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Why are there so few presolar grains in samples from
comet Wild2?
Stadermann F. J., Floss C., Kearsley A. T., and Burchell M. J.
(2009)
Geochim. Cosmochim. Acta 73 (13, Suppl.1), A1262.
ABSTRACT
When the solar system formed some 4.6 Ga ago, it incorporated
presolar grains that had condensed around other stars and that escaped
the widespread homogenization in the solar nebula. Such grains are
still present at ppm-levels in various types of solar system materials
and can be identified on the basis of their anomalous isotopic
compositions, which reflect their stellar formation conditions. Since
presolar grains can easily be destroyed by a variety of parent body
alteration processes, relatively high abundances of such grains can
attest to the primitive nature of its host material (e.g.,
interplanetary dust particles, primitive meteorites).
Among the most perplexing results of the analysis of Wild 2 cometary
matter, which was returned by the Stardust probe, is the fact that it
contains large contributions of high-temperature minerals, but only
very few presolar grains. Both observations are in apparent conflict
with the expectations for this comet whose residence in the cold Kuiper
belt should have provided ideal conditions for the preservation of
primitive early solar system materials.
To evaluate whether the low abundance of presolar grains in Wild 2
material could be due to preferential destruction of such matter during
sample collection, we are now performing 6.1 km/s test shots of
pulverized meteoritic material with known presolar grain abundances
into Stardust analog collector foils. Early results indicate that there
may indeed be a larger loss of such grains than previously thought, but
it remains to be seen whether this can fully account for the observed
low abundance. Alternatively, the low abundance may be due to a
significant dilution with material from the inner solar system. Various
scenarios will be discussed.
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