spatiale
Campus d'Orsay, Bat 121
91405 Orsay, cedex - France
Phone: +33 1 69 85 86 40
Fax: +33 1 69 85 86 75

Dr. Hugues Leroux
Laboratoire de Structure et Propriétés de l'Etat Solide, Bat C6
Université des Sciences et Technologies de Lille
59655 Villeneuve d'Ascq - France
Phone: +33 3 20 33 64 16

Press office:

Dr. Jennifer Martin
Journal Astronomy & Astrophysics
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Phone: +33 1 43 29 05 41

Released: February 14th, 2006

Interplanetary dust particles: reproducing GEMS-like structure in the
laboratory

"The origin of GEMS in IDPs as deduced from microstructural evolution of
amorphous silicates with annealing", by C. Davoisne et al.

To be published in Astronomy & Astrophysics.

For the first time, a team of French scientists [1] were able to reproduce
the structure of the exotic GEMS in the laboratory. The results of their
experiments will soon be published in Astronomy & Astrophysics. GEMS
(glass with embedded metal and sulphides) is a major component of
primitive interplanetary dust. To understand its origin is one of the
primary objectives of planetary science, and especially of the recently
successful Stardust mission.

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
Propriétés 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.

[1] The team includes C. Davoisne, H. Leroux (from LSPES, Lille, France),
Z. Djouadi, L. d'Hendecourt, A. P. Jones and D. Deboffle (from IAS, Orsay,
France).

The origin of GEMS in IDPs as deduced from microstructural evolution of
amorphous silicates with annealing
By C. Davoisne, Z. Djouadi, H. Leroux, L. d'Hendecourt, A. P. Jones, and
D. Deboffle
To be published in Astronomy & Astrophysics volume 448, issue 1, pp. L1-L4
(DOI number: 10.1051/0004-6361:200600002)

Full article available in PDF format,

http://www.edpsciences.org/articles/aa/pdf/press-releases/PRAA200603.pdf

IMAGE CAPTIONS:

[Fig. 1:
http://www.edpsciences.org/papers/aa/abs/press-releases/PRaa200603/prhl144_fig1.jpg
(51KB)]
Image of a GEMS in an interplanetary dust particle. Copyright: NASA

[Fig. 2:
http://www.edpsciences.org/papers/aa/abs/press-releases/PRaa200603/prhl144_fig2.jpg
(47KB)]
Iron grain embedded in silicate glass.


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