Micro-craters in aluminum foils: Implications for dust particles from comet Wild 2 on NASA's Stardust spacecraft

Abstract

a b s t r a c tDust impacts on aluminum foils during encounter of comet 81P/Wild 2 by the Stardust spacecraft in January 2004 have been simulated using spherical projectiles of monodispersive polymer, glass, and metals, and polydispersive mineral powders of diverse grain shapes. The encounter speed of the cometary particles was a constant and modest 6.1 km s À1 , well within the capabilities of light gas guns, permitting high fidelity experiments to infer dust size, density, and mass from quantitative dimensional analysis of both natural and experimental impact features. Specific interest focused on exceptionally small impactors, all <100 mm and some as small as 1.5 mm. To simulate the compound shape of many Stardust craters required novel, artificial aggregate projectiles of heterogeneous mass distribution. We demonstrate that the dimensional scaling obtained previously for millimeter sized impactors extends to particles as small as 10 mm at 6.1 km s À1 , all yielding a constant relationship for spherical soda lime glass projectiles of diameter (Dp) to crater diameter (Dc) in Al1100 of Dc ¼ 4.6 Dp; however, this ratio seems to decrease for projectiles (10 mm. The overwhelming majority of the Stardust craters are <20 mm in diameter, and substantial challenges remain in quantifying the exact size-frequency distribution of the Wild 2 comet dust. Nevertheless, the current experiments provide improved insights into some of the particles' physical properties.

Interpretation of Wild 2 dust fine structure: Comparison of Stardust aluminum foil craters to the three‐dimensional shape of experimental impacts by artificial aggregate particles and meteorite powders

“…(0.26 to 0.97; Fig. 21 of Kearsley et al 2008b), suggesting that projectile shape factor (aspect ratio) must be very high for major influence. Where particle shape is more equant, impactor density seems to control average depth/diameter , with higher density particles (e.g., steel) creating relatively deeper craters than, for example, low density polymer grains.…”

“…1). The De/Di values were compared to experimental data for mineral projectiles of known density, from Kearsley et al (2008b) and new data presented below. To exclude impact features created by gun debris, and to positively identify impacts by complex polymineralic aggregates, X-ray mapping and spectrum acquisition were performed using the SmartMap routines of the Oxford Instruments INCA system fitted to the SEM.…”

“…Six types of dust analogue aggregates were produced from mineral powders selected and prepared from the mineral projectile collection of the NHM, using variants of the adhesive spray droplet impregnation method described by Kearsley et al (2008b). Samples of the aggregates were examined by SEM to characterize textural and compositional internal heterogeneity and particularly to assess relative volumetric proportions of their components, and bulk aggregate porosity and density.…”

“…The shots of olivine, enstatite and polymineralic aggregates onto Al 6000 series each yielded a few small compound features. Some of the polymineralic impactor craters contained residue derived from more than one mineral component, as illustrated in Kearsley et al (2008b). The majority of impact features on the stub surfaces were simple craters of a wide size range, but with each only containing residue from a single mineral component.…”

Ejecta fragmentation in impacts into gypsum and water ice

“…For impacts into gypsum (at 90 o incident angle), the number of craters was roughly the same as the number of holes or even higher. According to Kearsley et al (2008), the diameter of the craters in the Al foil made by ice ejecta fragments corresponds to 3.5 times the ice ejecta size.…”