Impact Experiments with Al2O3 Projectiles into Aerogel

Introduction: The detailed mineralogical, compositional, and isotopic investigations of cometary dust retrieved by Stardust provide empirical evidence for the utility of aerogel in capturing analyzable residues of hyper velocity particles [1]. These residues are found along the walls and in the termini of unusually deep penetration “tracks” which typically form in such high-porosity, low-density materials as summarized by [2]. Nevertheless, a detailed theoretical understanding of the penetration process in aerogel is still in its infancy and constitutes a formidable problem in shock physics [3, 4, 5]. Specifically, the P-T histories of the recovered residues are difficult to ascertain, a prerequisite to evaluate potential particle alterations during the capture process. We therefore embarked on a series of impact experiments under simple, if not idealized, conditions conducive to theoretical analysis. First, we used aerogel of constant density (0.02 g cm), which was unlike the density-graded aerogel flown on Stardust. Second, we used spherical, high-purity, polycrystalline Al2O3 (α-corundum) projectiles. Not only are their relevant thermodynamic properties better known than those of soda-lime glass (SLG) projectiles used in most earlier experiments [6, 7, 8], but corundum is also highly refractory and very competent mechanically. The latter should minimize, possibly eliminate, mass loss due to ablation, as well as excessive deformation of the impactor [5], which may have affected all earlier SLG experiments. While these idealized boundary conditions may remove the present experiments to some degree from the penetration-behavior of natural impactors, they seem more suited for a basic examination of hypervelocity penetration in the highly porous aerogel. The increased complexities typical for natural impactors may be added incrementally to this baseline understanding in the future. This abstract represents a progress report on the dimensions and morphologies of penetration tracks produced under these simplified conditions. Experiments: The experiments were conducted with the 5-mm light-gas gun at NASA JSC, using four-piece sabots to "shotgun" an ensemble of small particles. We acquired Al2O3 spheres that averaged 35, 60, and 105 μm in diameter with a standard deviation of ~10% and sphericities of better than 0.95 for ~90% (on the basis of our own size analyses using an SEM). We conducted one experiment with each of the above sizes at a constant impact velocity of 6.1 km s; an additional series used the 60-μm projectiles at 3.11, 3.64, 5.21, and 6.04 km s in an effort to isolate the effects of velocity. Each impact was normal to the face of the aerogel target. Results: All tracks were exceptionally long and slender compared to those produced by SLG impactors [6, 7, 8]. A typical track is illustrated in Fig.1. Tracks tend to expand rapidly near the entrance hole to some maximum diameter (DTmax). The latter pertains over a sizeable portion of the track that is bounded by depth L1 and L2 (see Fig.1); the continuously tapering section at depth >L2 is typically >80% of the total track length (LT). Note some highly twisted and, on occasion, spiraled features in the track’s main cavity (top row in Fig.1). SEM observations on bisected tracks indicate that these are interconnected networks of tiny granules, mixed with and bonded by melts of aerogel. The sections above L2 seem dominated by melt phenomena, whereas all sections >0.3 LT display solid-state, mechanical deformations only. Indeed, the 3.11 km s case essentially repre-