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M. C. PRICE,
A. T. KEARSLEY, M. J. BURCHELL,
F. HÖRZ,
J. BORG,
J. C. BRIDGES,
M. J. COLE,
C. FLOSS,
G. GRAHAM,
S. F. GREEN,
P. HOPPE,
H. LEROUX,
K. K. MARHAS,
N. PARK,
R. STROUD,
F. J. STADERMANN,
N. TELISCH,
P. J. WOZNIAKIEWICZ
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ABSTRACT: Abstract– The fluence of dust particles <10 μm in diameter was recorded by impacts on aluminum foil of the NASA Stardust spacecraft during a close flyby of comet 81P/Wild 2 in 2004. Initial interpretation of craters for impactor particle dimensions and mass was based upon laboratory experimental simulations using projectiles less than >10 μm in diameter and the resulting linear relationship of projectile to crater diameter was extrapolated to smaller sizes. We now describe a new experimental calibration program firing very small monodisperse silica projectiles (470 nm–10 μm) at approximately 6 km s−1. The results show an unexpected departure from linear relationship between 1 and 10 μm. We collated crater measurement data and, where applicable, impactor residue data for 596 craters gathered during the postmission preliminary examination phase. Using the new calibration, we recalculate the size of the particle responsible for each crater and hence reinterpret the cometary dust size distribution. We find a greater flux of small particles than previously reported. From crater morphology and residue composition of a subset of craters, the internal structure and dimensions of the fine dust particles are inferred and a “maximum-size” distribution for the subgrains composing aggregate particles is obtained. The size distribution of the small particles derived directly from the measured craters peaks at approximately 175 nm, but if this is corrected to allow for aggregate grains, the peak in subgrain sizes is at <100 nm.
Meteoritics & Planetary Science. 08/2010; 45(9):1409 - 1428.
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C. Floss,
C. Allen,
S. Armes,
S. Bajt,
A. Ball,
R. Bastien,
H. Bechtel,
J. Borg,
F. E. Brenker,
J. C. Bridges, [......],
M. Trieloff,
J. Trigo-Rodriguez,
P. Tsou,
A. Tsuchiyama,
T. Tyliczszak,
B. Vekemans,
L. Vincze,
J. Warren,
A. J. Westphal,
M. E. Zolensky
Meteoritics and Planetary Science Supplement. 08/2010; 73:5270.
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J. Leitner,
C. Allen,
S. Armes,
S. Bajt,
A. Ball,
R. Bastien,
H. Bechtel,
J. Borg,
F. E. Brenker,
J. C. Bridges, [......],
M. Trieloff,
J. Trigo-Rodriguez,
P. Tsou,
A. Tsuchiyama,
T. Tyliczszak,
B. Vekemans,
L. Vincze,
J. Warren,
A. J. Westphal,
M. E. Zolensky
Meteoritics and Planetary Science Supplement. 08/2010; 73:5292.
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ABSTRACT: Preliminary results from a programme of impact experiments on simple ice
mixtures (CO2, NH3 and H2O) give a tantalising suggestion of the
successful shock synthesis of complex organics — including
glycine.
02/2010; 41:1830.
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ABSTRACT: In response to the recent report of phyllosilicates in comet 9P/Tempel
1, we explored survivability and alteration of phyllosilicates under
Stardust hypervelocity collection conditions for comet 81P/Wild 2 dust
and discuss the implications.
02/2010; 41:2357.
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ABSTRACT: Abstract— Here we demonstrate the use of Raman spectroscopy techniques to identify mineral particle fragments after their impact into aluminum foil at ˜6 km s−1. Samples of six minerals (olivine, rhodonite, enstatite, diopside, wollastonite, and lizardite) were fired into aluminum foil and the resulting impact craters were studied with a HeNe laser connected to a Raman spectrometer. Raman spectra similar to those of the raw mineral grains were obtained from the craters for impacts by olivine, rhodonite, enstatite, wollastonite, and diopside, but no Raman signals were found from lizardite after impact. In general, the impactors do not survive completely intact, but are fragmented into smaller fractions that retain the structure of the original body. Combined with evidence for SEM and FIB studies, this suggests that in most cases the fragments are relatively unaltered during impact. The survival of identifiable projectile fragments after impact at ˜6 km s−1 is thus established in general, but may not apply to all minerals. Where survival has occurred, the use of Raman spectroscopic techniques for identifying minerals after hypervelocity impacts into a metallic target is also demonstrated.
Meteoritics & Planetary Science. 01/2010; 43(1‐2):135 - 142.
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ABSTRACT: Abstract— New model organic microparticles are used to assess the thermal ablation that occurs during aerogel capture at speeds from 1 to 6 km s−1. Commercial polystyrene particles (20 μm diameter) were coated with an ultrathin 20 nm overlayer of an organic conducting polymer, polypyrrole. This overlayer comprises only 0.8% by mass of the projectile but has a very strong Raman signature, hence its survival or destruction is a sensitive measure of the extent of chemical degradation suffered. After aerogel capture, microparticles were located via optical microscopy and their composition was analyzed in situ using Raman microscopy. The ultrathin polypyrrole overlayer survived essentially intact for impacts at ∼1 km s−1, but significant surface carbonization was found at 2 km s−1, and major particle mass loss at ≥3 km s−1. Particles impacting at ∼6.1 km s−1 (the speed at which cometary dust was collected in the NASA Stardust mission) were reduced to approximately half their original diameter during aerogel capture (i.e., a mass loss of 84%). Thus significant thermal ablation occurs at speeds above a few km s−1. This suggests that during the Stardust mission the thermal history of the terminal dust grains during capture in aerogel may be sufficient to cause significant processing or loss of organic materials. Further, while Raman D and G bands of carbon can be obtained from captured grains, they may well reflect the thermal processing during capture rather than the pre-impact particle's thermal history.
Meteoritics & Planetary Science. 01/2010; 44(10):1407 - 1419.
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ABSTRACT: Abstract— Mineral particles analogous to components of cosmic dust were tested to determine if their Raman signatures can be recognized after hypervelocity capture in aerogel. The mineral particles were accelerated onto the silica aerogel by light-gas-gun shots. It was found that all the individual minerals captured in aerogel could be identified using Raman (or fluorescence) spectra. The laser beam spot size was ˜5 micrometers, and in some cases the captured particles were of a similar small size. In some samples fired into aerogel, a broadening and a shift in the wave numbers of some of the Raman bands was observed, a result of the trapped particles being at elevated temperatures due to laser heating. Temperatures of samples were also estimated from the relative intensities of Stokes and anti-Stokes Raman bands, or, in the case of corundum particles, from the wave number of fluorescence bands excited by the laser. The temperature varied greatly, dependent upon laser power and the nature of the particle. Most of the mineral particles examined had temperatures below 200 °C at a laser power of about 3 mW at the sample. This temperature is sufficiently low enough not to damage most materials expected to be found captured in aerogel in space. In the worst case, some particles were shown to have temperatures of 500–700 °C. In addition, selected meteorite samples were examined to obtain Raman signatures of their constituent minerals and were then shot into aerogel. It was possible to find Raman signatures after capture in aerogel and obtain a Raman map of a whole grain in situ in the aerogel. It is concluded that Raman analysis is indeed well suited for an in situ analysis of micrometer-sized materials captured in aerogel.
Meteoritics & Planetary Science. 01/2010; 41(2):217 - 232.
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M. J. Burchell,
S. A. J. Fairey,
P. Wozniakiewicz,
D. E. Brownlee,
F. Hörz,
A. T. Kearsley,
T. H. See,
P. Tsou,
A. Westphal,
S. F. Green,
J. M. Trigo-Rodríguez,
G. Domingúez
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ABSTRACT: Abstract— The cometary tray of the NASA Stardust spacecraft's aerogel collector was examined to study the dust captured during the 2004 flyby of comet 81P/Wild 2. An optical scan of the entire collector surface revealed 256 impact features in the aerogel (width >100 μm). Twenty aerogel blocks (out of a total of 132) were removed from the collector tray for a higher resolution optical scan and 186 tracks were observed (track length >50 μm and width >8 μm). The impact features were classified into three types based on their morphology. Laboratory calibrations were conducted that reproduced all three types. This work suggests that the cometary dust consisted of some cohesive, relatively strong particles as well as particles with a more friable or low cohesion matrix containing smaller strong grains. The calibrations also permitted a particle size distribution to be estimated for the cometary dust. We estimate that approximately 1200 particles bigger than 1 μm struck the aerogel. The cumulative size distribution of the captured particles was obtained and compared with observations made by active dust detectors during the encounter. At large sizes (>20 μm) all measures of the dust are compatible, but at micrometer scales and smaller discrepancies exist between the various measurement systems that may reflect structure in the dust flux (streams, clusters etc.) along with some possible instrument effects.
Meteoritics & Planetary Science. 01/2010; 43(1‐2):23 - 40.
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ABSTRACT: Abstract— Given that the Earth's surface is covered in around two-thirds water, the majority of impact events should have occurred in marine environments. However, with the presence of a water layer, crater formation may be prohibited. Indeed, formation is greatly controlled by the water depth to projectile diameter ratio, as discussed in this paper. Previous work has shown that the underlying target material also influences crater formation (e.g., Gault and Sonett 1982; Baldwin et al. 2007). In addition to the above parameters we also show the influence of impact angle, impact velocity and projectile density for a variety of water depths on crater formation and projectile survivability. The limiting ratio of water depth to projectile diameter on cratering represents the point at which the projectile is significantly slowed by transit through the water layer to reduce the impact energy to that which prohibits cratering. We therefore study the velocity decay produced by a water layer using laboratory, analytical and numerical modelling techniques, and determine the peak pressures endured by the projectile. For an impact into a water depth five times the projectile diameter, the velocity of the projectile is found to be reduced to 26–32% its original value. For deep water impacts we find that up to 60% of the original mass of the projectile survives in an oblique impact, where survivability is defined as the solid or melted mass fraction of the projectile that could be collected after impact.
Meteoritics & Planetary Science. 01/2010; 43(12):2015 - 2026.
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ABSTRACT: 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.
Meteoritics & Planetary Science. 01/2010; 44(10):1541 - 1559.
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ABSTRACT: We report on the results of an experimental shot programme using a light gas gun (LGG) the principal purpose of which was to extend the existing calibration of projectile vs. crater diameter to aid in the interpretation of very small (<10 μm) impact craters observed on Stardust aluminium (Al) foils. Stardust was a NASA mission which flew past a comet at 6.1 km s−1 in 2004 and collected freshly emitted cometary dust via impact onto its exposed surface. The results show an unexpected change in the profile of the calibration curve resulting in a need to readdress the fluence measurement for Comet 81P∕Wild‐2 and also gives an insight into the strain rate behaviour of Al‐1100 at the very high ( ∼ 109 s−1) rates experienced by the Al during impact of a micron sized projectile at 6.1 km s−1.
AIP Conference Proceedings. 12/2009; 1195(1):863-866.
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ABSTRACT: Abstract— New experimental results show that Stardust crater morphology is consistent with interpretation of many larger Wild 2 dust grains being aggregates, albeit most of low porosity and therefore relatively high density. The majority of large Stardust grains (i.e. those carrying most of the cometary dust mass) probably had density of 2.4 g cm−3 (similar to soda-lime glass used in earlier calibration experiments) or greater, and porosity of 25% or less, akin to consolidated carbonaceous chondrite meteorites, and much lower than the 80% suggested for fractal dust aggregates. Although better size calibration is required for interpretation of the very smallest impacting grains, we suggest that aggregates could have dense components dominated by μm-scale and smaller sub-grains. If porosity of the Wild 2 nucleus is high, with similar bulk density to other comets, much of the pore space may be at a scale of tens of micrometers, between coarser, denser grains.Successful demonstration of aggregate projectile impacts in the laboratory now opens the possibility of experiments to further constrain the conditions for creation of bulbous (Type C) tracks in aerogel, which we have observed in recent shots. We are also using mixed mineral aggregates to document differential survival of pristine composition and crystalline structure in diverse finegrained components of aggregate cometary dust analogues, impacted onto both foil and aerogel under Stardust encounter conditions.
Meteoritics & Planetary Science. 09/2009; 44(10):1489 - 1509.
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ABSTRACT: New model organic microparticles are used to assess the thermal ablation that occurs during aerogel capture at speeds from 1 to 6 km s(-1). Commercial polystyrene particles (20 mu m diameter) were coated with all ultrathin 20 nm overlayer of an organic conducting polymer, polypyrrole. This overlayer comprises only 0.8% by mass of the projectile but has a very strong Raman signature, hence its survival or destruction is a sensitive measure of the extent of chemical degradation suffered. After aerogel capture, microparticles were located via optical microscopy and their composition was analyzed in situ using Raman microscopy. The ultrathin polypyrrole overlayer survived essentially intact for impacts at similar to 1 km s(-1), but significant surface carbonization was found at 2 km s(-1), and major particle mass loss at >= 3 km s(-1). Particles impacting at similar to 6.1 km s(-1) (the speed at which cometary dust was collected in the NASA Stardust mission) were reduced to approximately half their original diameter during aerogel capture (i.e., a mass loss of 84%). Thus significant thermal ablation occurs at speeds above a few km s(-1). This suggests that during the Stardust mission the thermal history of the terminal dust grains during capture in aerogel may be sufficient to cause significant processing or loss of organic materials. Further, while Raman D and G hands of carbon can be obtained from captured grains, they may well reflect the thermal processing during capture rather than the pre-impact particle's thermal history.
Meteoritics & planetary science 01/2009; 44:1407-1420. · 2.72 Impact Factor
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A. T. Kearsley,
J. Borg,
G. A. Graham, M. J. Burchell,
M. J. Cole,
H. Leroux,
J. C. Bridges,
F. Hörz,
P. J. Wozniakiewicz,
P. A. Bland, [......],
N. Teslich,
T. See,
P. Hoppe,
P. R. Heck,
J. Huth,
F. J. Stadermann,
C. Floss,
K. Marhas,
T. Stephan,
J. Leitner
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ABSTRACT: Abstract— Aluminum foils of the Stardust cometary dust collector are peppered with impact features of a wide range of sizes and shapes. By comparison to laboratory shots of known particle dimensions and density, using the same velocity and incidence geometry as the Stardust Wild 2 encounter, we can derive size and mass of the cometary dust grains. Using scanning electron microscopy (SEM) of foil samples (both flown on the mission and impacted in the laboratory) we have recognized a range of impact feature shapes from which we interpret particle density and internal structure. We have documented composition of crater residues, including stoichiometric material in 3 of 7 larger craters, by energy dispersive X-ray microanalysis. Wild 2 dust grains include coarse (>10 μm) mafic silicate grains, some dominated by a single mineral species of density around 3–4 g cm−3 (such as olivine). Other grains were porous, low-density aggregates from a few nanometers to 100 μm, with an overall density that may be lower than 1 g cm−3, containing mixtures of silicates and sulfides and possibly both alkali-rich and mafic glass. The mineral assemblage is very similar to the most common species reported from aerogel tracks. In one large aggregate crater, the combined diverse residue composition is similar to CI chondrites. The foils are a unique collecting substrate, revealing that the most abundant Wild 2 dust grains were of sub-micrometer size and of complex internal structure. Impact residues in Stardust foil craters will be a valuable resource for future analyses of cometary dust.
Meteoritics & Planetary Science. 01/2008; 43(1‐2):41 - 73.
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Astronomy & Geophysics 11/2007; 48(6):6.27 - 6.31. · 0.61 Impact Factor
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ABSTRACT: CLSM can quantify aerogel track shape and particle location in
keystones, quickstones, and cm-scale unprepared blocks. It is suitable
for use at an early stage of curation and preparation of small features,
e.g., Stardust interstellar grain tracks.
02/2007; 38:1690.
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Meteoritics and Planetary ScienceMeteoritics and Planetary Science, United States; 01/2007
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A. T. Kearsley,
G. A. Graham, M. J. Burchell,
M. J. Cole,
Z. R. Dai,
N. Teslich,
J. P. Bradley,
R. Chater,
P. A. Wozniakiewicz,
J. Spratt,
G. Jones
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ABSTRACT: The known encounter velocity (6.1kms-1) and particle incidence angle (perpendicular) between the Stardust spacecraft and the dust emanating from the nucleus of comet Wild 2 fall within a range that allows simulation in laboratory light gas gun experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape and density range of mineral, glass, polymer and metal grains, have been fired to impact perpendicularly upon samples of Stardust Al1100 foil, tightly wrapped onto aluminium alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre-existing major and trace element composition of the foil, geometrical issues for energy dispersive X-ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density and composition for particles impacted upon the Stardust aluminum foils.
01/2007;
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ABSTRACT: The idea that life migrates naturally between planetary bodies has grown in strength in recent years. This idea (panspermia) is believed to be possible via the mechanism of impact events. Previous research on this topic has concentrated on small meteoroids (micrometres to centimetres in diameter), with giant objects (metres to kilometres in diameter) being relatively ignored. This is due to the common belief that the larger objects vaporize on impact with the Earth's surface, which in most studies is taken as rock. Here we examine experimentally whether hypervelocity impacts into water result in significant survival of the impactors. For this study the University of Kent's two-stage light gas gun was used to accelerate millimetre-sized shale projectiles obliquely into a relatively deep water layer, at approximately 5 km s−1. Two shots have been made with surviving fragments being recovered from each. The surviving fragments appear highly shocked and display clear signs of cracking. The fragments that have been isolated contribute to a significant percentage (~10%) of the original unfired projectile mass and are as large as ~20% of the original projectile diameter. This indicates that oceanic hypervelocity impact events of large asteroids may deliver significant volumes of solid material to the Earth and thus provide a possible mechanism for successful panspermia.
International Journal of Astrobiology. 06/2006; 5(03):261 - 267.
