Stardust impact analogs: Resolving pre- and postimpact mineralogy in Stardust Al foils

The grains returned by NASA’s Stardust mission from comet 81P/Wild 2 represent a valuable sample set that is significantly advancing our understanding of small solar system bodies. However, the grains were captured via impact at ?6.1 km s?1 and have experienced pressures and temperatures that caused alteration. To ensure correct interpretations of comet 81P/Wild 2 mineralogy, and therefore preaccretional or parent body processes, an understanding of the effects of capture is required. Using a two-stage light-gas gun, we recreated Stardust encounter conditions and generated a series of impact analogs for a range of minerals of cometary relevance into flight spare Al foils. Through analyses of both preimpact projectiles and postimpact analogs by transmission electron microscopy, we explore the impact processes occurring during capture and distinguish between those materials inherent to the impactor and those that are the product of capture. We review existing and present additional data on olivine, diopside, pyrrhotite, and pentlandite. We find that surviving crystalline material is observed in most single grain impactor residues. However, none is found in that of a relatively monodisperse aggregate. A variety of impact-generated components are observed in all samples. Al incorporation into melt-derived phases allows differentiation between melt and shock-induced phases. In single grain impactor residues, impact-generated phases largely retain original (nonvolatile) major element ratios. We conclude that both surviving and impact-generated phases in residues of single grain impactors provide valuable information regarding the mineralogy of the impacting grain whilst further studies are required to fully understand aggregate impacts and the role of subgrain interactions during impact.