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Stardust in STARDUST - the C, N, and O isotopic
compositions of Wild 2 cometary matter in Al foil
impacts.
Stadermann F. J., Hoppe P., Floss C., Heck P. R.,
Hörz F., Huth J., Kearsley A. T., Leitner J., Marhas K.
K., McKeegan K. D., and Stephan T. (2008)
Meteorit. Planet. Sci., in press.
ABSTRACT
In January 2006, the Stardust mission successfully
returned dust samples from the tail of comet 81P/Wild 2 in
two principal collection media, low density silica aerogel
and Al foil. While hypervelocity impacts at the Stardust
encounter velocity of 6.1 km/s into Al foils are generally
highly disruptive for natural, silicate-dominated impactors,
previous studies have shown that many craters retain
sufficient residue to allow a determination of the elemental
and isotopic compositions of the original projectile. We
have used two NanoSIMS ion microprobes to perform C, N, and
O isotope imaging measurements on four large (59 - 295
µm diameter) and on 47 small (0.32 - 1.9 µm
diameter) Al foil impact craters as part of the Stardust
Preliminary Examination. Most analyzed residues in and
around these craters are isotopically normal (solar) in
their C, N, and O isotopic compositions. However, the debris
in one large crater shows an average 15N enrichment of ~450
permil, which is similar to the bulk composition of some
isotopically primitive interplanetary dust particles and to
components of some primitive meteorites. A 250 nm grain in
another large crater has an 17O enrichment with
~2.65 times the solar 17O/16O ratio.
Such an O isotopic composition is typical for circumstellar
oxide or silicate grains from red giant or asymptotic giant
branch stars. The discovery of this circumstellar grain
clearly establishes that there is authentic 'stardust' in
the cometary samples returned by the Stardust mission.
However, the low apparent abundance of circumstellar grains
in Wild 2 samples and the preponderance of isotopically
normal material indicates that the cometary matter is a
diverse assemblage of presolar and solar system
materials.
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