Penetration tracks in aerogel produced by Al₂O₃ spheres

Abstract

) 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.

Impact experiments of exotic dust grain capture by highly porous primitive bodies

“…4 shows the relationshi p between the MW/PD ratio and the initial dynamic pressure, normalized by the projectile's tensile strength. We also plotted the results of various projectile s into aerogel, as obtained in previous studies (Burchell et al, 2008;Hörz et al, 2009;Kearsley et al, 2012;Niimi et al, 2012), using the tensile strength of soda-lime glass, copper (60 MPa, 195 MPa, respectively;Kaye and Laby, 1986 ), alumina (258 MPa;Shackelford and Alexander , 2000 ), carbonaceous chondrite meteorit es (637, 614, 614 MPa for Allende, Murchison, Orgueil, respectively ; Tsuchiya ma et al, private communicati on, 2012), and graphite (10.9 MPa; Manhani et al, 2007 ). The MW/ PD ratio increases with increasing dynamic pressure.…”

Capture effects in carbonaceous material: A Stardust analogue study

“…Based on observing mineralogical changes in captured grains in laboratory experiments, Noguchi et al (2007) estimated that during capture in aerogel of density 30 kg m −3 at 6.2 km s −1 , a heat pulse is delivered to the captured grain which raises its surface temperature to 500-600 °C for of order 1 µs. In separate experiments, Burchell et al (2009) show that even capture in aerogel (density 25 kg m −3 ) at the lower speed of 3.1 km s −1 is sufficient to melt and ablate the surface of a stainless steel projectile (melting point 1400 °C), and Hörz et al (2008Hörz et al ( , 2009 show that capture in 20 kg m −3 aerogel at 6 km s -1 , is sufficient to melt the surface of Al 2 O 3 spheres (melting point 2054 °C). Modelling suggests that a molten aerogel wrapping forms during capture, with a peak temperature of some 2100 °C for time scales of order 0.1 ms (Roskosz et al 2008).…”

Penetration into low-density media: In situ observation of penetration process of various projectiles

“…For more quantitative description, the depth L and the maximum diameter D m of final tracks are often measured as a function of the parameters varied in the experiments such as, projectile density q p , projectile diameter D p (radius R p ), projectile and target strengths, target density q t , and impact velocity v 0 . The features about L/D p and D m suggested by previous various experiments with low-density targets are roughly summarized as follows: L/D p increases with v 0 , and takes a peak, then decreases (e.g., Fechtig et al, 1980;Werle et al, 1981;Ishibashi et al, 1990;Tsou, 1990; Barrett et al, 1992;Kitazawa et al, 1999;Burchell et al, 2001;Hörz et al, 2009), and is scaled by q p /q t (e.g., Barrett et al, 1992;Hörz et al, 1993;Love et al, 1993;Burchell et al, 1999Burchell et al, , 2009Niimi et al, 2011Niimi et al, , 2012, and D m is proportional to D p regardless of q p /q t and proportional to v 0 (e.g., Ishibashi et al, 1990;Kitazawa et al, 1999;Burchell et al, 2008;Niimi et al, 2012).…”

“…Hypervelocity impact experiments on very low-density materials have been carried out to extend the cratering experiments (e.g., Cannon and Turner, 1967;Fechtig et al, 1980;Werle et al, 1981;Love et al, 1993;Trucano and Grady, 1995) and to develop and calibrate the instruments for intact capture of interplanetary dust samples using foams (e.g., Ishibashi et al, 1990;Tsou, 1990) and for aerogels (e.g., Barrett et al, 1992;Hörz et al, 1993Hörz et al, , 1998Hörz et al, , 2009Burchell et al, 1999Burchell et al, , 2001Burchell et al, , 2008Burchell et al, , 2009Kitazawa et al, 1999;Niimi et al, 2011Niimi et al, , 2012. These previous studies indicate that the impacts between high-density projectiles and low-density targets generate ''penetration tracks'': track diameter is small at the entrance (= impact point), then increases with depth, takes a peak, and decreases (this qualitative feature is common for any track).…”