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Dust from comet Wild 2: Interpreting particle
size, shape, structure and composition from impact features
on the Stardust aluminum foils.
Kearsley A. T., Borg J., Graham G. A., Burchell M. J.,
Cole M. J., Leroux H., Bridges J. C., Hörz F.,
Wozniakiewicz P. J., Bland P. A., Bradley J. P., Dai Z. R.,
Teslich N., See T., Hoppe P., Heck P. R., Huth J.,
Stadermann F. J., Floss C., Marhas K., Stephan T., and
Leitner J. (2008)
Meteorit. Planet. Sci., in press.
ABSTRACT
Aluminum foils of the Stardust cometary dust collector
are peppered with impact features of a wide range of sizes
and shapes. By comparison to laboratory shots of known
particle dimensions and density, using the same velocity and
incidence geometry as the Stardust Wild 2 encounter, we can
derive size and mass of the cometary dust grains. Using
scanning electron microscopy of foil samples (both flown on
the mission and impacted in the laboratory) we have
recognised a range of impact feature shapes, from which we
interpret particle density and internal structure. We have
documented composition of crater residues, including
stoichiometric material in 3 of 7 larger craters, by energy
dispersive X-ray microanalysis. Wild 2 dust grains include
coarse (>10 µm) mafic silicate grains, some
dominated by a single mineral species of density around 3 -
4 g cm-3 (such as olivine). Other grains were
porous, low density aggregates from a few nanometers to 100
µm, with overall density that may be lower than 1
gcm-3, containing mixtures of silicates and
sulfides and possibly both alkali-rich and mafic glass. The
mineral assemblage is very similar to the most common
species reported from aerogel tracks. In one large aggregate
crater the combined, diverse residue composition is similar
to CI chondrites. The foils are a unique collecting
substrate, revealing that the most abundant Wild 2 dust
grains were of sub-micrometer size, and of complex internal
structure. Impact residues in Stardust foil craters will be
a valuable resource for future analyses of cometary
dust.
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