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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. 43(1/2), 299-313
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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