Physical, Chemical, and Mineralogical Properties of Comet 81P/Wild 2 Particles Collected by Stardust |
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| Authors: | George James Flynn |
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| Institution: | (1) Department of Physics, State University of New York – Plattsburgh, 101 Broad St., Plattsburgh, NY 12901, USA |
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| Abstract: | NASA’s Stardust spacecraft collected dust from the coma of Comet 81P/Wild 2 by impact into aerogel capture cells or into Al-foils.
The first direct, laboratory measurement of the physical, chemical, and mineralogical properties of cometary dust grains ranging
from <10−15 to ∼10−4 g were made on this dust. Deposition of material along the entry tracks in aerogel and the presence of compound craters in
the Al-foils both indicate that many of the Wild 2 particles in the size range sampled by Stardust are weakly bound aggregates
of a diverse range of minerals. Mineralogical characterization of fragments extracted from tracks indicates that most tracks
were dominated by olivine, low-Ca pyroxene, or Fe-sulfides, although one track was dominated by refractory minerals similar
to Ca–Al inclusions in primitive meteorites. Minor mineral phases, including Cu–Fe-sulfide, Fe–Zn-sulfide, carbonate and metal
oxides, were found along some tracks. The high degree of variability of the element/Fe ratios for S, Ca, Ti, Cr, Mn, Ni, Cu,
Zn, and Ga among the 23 tracks from aerogel capture cells analyzed during Stardust Preliminary Examination is consistent with
the mineralogical variability. This indicates Wild 2 particles have widely varying compositions at the largest size analyzed
(>10 μm). Because Stardust collected particles from several jets, sampling material from different regions of the interior
of Wild 2, these particles are expected to be representative of the non-volatile component of the comet over the size range
sampled. Thus, the stream of particles associated with Comet Wild 2 contains individual grains of diverse elemental and mineralogical
compositions, some rich in Fe and S, some in Mg, and others in Ca and Al. The mean refractory element abundance pattern in
the Wild 2 particles that were examined is consistent with the CI meteorite pattern for Mg, Si, Cr, Fe, and Ni to 35%, and
for Ca, Ti and Mn to 60%, but S/Si and Fe/Si both show a statistically significant depletion from the CI values and the moderately
volatile elements Cu, Zn, Ga are enriched relative to CI. This elemental abundance pattern is similar to that in anhydrous,
porous interplanetary dust particles (IDPs), suggesting that, if Wild 2 dust preserves the original composition of the Solar
Nebula, the anhydrous, porous IDPs, not the CI meteorites, may best reflect the Solar Nebula abundances. This might be tested
by elemental composition measurements on cometary meteors. |
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| Keywords: | CI meteorites Wild 2 Comets Interplanetary dust particles IDPs Solar Nebula Meteor streams |
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