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

Dust 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−1, 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 μm and some as small as 1.5 μm. 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 μm at 6.1 km s−1, all yielding a constant relationship for spherical soda lime glass projectiles of diameter (Dp) to crater diameter (Dc) in Al1100 of Dc = 4.6 Dp; however, this ratio seems to decrease for projectiles ≪10 μm. The overwhelming majority of the Stardust craters are <20 μm 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.