Dust from comet Wild 2: Interpreting particle size, shape, structure, and composition from impact features on the Stardust aluminum foils

Abstract

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 (SEM) of foil samples (both flown on the mission and impacted in the laboratory) we have recognized 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 an overall density that may be lower than 1 g cm −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.

TOF‐SIMS analysis of crater residues from Wild 2 cometary particles on Stardust aluminum foil

“…Mn and especially Fe are strongly depleted in comparison to bulk CI chondritic values. The Mg/ Si ratio of 1.2, together with the absence of Ca and the very low Fe abundance (Table 2) indicates a high-Mg silicate, similar to enstatite, or, as concluded by Kearsley et al (2008), forsterite.…”

“…Thus, the low Si abundance observed in this study in the crater rim emphasizes the complex and heterogeneous structure of this impact feature. The main part of the silicates in this residue, as reported by Kearsley et al (2008) was not accessible by TOF-SIMS analysis and does not contribute to the elemental composition of the rim residue, generating the discrepancy between the TOF-SIMS and SEM-EDX results.…”

Identification of mineral impactors in hypervelocity impact craters in aluminum by Raman spectroscopy of residues

“…Also given in Table 1 are the particle sizes used along with the resultant crater sizes. These sizes are large compared to most of the craters that impacted the Stardust spacecraft; however, 63 craters larger than 20 μm in diameter have been identified so far, seven of which (sized 44-142 μm in diameter) have been studied in detail (Kearsley et al 2008).…”

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

“…The first order criteria (levels 0-2) are that the shape of the identified feature must be consistent with hypervelocity impact, and the captured particle or particle residue must have a composition that is consistent with formation in space, and inconsistent with spacecraft materials, or aerogel impurities. The trajectory of the particle is taken into consideration for the samples collected in aerogel, but not for the foils, because crater shapes depend strongly on the particle shape and composition, in addition to trajectory (22). The most definitive indication of an interstellar origin (level 3) for a particular particle would be an oxygen isotope composition inconsistent with solar system values.…”