identification of mineral impactors in hypervelocity

Meteoritics & Planetary Science 43, Nr 1/2, 135–142 (2008)Abstract available online at http://meteoritics.org

135 © The Meteoritical Society, 2008. Printed in USA.

Identification of mineral impactors in hypervelocity impact cratersin aluminum by Raman spectroscopy of residues

M. J. BURCHELL1*, N. J. FOSTER1, A. T. KEARSLEY2, and J. A. CREIGHTON1

1Centre for Astrophysics and Planetary Sciences, School of Physical Sciences, Ingram Building,University of Kent, Canterbury, Kent CT2 7NH, UK

2Impact and Astromaterials Research Centre, Department of Mineralogy, The Natural History Museum, London SW7 5BD, UK*Corresponding author. E-mail: [email protected]

(Received 20 February 2007; revision accepted 13 July 2007)

Abstract–Here we demonstrate the use of Raman spectroscopy techniques to identify mineralparticle 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 theresulting impact craters were studied with a HeNe laser connected to a Raman spectrometer. Ramanspectra similar to those of the raw mineral grains were obtained from the craters for impacts byolivine, rhodonite, enstatite, wollastonite, and diopside, but no Raman signals were found fromlizardite after impact. In general, the impactors do not survive completely intact, but are fragmentedinto smaller fractions that retain the structure of the original body. Combined with evidence for SEMand 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 ingeneral, but may not apply to all minerals. Where survival has occurred, the use of Ramanspectroscopic techniques for identifying minerals after hypervelocity impacts into a metallic target isalso demonstrated.

INTRODUCTION

The capture of cosmic dust in space can be achieved by avariety of means. Among the most simple is placing a smoothmetallic (ductile) material in space and retrieving it at a laterdate. Analysis of the surface will then show small impactcraters formed by the high-speed impacts. The speed of theimpact will depend on the relative speeds of the impactor andthe target. In low Earth orbit (LEO), these speeds willtypically be 7–11 km s−1 for man-made debris and higher(mean of 20–25 km s−1) for interplanetary dust particles(IDPs). Such impacts are in the speed range that is oftentermed “hypervelocity,” meaning that the speed is above the 1or 2 km s−1 that is the normal maximum traditionallyachievable on Earth. The results of an impact on a solid targetin this speed regime are that a crater forms in the target, theprojectile is crushed, heated, melted, even vaporized, andmuch of the projectile material may be removed from theimpact site as ejecta. This makes identifying the compositionof the projectile difficult. However, in hypervelocity impacts,it has long been known that some remnant of the impactor canline the resultant crater in the form of an impact melt, and onsome substrates (e.g., the borosilicate glass solar cell surfaces

of Kearsley et al. 2007a) a substantial residue fraction (>6%)can be found from even high-velocity impact (5–6 km s−1)and can yield the compositions of stoichiometric minerals. Anumber of essentially nondestructive in situ analysistechniques can be used on crater residues, the most obviousbeing scanning electron microscopy and energy dispersiveX-ray microanalysis (SEM-EDX; e.g., Kearsley et al. 2007b)and time-of-flight secondary ion mass spectrometry (ToF-SIMS; e.g., Hoppe et al. 2006), both of which can yielddetailed elemental information on the composition of theresidue, and in the latter case, isotopic analysis, albeit withsome surface ablation.

The recent return to Earth of cometary dust by the NASAStardust spacecraft (Brownlee et al. 2006) has generatedincreased efforts in analysis of cosmic dust. Although most ofthe cometary particles returned by Stardust were captured inaerogel (a low-density, porous solid), there were also manycraters in aluminum foils carried by the spacecraft (Hörz et al.2006). One unusual feature of dust collection by Stardust wasthat the impact speed was fixed by the encounter speed of thespacecraft during its fly-by of the comet and is thus wellconstrained as 6.1 km s−1, and is easily achievable in thelaboratory. Detailed analysis techniques have been developed

136 M. J. Burchell et al.

and tested in laboratory simulations of impacts at this speed to

study the impact residues in craters (e.g., Kearsley et al.

2007b), but these techniques rely on identifying the impactor by

its elemental composition. Here we assess whether an alternate

analysis method, Raman spectroscopy, can be successfully

employed to recognize characteristic molecular bonding attributes

within the projectile residue in hypervelocity impact craters on

aluminum targets.

Accordingly, in this paper we briefly discuss previous

applications of Raman spectroscopy to the study of solar

system materials, present evidence that intact fragments of

projectiles may survive impacts on metal surfaces, and

include examples of impacts in the laboratory on Stardust-

grade aluminum foils at speeds of 6 km s−1. The impact

craters in these samples were then subjected to examination

under a microRaman system with a view to determining if a

material structure based, rather than elemental based,

identification of the impactor can be made from the residues.

RAMA� SPECTROSCOPY OF MI�ERALS

Raman spectroscopy involves the illumination of a

sample by a monochromatic light source. Some of the light is

inelastically scattered and the spectrum of this component

reflects the vibrational energy levels of the bond structure in

the illuminated specimen. In many cases, the Raman

spectrum can be sufficiently distinct to serve as a means of

identifying the sample, and there is a very extensive literature

on this subject (e.g., Lewis and Edwards 2001; Smith and

Dent 2005), as well as several online spectral databases (e.g.,

http://rruff.info/index.php). Use of laser illumination

combined with a microscope allows study of materials at the

micron scale and has been a standard tool of mineralogical

analysis for many decades. It has been applied to minerals in

general (see, e.g., Frost et al. 1999 for a brief review) as well

as interplanetary dust particles (IDPs) (e.g., Wopenka 1988)

and meteoritic materials (e.g., Zinner et al. 1995). It is also

particularly useful for the study of carbonaceous materials

(see, e.g. Beyssac et al. 2003 and references therein). Use of

Raman techniques on particles captured in hypervelocity

impacts on aerogel was shown to be possible by Burchell

et al. (2001) for mineral grains and also for organic materials

(Burchell et al. 2004, 2006). In the case of organics, this was

successfully applied to particles captured in the Stardust

aerogel (Sandford et al. 2006). However, there has previously

been no demonstration as to whether impact crater residues in

aluminum foil are even in principle amenable to Raman

analysis.

Fig. 1. a) Soda lime glass residues in an impact crater. b) Wollastonite residues in impact craters. c) Olivine craters. d) Enstatite craters. Allcraters are in Al 1100 foil. (a) and (b) are secondary electron images taken at 2kV accelerating voltage, impact residues appear as dark areas.(c) and (d) are optical images from a Leica MZ stereo microscope.

Identification of mineral impactors in hypervelocity impact craters in aluminium 137

IMPACTS ON METALS

The widely held belief that impact on metallic substratesinvolves very high peak shock pressures may havecontributed to this oversight. It has been implicitly assumedthat not only will there will be very little projectile materialretained in the crater, but that what is present will haveundergone shock processing at pressures of many GPa andthat any residue will be impact melt or will have condensedfrom a vapor state (produced by the shock of the impact). Forthe majority of impacts in LEO this may well be true, but inthe case of the Stardust encounter, the impact velocity(6.1 km−1) is likely to generate peak pressures less than80 GPa, which is insufficient to completely melt, let alonevaporize, typical silicate impactors. The role of complexinternal grain boundaries may also result in inhomogeneouspartitioning of strain within the impactor, with “sacrificial”melting and vaporization in parts, and survival of solid (albeitdeformed) fragments from others.

Experimentally, a study of cratering by glass projectilesin aluminum by Hörz et al. (1995) showed that at 2 km s−1,projectile fragments could still be found in the crater, but thatabove this speed, melted glass dominated the residue. Indeed,at speeds of 6 km s−1 this impact melt was itself significantlyreduced in quantity, with none at the bottom of the craters andmost of it having been ejected. It may thus seem highlyunlikely that an analysis technique (such as Ramanspectroscopy) requiring projectile fragments at greater thanthe micron scale would be applicable. However, Hernandezet al. (2006) reported that they observed discrete projectilefragments of metal (stainless steel) projectiles in craters in thelaboratory for impact speeds of up to 5 km s−1, and thathydrocode simulations suggested these should still be presentat 6 km s−1. In their work, fragment size and number densitydecreased as impact speed increased, but fragments were stillfound at the bottom of the crater as well as on the walls. Thetargets were nickel, copper, and stainless steel (compared toaluminum for Stardust), but the results do suggest thatprojectile fragments (rather than melt) may still line craterseven in impacts at 6 km s−1.

The recent use of ion-stimulated (e.g., Graham et al.2006) or low–accelerating voltage electron-stimulatedsecondary electron imagery has allowed direct visualizationof the distribution of impact residues from silicate mineralsand glasses across the surface of aluminum foil impactcraters. From basaltic and artificial soda-lime glasses, there isevidence of an extensive but patchy, smooth, quenched meltsheet stretching from the crater floor, across the walls, andonto the uplifted rim. This is confirmed here by study ofimpacts at 6 km s−1 onto aluminum foil with soda-lime glassand wollastonite projectiles (Figs. 1a and 1b show soda-limeglass and wollastonite residues in craters). Focused ion beam(FIB) trenches through basalt glass residue (Fig. 2) show thatthe impactor melt may be interlayered with aluminum targetmaterial. Angular glass impactor fragments were alsoabundant, adhering to the internal crater walls. As the originalprojectile was amorphous (i.e., had no discernible crystallinestructure), it is very difficult to assign diagnostic featuresother than gross morphology, but the suggestion is that someof the original fragment survived unmelted. Crystallinesilicate impactors also create a sheet of quenched melt, buttheir craters also often contain remnant crystalline fragmentsof the impactor, either embedded within melt or as coarseclumps (Fig. 3). The retention of oriented, characteristiccrystal cleavage morphology in residues of wollastonite(Fig. 4) strongly suggests that the stoichiometric remnantshave not recrystallized from melt, but are genuine solidsurvivors from the original particle.

In separate work, transmission electron microscopy ofStardust small crater sections prepared by FIB microscopyalso shows the survival of crystalline silicate and sulfidemineral grains within quenched melt (Zolensky et al. 2006;Borg et al. 2007). In the case of mineral projectiles (ratherthan metals or glasses), such fragments may be amenable toRaman analysis. Indeed, Raman spectroscopy can be asensitive indicator of the extent of impact shock

Fig. 2. Ion-induced secondary electron image of residue (dark, shownarrowed) in a vertical FIB section cut into metal (bright) of the floorof an impact crater (basalt glass projectile at ~6.1 km s−1).

Fig. 3. Backscattered electron image of vertical section of impactcrater in aluminum foil from impact of diopside at 6 km s−1.Residues are shown arrowed.


processing, for example, in feldspars, where the distinctivefine peak structure becomes suppressed as maskelynite isformed.

METHOD

The impacts were generated using a two-stage light-gasgun at the University of Kent (Burchell et al. 1999). As wellas firing single projectiles in a shot, this gun can also fire acloud of projectiles in a “buckshot” (mono-dispersive) or“powder” (poly-dispersive) type of shot. This was done forthe experiments reported herein, yielding several craters toanalyze from each shot. The shot speed was found in eachshot using passage of the projectiles through laser lightcurtains (see Burchell et al. 1999 for details) and thedispersion of speeds across the cloud of projectiles was lessthan 2%. Details of the shot program are given in Table 1.Projectile materials were selected to include a variety ofminerals, i.e., olivine ([Mg,Fe]2SiO4, a common mineral,found in meteorites and with a high melting point), diopside(MgCaSi2O6, a rock-forming mineral found in meteorites andthe Ca-Mg end member of the pyroxene family), rhodonite([Mn,Fe,Mg, Ca]SiO3, a mineral used as an internalstandard in our laboratory and which gives strong Ramansignals), enstatite (MgSiO3, the Mg endmember pyroxene,

found in meteorites and which gives a moderate strengthRaman signals), wollastonite (CaSiO3, which we use as aprojectile with high aspect ratio as it cleaves readily intoelongated fragments and is fairly refractory), and lizardite(Mg3Si2O5[OH]4, a phyllosilicate with about 13% watercontent, and a polymorph of serpentine). Although most shotshad only one mineral type per shot, one shot included amixture of rhodonite and olivine as a blind test of the analysis.Also given in Table 1 are the particle sizes used along withthe resultant crater sizes. These sizes are large compared tomost of the craters that impacted the Stardust spacecraft;however, 63 craters larger than 20 μm in diameter have beenidentified so far, seven of which (sized 44–142 μm indiameter) have been studied in detail (Kearsley et al. 2008).

Targets were sheets of flight spare Stardust aluminumfoil. This was type Al l100, with a thickness of 0.1 mm and asurface area of 1 × 1 cm (see Kearsley et al. 2007b for furtherdetails). The targets were mounted on a larger aluminum plateand placed in the gun so that impacts were at normalincidence. During the shots, the target chamber wasevacuated to typically 0.2 mbar.

The Raman system used a Jobin-Yvon microRamanmodule coupled with an Olympus BX40 microscope. It alsocomprised a HR640 spectrometer with 1200 g/mm gratingand a liquid nitrogen cooled CCD (1024 × 256 pixels).Illumination was from a HeNe (632.8 nm), laser whichdelivered 3 mW of power at the sample in a spot size of 4–5 microns, yielding a power intensity at the sample ofapproximately 189 MW m−2. Collection times for raw grainswere typically 4 × 30 s and 8 × 30 s for craters.

RESULTS

After each shot, the target foils were examined andimpact craters found. Typical craters are shown in Figs. 1and 4. Craters were usually 60 to 300 microns across. Acrater-projectile size calibration for impacts at this speed in

Fig. 4. Fine structure on walls of crater in aluminum foil made by impact of wollastonite at 6 km s−1. The crater is shown full size (left) andthe region highlighted is expanded (right). In the expanded view fine elongate fragments are shown arrowed (apparently embedded in impactmelt) with similar orientation to the main axis of the original (elongated) crater.

Table 1.

ProjectileImpact speed(km s−1)

Approximategrain size(μm)

Crater size(μm)

Olivine 5.89 75–175 220–300Olivine and rhodonite

6.06 75–90 and 53–125 250–300

Enstatite 5.85 >38 60–300Diopside 6.01 <125 200Wollastonite 5.74 20–350 100–150Lizardite 5.85 53–125 ~300


Al 1100 was given in Kearsley et al. (2006). It suggests thatcrater rim-rim diameters should be 4.6 times projectilediameter for glass impactors with density 2.4 g cm−3,extrapolation from density-dependent crater scaling(Kearsley et al. 2007b) gives approximately 5 timesprojectile diameter for the five minerals in our experiments.Using this relation suggests that the impactors here were oforder 12 to 60 μm in diameter, which is roughly in line withexpectations (Table 1).

Raw Raman spectra were obtained from each mineralsample before firing. After a shot the craters were thenexamined with the Raman system. For the olivine,rhodonite, and diopside shots, identifiable Raman spectra ofthe projectile material were readily obtained from thecraters. Examples are given in Fig. 5. There is a clearidentification of the Raman peaks from the raw grains

(pre-shot) with the signals obtained from the craters.Several craters on each target were studied in this fashion;not all gave Raman spectra. For example, on the firstolivine shot one out of the three craters studied gave agood signal (shown here), one a weak signal (just abovenoise levels) and one no signal. It may be possible thatlonger integration times on the spectrometer would improvethese statistics. In the shot which fired both rhodonite andolivine, all craters studied gave spectra, several for olivineand several for rhodonite. Given the size of the craterscompared to the Raman beam size, it was possible to takespectra from several distinct sites inside individual craters.Raman spectra showing rhodonite, for example, wereobtained from the floor (bottom) of a crater and from therising walls (edges), indicating a wide dispersion of theresidue on the crater walls.

Fig. 5. Raman spectra from craters in Al 1100 foils from made from impacts of (a) olivine, (b) rhodonite, (c) diopside, (d) enstatite, and (e)wollastonite projectiles. In each case the spectrum from the crater is the upper trace, accompanied by a lower trace of a spectrum from a rawgrain of the same material pre-shot.


For the enstatite shot, initially no Raman signals forenstatite were obtained from the craters. Re-examination of thecraters with longer accumulation times (15 × 60 s) did thenresult in successful acquisition of an enstatite spectrum froma 68 μm diameter crater. This spectrum (Fig. 5d) wasobtained from the internal wall of the crater, where a featurewhose morphology was suggestive of an accumulation ofresidue had been seen under the microscope. It may be that forsome materials, longer integration times combined withselectivity in the site to be illuminated are thus required toobtain signals above the noise level. By contrast, thewollastonite impact craters that were studied only gave goodRaman spectra for wollastonite from one out of six craters(using the same integration times as for olivine, rhodonite, anddiopside).

In the lizardite shots, no Raman spectra showed distinctpeaks indicative of the projectiles. Several craters were testedand multiple sites were tested in the craters and on their rims,with longer accumulation times tried, but still no spectra forlizardite were obtained, nor indeed were any distinctiveRaman peaks found in the observed spectra. Raw grains oflizardite did, however, readily give Raman spectra.

DISCUSSION

Following the work of Hernandez et al. (2006) thesuggestion arose that discrete grains of projectile materialmay survive impacts at speeds above 5 km s−1. As indicated inthe introduction here, at speeds of 6 km s−1, laboratoryimpacts into aluminum foil now provide strong evidence fordiscrete regions in the impact melt lining craters, which issuggestive of fragments of the incident projectile.Furthermore, FIB cross sections from Stardust craters clearlydemonstrate survival of crystalline material (see above). Theresult found here is that for olivine, diopside, enstatite, andrhodonite impacting at 6 km s−1, Raman spectra from residueafter the impact can be obtained with the same characteristicsas the impactor. This does not a priori confirm thatmacroscopic fragments of projectile survive intact at thesespeeds. It could be argued that re-crystallized melt hasformed. However, this would have to retain the samecomposition and structure as the original material and weconsider it unlikely. Furthermore, the projectile would haveto be heated to a sufficiently high enough temperature(compared to its melting point) for a prolonged period for fullmelting of the entire projectile to have occurred. It is morelikely that the projectile has been partly melted and partlybroken into fragments, with local regions of higher/lowerpressure and temperatures occurring non-uniformly acrossthe body. In addition, rapid cooling will have occurred due tocontact with the metal target with a lower melting point.

The success of Raman spectroscopy on aluminum craterscurrently applies to five minerals: olivine, diopside,rhodonite, wollastonite, and enstatite. More work is required,

particularly to extend the range of appropriate mineral species(such as feldspars and sulfides) or to various members insidea mineral family (for example, Kuebler et al. 2005 haveshown that it is possible to use Raman spectroscopy on rawgrains to sensitively determine the composition of olivines interms of their Mg and Fe content; in future work we willcompare the shifts and any broadening from residual strainwith those due to varying family member composition), toincrease the sensitivity of the Raman analysis (for example,by longer accumulation times to see if this reveals any weakspectral features), and using impactors of varying sizes todetermine the size dependence of structural preservation.Preliminary further work using smaller projectiles of olivine,has shown that craters just 25 μm in diameter (and henceprojectile diameters of approximately 5 μm) still yieldrecognizable Raman spectra for olivine.

The lack of distinctive Raman peaks of any kind fromlizardite impact residue is a particularly significant result as itis a member of the serpentine group of hydrous phyllosilicateminerals and decomposes with loss of water at a relativelylow temperature, usually with generation of olivine.Explaining the absence of a signal from the lizardite is notstraightforward. Material strength and melting point forexample are a function of pressure, and, during impacts,extreme pressures occur for short durations. Therefore a fullsimulation of the material response to extreme shockpressures is required. It may be that lizardite as aphyllosilicate is less prone to survival under these impactconditions. It appears that this mineral group, common inType 1 and 2 carbonaceous chondrite meteorites and usuallyattributed to hydration of anhydrous mafic silicateprecursors during parent-body (asteroidal) processing,may not survive impact under Stardust encounterconditions in a form that can be recognized by Ramanspectroscopy of craters. It thus may not be possible to useRaman spectroscopy to distinguish residue fromphyllosilicates (indicating substantial aqueous activity) fromamorphous anhydrous mafic silicate glass of pre-nebularorigin. Further quantitative SEM-EDS work is needed on thelizardite craters to see if their residue contents give a differentanalytic totals to the original projectiles, and whether this canbe attributed to loss of water or even generation of anhydroussilicate mineral species

In summary, however, it is clear that several of the mostcommon minerals to be found in solar system bodies,especially olivine and pyroxenes, can survive as identifiablecrystalline residues after hypervelocity impacts at 6 km s−1.

CONCLUSIONS

The results show that for 5 minerals (olivine, diopside,rhodonite, wollastonite, and enstatite), Raman spectra mayreadily be obtained after a projectile strikes an aluminumtarget at speeds of some 6 km s−1. For some minerals, all of


the craters studied readily gave recognizable Raman spectrafor the impactor. In others, however, longer integration timeswere needed to produce spectra and some yielded norecognizable spectra, even though neighboring craters in thesame shot did so. This suggests that either the survival ofcrystalline fragments is not uniform or that they may beburied in the impact melt below a layer of aluminum (thetarget material). The smallest craters so far studied that gaveRaman signals for the impactors were 25 μm in diameter,corresponding to grains of olivine approximately 5 μmin diameter. In the case of one mineral, lizardite, noRaman spectra were obtained from the impact craters.With just one counter-example, it is hard to be definitiveas to the cause, but it may lie in the delicatecrystallographic structure of this phyllosilicate projectile(with consequent weak bonding between planes) or thepresence of a significant water content altering the behavioron impact.

These results suggest that projectile fragments maysurvive impact, but that this may not be true for all materials.This novel result is of interest in its own right. It both changesthe view as to the degree of melt and alteration in impacts atthese speeds and also introduces a new analysis technique forstudying impact crater residues. It suggests that the NASAStardust space mission analysis program should use Ramantechniques to examine the Al 1100 foils exposed during theflyby of comet 81P/Wild 2 in 2004 (at an encounter speed of6.1 km s−1). There are many large craters reported on thosefoils, with at least seven greater than 50 microns in size (i.e.,in the size range used here) already studied in great detail withSEM-EDS. A major portion of the grains impacting Stardustwere silicate materials such as olivine and pyroxene(Brownlee et al. 2006; Hörz et al. 2006) and olivine anddiopside (the Ca-Mg end member of the pyroxene family)provided positive results in the analysis here. Such an analysisapplied to Stardust craters would offer the chance of aspectroscopic identification of the impactors based on theirstructure, rather than just on elemental composition, as iscommon at present. Although the bulk of the mass (over 90%)that hit Stardust occurred in the larger particles (and hencelarger craters), the great majority of particles (and craters) aremuch smaller. However, for particles of 1 μm and below it isnot yet clear if this technique can be applied as an analysistool.

Acknowledgments—We thank the Particle Physics andAstronomy Research Council for grants to operate the light-gas gun. We thank Mr. M. Cole for the firing of the gun.F. Hörz is thanked for supplying the Stardust-grade aluminumfoil. Richard Chater of the Department of Materials at ImperialCollege London kindly prepared and imaged the FIB trenchshown in Fig. 2. N. J. F. thanks the University of Kent alumnifor funding.

Editorial Handling—Dr. Scott Sandford

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identification of mineral impactors in hypervelocity

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