Implications for the Wild-2 dust distribution of the statistical results obtained by SEM-EDS from nearly 300 impact craters on aluminium foils of the Stardust sample tray assembly

Borg, J.

Bridges, J. C.

Burchell, M. J.

Djouadi, Z.

Floss, C.

Graham, G. A.

Green, S. F.

Heck, P. R.

Hoppe, P.

Huth, J.

Hörz, F.

Kearsley, A.

Leroux, H.

Marhas, K.

Stadermann, F. J.

Teslich, N.

English

Bridges, J.

Burchell, M.

Graham, G.

Green, S.

Heck, P.

Stadermann, F.

MPG.PuRe

SEM-EDS analyses of small craters in Stardust aluminum foils: Implications for the Wild-2 dust distribution

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsSEM-EDS analyses of small craters in stardustaluminium foils: implications for the Wild-2 dustdistributionConference or Workshop ItemHow to cite:Borg, J.; Ho¨rz, F.; Bridges, J. C.; Burchell, M. J.; Djouadi, Z.; Floss, C.; Graham, G. A.; Green, S. F.; Heck,P. R.; Hoppe, P.; Huth, J.; Kearsley, A.; Leroux, H.; Marhas, K.; Stadermann, F. J. and Teslich, N. (2007). SEM-EDSanalyses of small craters in stardust aluminium foils: implications for the Wild-2 dust distribution. In: 38th Lunar andPlanetary Science Conference, 12-16 Mar 2007, League City, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1592.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukSEM-EDS ANALYSES OF SMALL CRATERS IN STARDUST ALUMINIUM FOILS: IMPLICATIONSFOR THE WILD-2 DUST DISTRIBUTION.  J. Borg1, F. Hörz2, J.C. Bridges3, M.J. Burchell4, Z. Djouadi1, C.Floss5, G.A. Graham6, S.F. Green3, P.R. Heck7, P. Hoppe7, J. Huth7 , A. Kearsley8, H. Leroux9, K. Marhas5, F.J. Sta-dermann5 and N. Teslich6. 1Institut d’Astrophysique Spatiale, CNRS, Université Paris-Sud, UMR8617, F-91405 Orsay-Cedex, France,    janet.borg@ias.u-psud.fr   , 2KT NASA Johnson Space Center, Houston, TX 77058, USA,3PSSRI, Open University, Milton Keynes, MK7 6AA, UK, 4School of Physical Sciences, University of Kent, Canterbury,Kent CT2 7NH, UK, 5Laboratory for Space Sciences and Physics Department, Washington University, St. Louis MO 63130,USA, 6IGPP, Lawrence Livermore National Laboratory, 7000 East Avenue, L413, Livermore, CA 94550, USA, 7Max-Planck-Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany, 8IARC, Department of Mineralogy, The Natural History Mu-seum, London SW7 5BD, UK, 9Laboratoire de Structure et Propriétés de l’Etat Solide, UMR 8008, Université des Sciences etTechnologies de Lille F59655 Villeneuve d’Ascq, France.Introduction: Aluminium foils were used on Star-dust to stabilize the aerogel specimens in the modularcollector tray.  Part of these foils were fully exposed tothe flux of cometary grains emanating from Wild 2.Because the exposed part of these foils had to be har-vested before extraction of the aerogel, numerous foilstrips some 1.7 mm wide and 13 or 33 mm long weregenerated during Stardusts’s Preliminary Examination(PE). These strips are readily accommodated in theirentirety in the sample chambers of modern SEMs, thusproviding the opportunity to characterize in situ thesize distribution and residue composition - employingEDS methods - of statistically more significant num-bers of cometary dust particles compared to aerogel,the latter mandating extensive sample preparation [1].We describe here the analysis of nearly 300 impactcraters and their implications for Wild 2 dust.Experimental set-ups: During PE, some 25 foilswere analysed by  SEM/EDS techniques in a variety oflaboratories, substantially following [e.g.2,3]. In ourlaboratories, secondary electron imaging was carriedout at 5 or 20 kV, while EDS was performed at 20 kVfor heavy elements investigation and at 5 or 7 kV forlight elements. Spatial resolution varied, and rangedfrom low resolutions of the entire foil strip, overlap-ping with optical microscope surveys of craters > 20µm in size,  to high resolution scans at crater sizes of100 nm.The goals of this investigation were to:  i) carefullylocate all the impact events to determine  their spatialdistribution, ii) characterize the morpholo-gies(diameter, depth, surface roughness) of the impactcraters and relate them to the physical properties of theincident particles (size, density) based on dedicatedcalibration experiments [4,5] at Stardust’s constantencounter velocity of 6.1 km/s and iii) obtain compo-sitional information on the melted residues on the cra-ter bottoms, walls and rims, which are in part inti-mately mixed with molten Al.Calibrations performed prior to Stardust returnwith sodalime glass beads impacted at ~ 6 km/s gave aratio of 4.6 between the lip-to-lip diameter of the craterand the mean diameter of the incident grain [5]. Addi-tional shots with various materials of different densi-ties and/or porosities indicate that this ratio does notvary greatly for silicate impactors, while the depth ofthe crater and the rugosity of the crater walls and bot-tom can be affected [6]. Concerning the chemicalanalysis, laboratory simulations performed with vari-ous minerals (olivine, diopside, plagioclase feldspar,pyrrhotite, etc.) indicate that the residue thickness maybe only a few tens of nanometers, when the incidentparticles are in the micron size range, as is the case formost of the Wild-2 grains (see paragraph on “Re-sults”). Such a thickness is much smaller than thedepth of the primary electron beam interaction volumeand complex absorption effects then take place, thatcannot be quantified, so only qualitative identificationof the major elements is possible and accurate deter-mination of elemental ratios is not feasible in situ [6].Results:  During PE, 25 strips of foils were thor-oughly scanned by the various members of the PE,allowing the characterization of 292 craters, in terms oflocalization, size and composition, on a total area of~ 10 cm2 of foil surveyed.Clustering. A large variation in the crater distribu-tion was found at the mm2 scale, with a range from 0 tomore than 50 craters identified per foil [1]. Three foils(C2008N, C2020W and C2044N) are heavily clus-tered, holding about one third of all the craters ana-lysed by SEM/EDS in this study; furthermore, fivefoils (C2037N, C2044W, C2052N, C2055N andC2068W) each contain more than 10 craters, so thatthe eight “crater - clustered foils” contain ~ 85% of thetotal number of craters, in less than one third of thescanned area. This clustering of impacts and the strongvariation in the spatial distribution of impact features isdiscussed in [7] and is not yet fully understood.Size distribution. More than 90% of the identifiedcraters are smaller than 5 µm in diameter, yet most areof submicron size. This implies that the size distribu-tion of the Wild-2 dust grains is dominated by particlesof submicron sizes, as small as a few tens of nanome-ters. Such small grains cannot easily be identifed in theLunar and Planetary Science XXXVIII (2007) 1592.pdfaerogel collectors and have not been extracted to date.So the study of their impact features on foil remainsthe only possibility for characterizing the dominantpopulation of Wild-2 dust. High resolution images ofthese craters show a non-uniform melting of their inte-rior (fig 1) and even, in some cases, the presence ofsmall crystalline residues, as confirmed by FocusedIon Beam – Transmission Electron Microscope (FIB-TEM) analyses [8]. Also, many of the craters, (e.g.figure 1) are clearly due to impact by composite grains(i.e. containing diverse internal components), as theypresent a markedly non-circular outline, not the simplecircular bowl shapes of [6]. Figure 1 – High resolution SEM image of one crater on foilC2043N. Its interior is not fully melted and its shape is notround, both features characteristic of impact by a compositegrain.Chemical identification. EDS analysis was per-formed on 263 craters, all smaller than 5 µm. All theelemental signatures found are potentially extraterres-trial in origin; no man made components were identi-fied and thus there was no evidence of secondaryejecta from impacts elsewhere on the Stardust space-craft. Nearly 50% of the projectiles appear to be com-posite grains containing variable amounts of Mg, Fesilicates (probably olivines and/or pyroxenes) and Fesulfides, while ~ 25%  are Mg,Fe silicates alone and~ 18% pure Fe sulfides alone (fig 2).Figure 2 – Mineralogical distribution of residues in the craters.The few remaining residues either show no EDSsignature or dominance by other elements (Ca, Na, ….)in various proportions. C and O have not been consid-ered in this statistical evaluation, as the analyticalmethod varies from group to group. Figure 3 showsthat the sub-micron particles also display the samedomination by composites seen in the larger ones.Figure 3 – Mineralogy distribution as a function of the size of thecrater, clearly showing that the smallest impactors are mainlycomposite grains.Conclusion: SEM/EDS, routinely used as an in-vestigation tool of impact craters in metallic targetsappears to be a very useful and sensitive tool to obtainstatistical surveys of the Wild-2 dust. It permits for thecharacterization of the smallest size fraction of thisdust, by far the most numerous, that cannot be studiedcurrently when trapped in aerogel. It must be consid-ered as a first tool to be followed by high resolutionanalytical techniques such as NanoSIMS [9], TOF-SIMS [10], FIB-SEM [11] or FIB-TEM [8], used dur-ing the PE on some craters, providing a more completeunderstanding in terms of chemical, mineralogical orisotopical dust compositions. In the future, SEM/EDSwill continue as a survey tool to possibly identify com-positionally rare and interesting particles as candidatesfor these more quantitative, high resolution characteri-zations of Wild-2 dust.References: [1] Hörz et al. (2006) Science, 314 ,1716-1719 [2] Graham G.A. et al. (2001) Proc. 3rdEuropean Space Debris Conf., ESA Special Publication473, 197-203. [3] Kearsley A.T. et al. 2005 LPS XXXVI,Abstract #1670, [4] Leroux H. et al. (2006) MAPS,41.2, 181-196, [5] Kearsley A.T. et al. (2006) MAPS,41.2, 167-180, [6] Kearsley A.T. et al, (2007) MAPS42.2 in press, [7] Westphal et al. (2007) submitted toMAPS, [8] Leroux H. et al. (2007) to submit to MAPS ,[9] Hoppe P.et al. (2006).MAPS 41.2  197-210, [10]Stephan T. et al. (2006) MAPS 41.2, 211-216, [11]Bridges J.C. et al. (2007) submitted to LPS XXXVIII.Lunar and Planetary Science XXXVIII (2007) 1592.pdf

SEM-EDS analyses of small craters in stardust aluminium foils: implications for the Wild-2 dust distribution

Aluminium foils were used on Stardust to stabilize the aerogel specimens in the modular collector tray. Part of these foils were fully exposed to the flux of cometary grains emanating from Wild 2. Because the exposed part of these foils had to be harvested before extraction of the aerogel, numerous foil strips some 1.7 mm wide and 13 or 33 mm long were generated during Stardusts's Preliminary Examination (PE). These strips are readily accommodated in their entirety in the sample chambers of modern SEMs, thus providing the opportunity to characterize in situ the size distribution and residue composition - employing EDS methods - of statistically more significant numbers of cometary dust particles compared to aerogel, the latter mandating extensive sample preparation. We describe here the analysis of nearly 300 impact craters and their implications for Wild 2 dust

Horz, F.

Kearsley, A

NASA Technical Reports Server

SEM-EDS ANALYSES OF SMALL CRATERS IN STARDUST ALUMINIUM FOILS: IMPLICATIONS
FOR THE WILD-2 DUST DISTRIBUTION.  J. Borg1, F. Hörz2, J.C. Bridges3, M.J. Burchell4, Z. Djouadi1, C.
Floss5, G.A. Graham6, S.F. Green3, P.R. Heck7, P. Hoppe7, J. Huth7 , A. Kearsley8, H. Leroux9, K. Marhas5, F.J. Sta-
dermann5 and N. Teslich6. 1Institut d’Astrophysique Spatiale, CNRS, Université Paris-Sud, UMR8617, F-
91405 Orsay-Cedex, France,    janet.borg@ias.u-psud.fr   , 2KT NASA Johnson Space Center, Houston, TX 77058, USA,
3PSSRI, Open University, Milton Keynes, MK7 6AA, UK, 4School of Physical Sciences, University of Kent, Canterbury,
Kent CT2 7NH, UK, 5Laboratory for Space Sciences and Physics Department, Washington University, St. Louis MO 63130,
USA, 6IGPP, Lawrence Livermore National Laboratory, 7000 East Avenue, L413, Livermore, CA 94550, USA, 7Max-Planck-
Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany, 8IARC, Department of Mineralogy, The Natural History Mu-
seum, London SW7 5BD, UK, 9Laboratoire de Structure et Propriétés de l’Etat Solide, UMR 8008, Université des Sciences et
Technologies de Lille F59655 Villeneuve d’Ascq, France.
Introduction: Aluminium foils were used on Star-
dust to stabilize the aerogel specimens in the modular
collector tray.  Part of these foils were fully exposed to
the flux of cometary grains emanating from Wild 2.
Because the exposed part of these foils had to be har-
vested before extraction of the aerogel, numerous foil
strips some 1.7 mm wide and 13 or 33 mm long were
generated during Stardusts’s Preliminary Examination
(PE). These strips are readily accommodated in their
entirety in the sample chambers of modern SEMs, thus
providing the opportunity to characterize in situ the
size distribution and residue composition - employing
EDS methods - of statistically more significant num-
bers of cometary dust particles compared to aerogel,
the latter mandating extensive sample preparation [1].
We describe here the analysis of nearly 300 impact
craters and their implications for Wild 2 dust.
Experimental set-ups: During PE, some 25 foils
were analysed by  SEM/EDS techniques in a variety of
laboratories, substantially following [e.g.2,3]. In our
laboratories, secondary electron imaging was carried
out at 5 or 20 kV, while EDS was performed at 20 kV
for heavy elements investigation and at 5 or 7 kV for
light elements. Spatial resolution varied, and ranged
from low resolutions of the entire foil strip, overlap-
ping with optical microscope surveys of craters > 20
µm in size,  to high resolution scans at crater sizes of
100 nm.
The goals of this investigation were to:  i) carefully
locate all the impact events to determine  their spatial
distribution, ii) characterize the morpholo-
gies(diameter, depth, surface roughness) of the impact
craters and relate them to the physical properties of the
incident particles (size, density) based on dedicated
calibration experiments [4,5] at Stardust’s constant
encounter velocity of 6.1 km/s and iii) obtain compo-
sitional information on the melted residues on the cra-
ter bottoms, walls and rims, which are in part inti-
mately mixed with molten Al.
Calibrations performed prior to Stardust return
with sodalime glass beads impacted at ~ 6 km/s gave a
ratio of 4.6 between the lip-to-lip diameter of the crater
and the mean diameter of the incident grain [5]. Addi-
tional shots with various materials of different densi-
ties and/or porosities indicate that this ratio does not
vary greatly for silicate impactors, while the depth of
the crater and the rugosity of the crater walls and bot-
tom can be affected [6]. Concerning the chemical
analysis, laboratory simulations performed with vari-
ous minerals (olivine, diopside, plagioclase feldspar,
pyrrhotite, etc.) indicate that the residue thickness may
be only a few tens of nanometers, when the incident
particles are in the micron size range, as is the case for
most of the Wild-2 grains (see paragraph on “Re-
sults”). Such a thickness is much smaller than the
depth of the primary electron beam interaction volume
and complex absorption effects then take place, that
cannot be quantified, so only qualitative identification
of the major elements is possible and accurate deter-
mination of elemental ratios is not feasible in situ [6].
Results:  During PE, 25 strips of foils were thor-
oughly scanned by the various members of the PE,
allowing the characterization of 292 craters, in terms of
localization, size and composition, on a total area of
~ 10 cm2 of foil surveyed.
Clustering. A large variation in the crater distribu-
tion was found at the mm2 scale, with a range from 0 to
more than 50 craters identified per foil [1]. Three foils
(C2008N, C2020W and C2044N) are heavily clus-
tered, holding about one third of all the craters ana-
lysed by SEM/EDS in this study; furthermore, five
foils (C2037N, C2044W, C2052N, C2055N and
C2068W) each contain more than 10 craters, so that
the eight “crater - clustered foils” contain ~ 85% of the
total number of craters, in less than one third of the
scanned area. This clustering of impacts and the strong
variation in the spatial distribution of impact features is
discussed in [7] and is not yet fully understood.
Size distribution. More than 90% of the identified
craters are smaller than 5 µm in diameter, yet most are
of submicron size. This implies that the size distribu-
tion of the Wild-2 dust grains is dominated by particles
of submicron sizes, as small as a few tens of nanome-
ters. Such small grains cannot easily be identifed in the
Lunar and Planetary Science XXXVIII (2007) 1592.pdf
https://ntrs.nasa.gov/search.jsp?R=20070011620 2019-08-30T00:38:01+00:00Z
aerogel collectors and have not been extracted to date.
So the study of their impact features on foil remains
the only possibility for characterizing the dominant
population of Wild-2 dust. High resolution images of
these craters show a non-uniform melting of their inte-
rior (fig 1) and even, in some cases, the presence of
small crystalline residues, as confirmed by Focused
Ion Beam – Transmission Electron Microscope (FIB-
TEM) analyses [8]. Also, many of the craters, (e.g.
figure 1) are clearly due to impact by composite grains
(i.e. containing diverse internal components), as they
present a markedly non-circular outline, not the simple
circular bowl shapes of [6].
 Figure 1 – High resolution SEM image of one crater on foil
C2043N. Its interior is not fully melted and its shape is not
round, both features characteristic of impact by a composite
grain.
Chemical identification. EDS analysis was per-
formed on 263 craters, all smaller than 5 µm. All the
elemental signatures found are potentially extraterres-
trial in origin; no man made components were identi-
fied and thus there was no evidence of secondary
ejecta from impacts elsewhere on the Stardust space-
craft. Nearly 50% of the projectiles appear to be com-
posite grains containing variable amounts of Mg, Fe
silicates (probably olivines and/or pyroxenes) and Fe
sulfides, while ~ 25%  are Mg,Fe silicates alone and
~ 18% pure Fe sulfides alone (fig 2).
Figure 2 – Mineralogical distribution of residues in the craters.
The few remaining residues either show no EDS
signature or dominance by other elements (Ca, Na, ….)
in various proportions. C and O have not been consid-
ered in this statistical evaluation, as the analytical
method varies from group to group. Figure 3 shows
that the sub-micron particles also display the same
domination by composites seen in the larger ones.
Figure 3 – Mineralogy distribution as a function of the size of the
crater, clearly showing that the smallest impactors are mainly
composite grains.
Conclusion: SEM/EDS, routinely used as an in-
vestigation tool of impact craters in metallic targets
appears to be a very useful and sensitive tool to obtain
statistical surveys of the Wild-2 dust. It permits for the
characterization of the smallest size fraction of this
dust, by far the most numerous, that cannot be studied
currently when trapped in aerogel. It must be consid-
ered as a first tool to be followed by high resolution
analytical techniques such as NanoSIMS [9], TOF-
SIMS [10], FIB-SEM [11] or FIB-TEM [8], used dur-
ing the PE on some craters, providing a more complete
understanding in terms of chemical, mineralogical or
isotopical dust compositions. In the future, SEM/EDS
will continue as a survey tool to possibly identify com-
positionally rare and interesting particles as candidates
for these more quantitative, high resolution characteri-
zations of Wild-2 dust.
References: [1] Hörz et al. (2006) Science, 314 ,
1716-1719 [2] Graham G.A. et al. (2001) Proc. 3rd
European Space Debris Conf., ESA Special Publication
473, 197-203. [3] Kearsley A.T. et al. 2005 LPS XXXVI,
Abstract #1670, [4] Leroux H. et al. (2006) MAPS,
41.2, 181-196, [5] Kearsley A.T. et al. (2006) MAPS,
41.2, 167-180, [6] Kearsley A.T. et al, (2007) MAPS
42.2 in press, [7] Westphal et al. (2007) submitted to
MAPS, [8] Leroux H. et al. (2007) to submit to MAPS ,
[9] Hoppe P.et al. (2006).MAPS 41.2  197-210, [10]
Stephan T. et al. (2006) MAPS 41.2, 211-216, [11]
Bridges J.C. et al. (2007) submitted to LPS XXXVIII.
Lunar and Planetary Science XXXVIII (2007) 1592.pdf


SEM-EDS Analyses of Small Craters in Stardust Aluminum Foils: Implications for the Wild-2 Dust Distribution

http://oro.open.ac.uk/8735/1/SEM-EDS_analyses_of_small_craters_in_Stardust_aluminium_foils.pdf

SEM-EDS analyses of small craters in stardust aluminium foils: implications for the Wild-2 dust distribution

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