Impact features and projectile residues in aerogel exposed on Mir

Approximately 0.63 m(2) of SiO2-based aerogel (0.02 g cm(-3)) was exposed for 18 months on the Mir Station to capture hypervelocity particles from both man-made and natural sources. Optical inspection revealed two major classes of hypervelocity impact features in the aerogel: (1) long, carrot-shaped tracks, well known from laboratory impact experiments, that exhibit a depth- (t) to-diameter (D) relationship of t/D > 10, typically 20-30, and (2) shallow pits (t/D < 10; typically 1-3) that have no laboratory analog. Blunt-nosed, yet deep (t/D = 5-10), cylindrically shaped cavities suggest the existence of transitional morphologies between these tracks and pits. All tracks contain projectile residues that are unmelted, while pits rarely contain even traces of projectile material. These and other observations suggest that slender tracks form at lower impact velocities than the shallow pits. In addition, we observed that the measured track-length does not systematically correlate with the size of the projectile residue. This renders the reconstruction of encounter velocity and/or projectile mass from measured track dimensions not feasible at present. Recovery of particles from individual tracks is time-consuming, yet readily accomplished by operators familiar with the handling of individual, micrometer-sized particles. Compositional analyses by SEM-EDS identified a variety of man-made and natural particles. A few natural particles were embedded in epoxy, microtomed, and analyzed by TEM. All were polymineralic aggregates that contained olivine exhibiting sharp electron-diffraction spots, and suggesting that the materials had experienced only minimal shock-deformation, if any. One natural particle contained olivine, augite, diopside, troilite, chromite/magnetite, and hercynite, the latter existing as pristine, undeformed octahedral crystals. The olivine in two of the particles were Fo(60-70) and Fo(39-53), and thus, more equilibrated than olivines in most stratospheric particles (Fo(80-100)) These results illustrate that particle collections in Earth orbit are highly complementary to ground-based collections of cosmic dust. (C) 2000 Academic Press.