We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s−1 with a light‐gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less‐refractory Si3N4 grains. In small (<2 μm) craters that are formed by single grain impacts, the entire impacting crystalline grain is often preserved intact with minimal modification. While they also survive in crystalline form, grains at the bottom of larger craters (>5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI‐like minerals.

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft's encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet's fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events.

NASA's Stardust spacecraft flew through the coma of comet 81P/Wild 2 at 6.1 km/s and successfully returned to Earth with the first unambiguous cometary material. However, the collected cometary material experienced extensive alteration due to the high collection velocity [1]. Previous investigations of the Stardust collector foils have succeeded in returning surviving crystalline material [2,3], but further investigations are required to better characterize the comet's fine component.We imaged two Stardust foils (C2113N-B and C2118N-B) at 1k to 2.5k magnification with a Mira Tescan Scanning Electron Microscope (SEM), locating two craters on each foil with diameters between 1.5 and 4.8 μm. The crater residues were elementally characterized with energy dispersive x-ray spectroscopy (EDS) using Cliff-Lorimer analysis. Cross sections of these craters were extracted and thinned to 100-150 nm with a FEI Quanta 3D Focused Ion Beam (FIB) operated at 30kV, maximizing the preservation of cometary material for potential future isotope analyses. A final polish was performed with a FEI Helios operated at 8 kV. We obtained high-resolution images of the crater cross sections using a Nion UltraSTEM 200 aberration-corrected scanning transmission electron microscope (STEM), and STEM-EDS maps with the JEOL 2200 FS STEM, equipped with an Oxford Aztec SDD-EDS, at the Naval Research Laboratory.SEM images of the craters showed that three of the craters contained a single, rounded crater bottom suggestive of a compact impactor, whereas one crater contained a double indentation indicative of a complex aggregate grain of varying density. SEM-EDS analysis indicated that all four craters resulted from aggregate impactors that were dominated by Mg-and Si-rich materials coupled with iron sulfides. STEM imaging revealed narrow (10-100 nm) bands of residue blanketing the crater floors. Z-contrast STEM images, coupled with STEM-EDS mapping, showed heterogeneous melts of Si-, Mg-, and Ferich impactor material. EDS analysis of the residues showed Mg/Si ratios in the craters ranged from 1.42 ± 0.13 to 0.86 ± 0.07, and Fe/S ratios in the craters ranged from 1.75 ± 0.44 to 1.14 ± 0.15 (Fig. 1). Assuming a Fe/S ratio of 1 for iron sulfide, O/(Mg+Si+Fe) ratios in the residues ranged from 1.21 ± 0.25 to 2.13 ± 0.40, and (Mg + Fe)/Si ratios ranged from 1.01 ± 0.10 to 1.70 ± 0.18. Trace Ni was visible in three of the craters while trace Ca was only visible in a single crater.Previous investigations of Stardust analog impactors have shown that S-loss is common in high-velocity impacts [4,5], suggesting that much of the observed Fe originates from iron sulfides rather than from silicate impactors. Additionally, previous analog studies have also shown post-impact abundances of Mg, Si, and Fe to be minimally altered, with only small losses of O relative to these elements [4,6]. The elemental compositions of the impactors are consistent with Mg-rich pyroxene and olivine combined with troilite or pyrrhotite. High O abundances relative to Fe, Si, and Mg are lik...

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