Advances in Space Research 34 (2004) 2292–2298

www.elsevier.com/locate/asr

Extraction and microanalysis of cosmic dust captured duringsample return missions: laboratory simulations

G.A. Graham a,b,*, A.T. Kearsley b, A.L. Butterworth a,c, P.A. Bland d, M.J. Burchell e,D.S. McPhail f, R. Chater f, M.M. Grady b, I.P. Wright a

a Planetary and Space Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, UKb Department of Mineralogy, The Natural History Museum, London, SW7 5BD, UK

c Space Science Laboratory, University of California at Berkeley, Berkeley, CA 94720, USAd Department of Earth Sciences and Engineering, Imperial College, London, SW7 2BP, UK

e Centre for Astrophysics and Planetary Sciences, University of Kent at Canterbury, Kent, CT2 7NR, UKf Department of Materials, Imperial College, London, SW7 2BP, UK

Received 11 October 2002; received in revised form 30 June 2003; accepted 7 July 2003

Abstract

Particles of cometary and asteroidal origin collected at source using dedicated capture cell technologies will be returned to Earth

within the next 8 years. Furthermore, coincidental capture of interplanetary dust particles will occur on the exposed surfaces of the

Genesis spacecraft. Laboratory simulations using both light-gas-gun and Van de Graaff accelerators have impacted dust analogues

at velocities ranging from 5 km s�1 to ca. 72 km s�1 into comparable silicon and aerogel targets. Analysis of the impacts on silicon

has shown complete spallation of impact residues for silicate projectiles of 38–53 lm in diameter, however craters formed by 1 lmiron projectiles show that near-intact residues can be preserved. An olivine grain embedded in aerogel has been characterized in situ

using Raman micro-spectroscopy. Monte Carlo simulations and laboratory experiments have shown that analytical scanning

electron microscopy can also be used to characterize embedded grains. Development of a novel particle extraction methodology

using a 266 nm UV laser micro-dissection system has resulted in the recovery of an olivine grain. The extracted particle was then

‘‘cleaned up’’ using focused ion beam (FIB) milling to remove excess aerogel that was fused on the grain surface.

� 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Cosmic dust; Sample return missions; Extraction and microanalysis; Laboratory simulations

1. Introduction

Microprobe technique development over the past

30 years has enabled the analysis of smaller and smaller

materials, and application has been especially successful

in analysis of cosmic dust (Zolensky et al., 2000). De-tailed interpretation has been achieved on the compo-

sition of nanometre features within individual grains

(e.g. Dai et al., 2002). However, despite these milestones,

most techniques still have not presented detailed and

* Corresponding author. Present address: Institute of Geophysics

and Planetary Physics, Lawrence Livermore National Laboratory,

P.O. Box 808 L413, Livermore, CA 94550, USA. Tel.: +1-925-423-

5523; fax: +1-925-423-5733.

E-mail address: graham42@llnl.gov (G.A. Graham).

0273-1177/$30 � 2004 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2003.07.066

unambiguous evidence as to the parent body origin of

many captured particles. In order to resolve such fun-

damental unanswered questions this decade sees two

sample missions that will return cometary (Brownlee et

al., 2000) and asteroidal particles (Kawaguchi et al.,

1999) collected directly from source, providing ‘ground-truth’ data. The yield of information that these samples

could offer is potentially enormous, yet some important

issues need to be resolved quickly, before their return.

Two immediate problems are those of primary charac-

terization for curation, and the subsequent handling of

small yet precious samples, arising because the missions

are likely to return particles that are dominantly of

micrometer or smaller scale. Herein we discuss possibleanalytical strategies and an extraction method for par-

ticles embedded in silica aerogel. As the Genesis space-

ved.

G.A. Graham et al. / Advances in Space Research 34 (2004) 2292–2298 2293

craft will encounter interplanetary dust particles during

the mission lifetime we also discuss the potential for

recovery and interpretation of hypervelocity impact-

derived residues that may be preserved on several of the

collector surfaces.

2. Simulating potential hypervelocity impact collision on

the surfaces of the genesis spacecraft

The study of space-exposed surfaces that have been

impacted by micrometeoroids at hypervelocity speeds

has usually focused on those recovered from low Earth

orbit (e.g. Graham et al., 2001a). However any exposedsurface in any extra-atmospheric location will have the

potential of experiencing hypervelocity collision events,

thus the Genesis spacecraft whilst capturing solar wind

at L1 Lagrangean point (Clark, 2001) is also likely to

encounter interplanetary dust particles and b-meteor-

oids. Silicon wafers and high purity molybdenum-coated

platinum surfaces on Genesis may be acting as non-

dedicated capture media and could preserve impact-derived remnants of dust particles. We have simulated

the potential collision events that may occur between the

dust particles and silicon wafers by using a light-gas-gun

facility to impact 38–53 lm diameter rhodonite (man-

ganese silicate) projectiles into the targets at 5.7 km s�1

using a buck-shot technique (Burchell et al., 1999).

Rhodonite was chosen as it has very similar physical

properties to some natural extra-terrestrial minerals, yethas a very distinctive chemical composition that allows

unambiguous recognition of very small quantities of

residue. The impact craters are complex (Fig. 1(a)), and

show extensive shattering and spallation of material,

with a characteristic ‘‘Maltese Cross’’ outline as has

been noted in other studies (Taylor et al., 2001). De-

tailed X-ray elemental mapping of the numerous impact

craters failed to identify any remnants of the originalprojectiles. A second impact experiment was carried out

Fig. 1. (a) Back-scattered electron image of a typical impact feature on the si

by a 1 lm diameter projectile accelerated into the silicon target using the Van

is clearly visible in the image.

using a Van de Graaff accelerator to impact 1 lm iron

projectiles into the silicon target at velocities up to and

exceeding 72 km s�1. The required experimental condi-

tions limit the selection of projectile material used in the

Van de Graaff experiments (Burchell et al., 1999). The

higher velocities were used to simulate potential impactsfrom b-meteoroids that the exposed experimental sur-

faces from the Genesis spacecraft might encounter. The

impact encounter velocities range to much higher values

than those generated by a light-gas-gun (typical experi-

mental parameters are described in Burchell et al., 1999).

A significant number of these smaller craters revealed

Fe-projectile material embedded within a central melt

pit (Fig. 1(b)), apparently having escaped severe meltingor vaporization. We intend further work on these im-

pact features, however our preliminary experiments

suggest that the silicon wafers on the Genesis spacecraft

do have the potential to retain some chemical signature

of smaller impacting particles, even if they had a rela-

tively high velocity.

3. Laboratory simulation of dust capture in aerogel

One of the principle goals of the Stardust discovery

mission is to sample dust particles from Comet Wild 2

(Brownlee et al., 2000). Intact collection of particles

without experiencing extreme shock, melting or vapor-

ization where the relative encounter velocities of will be

approximately 6.2 km s�1, is an impossible task. Tsou(1995) showed that silica aerogel could be used to cap-

ture cosmic dust, and this material was adopted as a

dedicated, low-density capture cell. In the interim period

between launch and return, it is important to develop

optimum sample-handling, extraction and microanalysis

strategies. To simulate the capture of dust particles,

a variety of aerogel targets (densities ranging from

20 to 96 mg cm�3) supplied to the Open University byJPL/NASA and the University of Kent at Canterbury

licon target. (b) Secondary electron image of a smaller crater generated

de Graaff accelerator. The near-intact remnant of the original projectile

2294 G.A. Graham et al. / Advances in Space Research 34 (2004) 2292–2298

have been impacted using mineralogical analogues

ranging from well-characterized single olivine grains to

heterogeneous, crushed Allende meteorite powders. The

projectiles have been impacted at a speed of 5.1 km s�1

using a light-gas-gun; an experimental description is

given in Burchell et al. (1999). As the preliminary studiesof the returned material from Stardust may focus on

characterization of bulk chemistry and mineralogy, it is

important to assess suitable non-destructive techniques

that can be employed for early in situ measurements

prior to particle extraction.

4. In situ Raman microanalysis

There are numerous analytical techniques that can be

used to acquire bulk chemical and mineralogical infor-

mation of an isolated and exposed individual grain

(Zolensky et al., 2000), however it is important that the

initial characterization of precious materials does not

contaminate or damage the particles. Burchell et al.

(2001), suggested that due to the non-destructive natureand limited sample preparation requirements, Raman

spectroscopy would be well-suited to the preliminary

analysis of sample return materials, an idea supported

by previous studies of the carbon chemistry of individ-

ual interplanetary dust particles (Wopenka, 1988). As a

progression from the individual point-analysis spectra

acquired by Burchell et al. (2001), we have employed a

Jobin Yvon Horiba LabRam HR microprobe for in situ

Fig. 2. (a) An optical micrograph of the preserved hypervelocity track and th

surrounding aerogel and the captured olivine grain. The grain is approximate

signal from the grain can still be acquired. (c) A Y-modulated Raman inte

olivine.

imaging of an olivine grain embedded in aerogel. The

confocal sampling of this microprobe allows spectral

acquisition from a small, well-defined volume, as little as

2 lm in diameter. Beam scanning and automated stage-

movement permit rastering in three dimensions. So it is

possible to build two or three dimensional mineralogicalmaps of an embedded grain. To acquire the Raman

image (Fig. 2), a 514.5 nm laser excitation (argon ion gas

laser at 8 mW) is focused onto the grain using a 50�long working distance objective. The successful miner-

alogical characterization of an embedded crushed

Allende meteorite fragment using confocal Raman mi-

croscopy (Graham et al., 2001b) has shown that the

technique is well-suited to mapping complex polymi-neralic textures such as may be encountered in returned

samples.

5. In situ microanalysis using analytical scanning electron

microscopy

Confocal Raman microscopy and mapping can re-veal the mineralogical composition of some particles

through encasing aerogel, however material potentially

captured during the encounter with Comet Wild

2 may not yield particularly strong or any Raman

signals at all, therefore other techniques must be in-

vestigated. X-ray fluorescence stimulated by intensely

focused X-rays (e.g. from a synchrotron source, Flynn

et al., 2000) can be used to determine the elemental

e olivine grain at the terminus. (b) The Raman spectra obtained for the

ly 1mm beneath the surface of the aerogel yet a relative strong spectral

nsity map generated for the characteristic 825 cm�1 Raman band for

Fig. 3. (a) Monte Carlo simulation of 40 keV electron beam on quartz.

Note 20 lm scale bar. (b) Monte Carlo simulation of 40 keV electron

beam on silica aerogel. Note the scale bar of 1 mm. The Monte Carlo

simulation of Silicon X-ray emission from silica aerogel under 40 keV

electron beam is also shown, generated to a depth of approximately

700 lm.

G.A. Graham et al. / Advances in Space Research 34 (2004) 2292–2298 2295

composition deep within aerogel, but requires bulky,

expensive and unusual instrumentation. Surprisingly,

the characteristics of aerogel may, under some cir-

cumstances, allow electron-stimulated X-ray fluores-cence to be applied successfully upon particles below

the surface. Conventional X-ray microanalysis relies

upon the stimulation of characteristic X-rays in a very

small sample volume immediately beneath a focused

Fig. 4. (a) Backscattered electron image (BEI) showing the ‘end-on’ view of

maps for Aluminium Ka, Silicon Ka and Copper Ka emission from surface

electron beam ‘spot’. The scattering of beam electrons

within the sample can be modelled by Monte Carlo

methods, allowing prediction of the volume of ener-

getic electron penetration, the distribution of electron

back-scattering and the location of characteristic

X-ray emission. In a typical substrate such as silica(e.g. the mineral quartz) 40 keV beam electrons can

penetrate to a maximum depth of about 40 lm below

the surface (Fig. 3(a)). However, the bulk of beam-

stimulated X-radiation that reaches the X-ray detector

comes from a depth of substantially less than 10 lm(usually about 5 lm), especially as absorption within

the sample further reduces the flux. This restricted

sample volume is a desirable property where highspatial resolution is required. By contrast, the

extraordinarily low density of flight-grade silica aero-

gel (for the simulation the density value of the aerogel

was 58 mg cm�3, which is a little higher than the ac-

tually density of the Stardust aerogel) permits much

greater penetration of the high-energy electron beam,

well-beyond 1 mm in depth. X-ray emission also

occurs within a broad zone to great depth (shown inFig. 3(b)), and little X-ray absorption takes place

along paths to the detector.

Laboratory experiments using a 40 keV electron

beam focused onto the surface of a triangular wedge of

aerogel (Fig. 4(a)) reveal that detectable excitation of

aluminium Ka X-ray radiation (Fig. 4(b)) from the

underlying surface of a metal stub can be achieved

routinely and quickly through a depth of at least 400 lmof aerogel. Aluminium Ka radiation (Fig. 5) can be

distinguished through 450 lm of aerogel (three times

background count rate, brehmstrahlung plus noise) after

only 10 ms. This count rate allows X-ray maps of a

sample area 200� 150 lm in dimension to be collected

in less than 10 min duration, sufficient to locate a 5 lm-

impacted particle (e.g. olivine, a natural magnesium

silicate component) more than 300 lm beneath thesurface.

aerogel wedge mounted upon an aluminium stub. (b) BEI and X-ray

and depth below aerogel slice.

Fig. 6. Secondary electron image of the tracks cut into aerogel with 266

nm, 10 Hz pulsed laser show that power is critical to a clean cut; here

two tracks were rastered using 4 mJ/pulse (track 1) and then 3 mJ/pulse

(track 2). The image also shows how brittle aerogel is at the sub-mil-

limetre scale.Fig. 5. Line scan of Aluminium Ka emission along a line crossing the

aerogel wedge, with thickening aerogel above the aluminium from left

to right. Note loss of signal above background when aerogel exceeds

600 lm thickness.

2296 G.A. Graham et al. / Advances in Space Research 34 (2004) 2292–2298

Although the beam energy in most scanning electron

microscopes cannot be raised sufficiently to allow

sampling to the same depth as synchrotron sources or

Raman microscopes, the sub-surface mapping does haveparticular potential in the location of shallow particles,

especially as it could be employed in stereo pair imagery.

6. Particle extraction

Probably the most significant challenge to overcome

before the actual return of Stardust samples is the ex-traction procedure of particles from the aerogel. Several

novel techniques have previously been applied to the

problem, (e.g. Stadermann and Floss, 2000) with varying

degrees of success. Recent work by Westphal et al.

(2002), using micro-needles and micro-tweezers has

shown that particles can be removed routinely from

aerogel, however the entire extraction process can be

time consuming. We have concurrently investigated theuse of a specialized laser system developed as an ex-

traction procedure for Genesis return samples (Butter-

worth et al., 2000). The laser is a 266 nm, 10 Hz pulsed,

quadrupled Nd:YAG laser (Spectron Laser Systems

SL404) with a microscope machine head and a color

video camera (Hitachi VK-C180E). Early tests identified

that this laser frequency was suitable for ablating aerogel

(Fig. 6). The manually focused beam, of 50–100 lm spotsize, is held in a fixed vertical orientation, which allows

the top surface of a block of aerogel to be ablated. The

aerogel can be moved relative to the laser beam on a 1 lmprecision, fixed-height, programmable two dimensional

stage (M€arzh€auser Wetzlar MultiControl 2000). Com-

puter-controlled speed and movement of the X–Y stage

allowed tracks and shapes to be cut.

The system was used to attempt the recovery of an

individual olivine grain. The UV laser ablation removed

up to �0.2 mm depth of aerogel in one pass, therefore it

was necessary to raster many times to slice out a 2 mm

plug of aerogel containing the embedded grain (Fig. 7).

The whole block was also rotated to carve out the plugfrom different directions and the height of the ablated

surface had to be constantly adjusted to remain within

the narrow focal distance of the beam. Cutting deep

tracks in aerogel was sensitive to beam optics and laser

power; both were critical to ensure efficient ablation, but

avoiding the formation of an opaque ‘‘snow’’ (which

made visual checks and manual adjustments more la-

borious). The aerogel in the plug showed little or noradiation damage; therefore we assumed that the grain

was also unaffected. The removed plug was still a

practical size to be handled, but fragile and brittle. In

the case shown, the plug snapped across the direction of

the grain’s entry track, thus leaving the olivine grain

completely exposed and in a suitable location to extract

using micromanipulators.

7. Focused beam ion milling of extracted grain

Extracted particles that have been embedded into

silica aerogel during hypervelocity collision in the lab-

oratory usually become coated in a layer of degraded

aerogel (Fig. 8(a)). The same observation has been made

on particles recovered from the aerogel in the originallaboratory feasibility studies (Barrett et al., 1992) and in

low-Earth orbit (H€orz et al., 1999). Potentially this

partial covering could be problematic when attempting

to obtain high precision microanalysis measurements

and therefore it is desirable to be able to remove the

melted layer of the aerogel. The olivine grain isolated by

Fig. 8. (a) Secondary electron image of an extracted olivine grain from aerogel. The smooth, melted aerogel coat fused to the surface of the grain is

clearly visible. (b) Secondary electron image after FIB milling.

Fig. 7. (a) Optical micrograph of an area of aerogel subjected to UV laser ablation. (b) Optical micrograph of the top of a 2 mm plug of aerogel,

which has the olivine grain now exposed on the surface.

G.A. Graham et al. / Advances in Space Research 34 (2004) 2292–2298 2297

laser cutting (described above) was removed from the

aerogel surface and examined using a FEI focused ionbeam (FIB) 200 TEM workstation. FIB microscopy is a

powerful new technique for preparing and manipulating

materials at the sub-micrometer scale. The focused beam

of gallium ions was used to ablate the melted aerogel in

precise locations and at carefully controlled rates on the

grain surface (Fig. 8(b)). However it should be pointed

out that the ion milling process does deposit gallium

from the primary beam and ablated material on thesample that are potential sources of contamination

during later microanalysis. Furthermore the high ac-

celerating voltages that can be used during the milling

can form an amorphous film on the sample although it is

possible to ‘‘clean-up’’ the sample using lower acceler-

ating voltages (Lee et al., 2003).

8. Future developments

So far our investigation into best practice strategies for

handling, extracting and analysing recovered particles

from aerogel has focused on the preliminary character-

ization stages. Also, it should be pointed out that in ourexperiments we have been using projectiles in the 38–50

lm diameter size range. This is a good size range to de-

velop the extraction and microanalysis techniques, but

the particles captured by Stardust collectors will be

smaller, ranging up to 20 lm in diameter. It is now im-

portant to assess new and developing techniques thatmay

assist in the analysis of more representative analogues to

the recovered particles. For example, the development ofa new type of high-brightness X-ray sourcemicroprobe at

the Natural History Museum will allow both rapid phase

identification and quantification of abundance in situ for

small samples, and has the potential for automated phase

mapping, imaging and analysis (Bland et al., 2001).

9. Conclusions

Light-gas-gun and Van de Graaff accelerators have

enabled laboratory simulation of dust capture on a

variety of substrates. Such experiments are vital as

2298 G.A. Graham et al. / Advances in Space Research 34 (2004) 2292–2298

preparation for deciphering the mineralogy and chemis-

try of materials returned to Earth within the next 8 years,

whether captured apropos on the Genesis Sample Return

Capsule, or cometary particles in dedicated aerogel col-

lectors on the Stardust spacecraft. Herein we have shown

that particles captured in aerogel can be characterized insitu, using confocal Raman microscopy and analytical

scanning electron microscopy. A novel laser ablation

system can successfully extract grains embedded in

aerogel. Melted aerogel can be removed from the surface

of a grain by high precision, FIB milling. While the best-

practice strategies for the preliminary stages of handling

materials such as Stardust samples are now in place, it is

important to continue to assess new technologies onappropriate analogues so that the maximum yield of in-

formation can be acquired from these unique materials.

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