Bulbous tracks arising from hypervelocity capture in aerogel

Self Cite

) with spherical corundum projectiles. The highly refractory nature and mechanical strength of corundum minimizes projectile deformation and continuous mass loss by ablation that might have affected earlier experiments with soda-lime glass (SLG) impactors into aerogel targets. We find that corundum is a vastly superior penetrator producing tracks a factor of 2.5 longer, yet similar in diameter to those made by SLG. At velocities >4 km s −1 a cylindrical "cavity" forms, largely by melting of aerogel. The diameter and length of this cavity increase with velocity and impactor size, and its volume dominates total track volume. A continuously tapering, exceptionally long and slender "stylus" emerges from this cavity and makes up some 80-90% of the total track length; this stylus is characterized by solid-state deformations. Tracks formed below 4 km s −1 lack the molten cavity and consist only of a stylus. Projectile residues recovered from a track's terminus substantially resemble the initial impactors at V < 4 km s −1 , yet they display two distinct surfaces at higher velocities, such as a blunt, forward face and a well-preserved, hemispherical trailing side; a pronounced, circumferential ridge of compressed and molten aerogel separates these two surfaces. Stringers and patches of melt flow towards the impactor's rear where they accumulate in a characteristic melt tip. SEM-EDS analyses indicate the presence of Al in these melts at velocities as low as 5.2 km s −1 , indicating that the melting point of corundum (2054 °C) was exceeded. The thermal model of aerogel impact by Anderson and Cherne (2008) suggests actual aerogel temperatures >5000 K at comparable conditions. We therefore propose that projectile melting occurs predominantly at those surfaces that are in contact with this very hot aerogel, at the expense of viscous heating and associated ablation. Exposure to superheated aerogel may be viewed as extreme form of "flash heating." This seems consistent with observations from the Stardust mission to comet Wild 2, such as relatively pristine interiors of rather large, terminal particles, yet total melting of most fine-grained dust components.

Section: Introductionmentioning

confidence: 99%

Penetration tracks in aerogel produced by Al₂O₃ spheres

Hörz

Cintala

See³

et al. 2009

Self Cite

“…The success of mineral interpretation for terminal particles extracted from aerogel (e.g., Zolensky et al 2006), shows that larger grains pose no great problems. However, the admixture of aerogel and remnants of finer particles along aerogel tracks (Zolensky et al 2006;Trigo-Rodríguez et al 2008) suggests that it may be useful to determine the composition of smaller grains within the aluminium foil craters where the impactor material may be retained in a limited area, and not dispersed over a large volume as is the case in some Stardust aerogel tracks where the impactor has broken up during capture. In addition, particles collected by aerogel cannot be readily analyzed in situ, generally requiring extraction and preparation for examination.…”

Section: Introductionmentioning

confidence: 99%

In situ analysis of residues resulting from laboratory impacts into aluminum 1100 foil: Implications for Stardust crater analyses

Wozniakiewicz

Kearsley

Burchell

et al. 2009

Abstract-The encounter between the Stardust spacecraft and particles from comet 81P/Wild 2 gave impacts at a relative velocity of 6.1 km s −1 and near perpendicular incidence to the collector surface. Such conditions are well within the performance limits of light gas gun laboratory simulations. For this study, two series of shots were conducted at the University of Kent, firing magnesium silicates (Mg end-member forsterite, enstatite, diopside and lizardite), followed by a suite of increasingly Ferich olivines (through to Fe end-member fayalite) into Stardust flight-spare foils. Preserved residues were analysed using scanning electron microscopy combined with energy dispersive X-ray analyses (SEM/EDX). X-ray count integrals show that mineral compositions remain distinct from one another after impact, although they do show increased scatter. However, there is a small but systematic increase in Mg relative to Si for all residues when compared to projectile compositions. While some changes in Mg:Si may be due to complex analytical geometries in craters, there appears to be some preferential loss of Si. In practice, EDX analyses in craters on Stardust Al 1100 foil inevitably include contributions from Fe-and Si-rich alloy inclusions, leading to further scattering of element ratios. Such inclusions have complicated Mg:Fe data interpretation. Compositional heterogeneity in the synthetic olivine projectiles also introduces data spread. Nevertheless, even with the preceding caveats, we find that the main groups of mafic silicates can be easily and reliably distinguished in EDX analyses performed in rapid surveys of foil craters, enabling access to a valuable additional collection of cometary materials.

“…Burchell et al (2009) have suggested an analysis of the tracks in the Stardust aerogel implies the aerogel on average behaved as if it had a mean density of just under 20 kg m −3 . Trigo-Rodríguez et al (2008) estimate peak shock pressures during capture in aerogel of 20 kg m −3 of just under 1 GPa. In its rapidity and thermal effects, aerogel capture amounts to shock processing and may have the dual effects of affecting both structure (via amorphization) and composition (via devolatilization) of cometary carbonaceous materials.…”

Section: Stardust Aerogelmentioning

confidence: 99%

Capture effects in carbonaceous material: A Stardust analogue study

Fries

Burchell

Kearsley

et al. 2009

Abstract-It is reasonable to expect that cometary samples returned to Earth by the Stardust space probe have been altered to some degree during capture in aerogel at 6.1 km/s. In order to help interpret the measured structure of these particles with respect to their original cometary nature, a series of coal samples of known structure and chemical composition was fired into aerogel at Stardust capture velocity. This portion of the study analyzed the surfaces of aerogel-embedded particles using Raman spectroscopy. Results show that particle surfaces are largely homogenized during capture regardless of metamorphic grade or chemical composition, apparently to include a devolatilization step during capture processing. This provides a possible mechanism for alteration of some aliphatic compoundrich phases through devolatilization of cometary carbonaceous material followed by re-condensation within the particle. Results also show that the possibility of alteration must be considered for any particular Stardust grain, as examples of both graphitization and amorphization are found in the coal samples. It is evident that Raman G band (~1580 cm −1 ) parameters provide a means of characterizing Stardust carbonaceous material to include identifying those grains which have been subjected to significant capture alteration.