Experimental Impact Craters in Aluminum Foils: Insights for Cometary Sample Return

Introduction: The aluminum foils wrapped around the sample tray assembly (STA) of the Stardust spacecraft will contain a record of both cometary and interstellar dust hypervelocity collisions in the form of craters [1-2]. While the primary goal of the cratering preliminary examination (PE) is to define the cometary particle flux [2], previous studies of exposed metallic foils in space have shown that substantial information on projectile composition can be obtained from the preserved residues [e.g. 3]. Coupled with recent advances in sample preparation and microanalysis techniques [4-6], detailed analysis of the impact residue preserved in the STA foils may contribute significantly to the overall understanding of the compositional properties of comet Wild-2. Here we summarize sample recovery methods and the analysis of experimental impact residues to support Stardust studies. Initial electron microscopy of the foils was carried out using a JEOL 5900LV SEM fitted with an Oxford Inca Energy Dispersive Spectrometer (EDS). Focused ion beam microscopy was performed using both an FEI FIB200 TEM workstation and a FEI Nova 600 DualBeam FIB-FESEM microscope. The FIB prepared TEM sections were recovered from the bulk material using both in-situ and ex-situ methods [8]. Analytical electron microscopy was performed using a 200 kV FEI Tecnai G2 F20 XT (S)TEM fitted with an EDAX EDS and FEI TIA spectral processing software. Ion Imaging: The distribution of residues within impact craters is not uniform therefore the critical first order task is to locate the material. Due to complex topography of the craters, the compositional contrast between the residue material and the aluminum substrate is often masked when using back-scattered electron imaging. This problem can be overcome by acquiring ion-induced secondary electron images that are surface sensitive and show an increase in material contrast (Fig. 2). At low magnifications and/or with low ion currents, there is negligible sample damage during ion image acquisition. However prolonged use may increase implantation of Ga generating additional unwanted peaks in the subsequent EDS analysis. Methods: A range of projectiles including sodalime glass, crushed meteoritic material and single composition mineral grains were used in laboratory simulations. The projectiles were accelerated into Stardust flight-grade aluminum foil (1100 series) targets by light gas guns using a buckshot technique [4-5,7] (Fig. 1). Impact velocities were within the range of the Stardust encounter with Comet Wild-2 of ~6 km/s. [4-5,7].