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Acta Astronautica

Acta Astronautica 107 (2015) 163–176

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Physicochemical properties of respirable-size lunar dust

D.S. McKay †,a, B.L. Cooper b,n, L.A. Taylor c, J.T. James d, K. Thomas-Keprta e,C.M. Pieters f, S.J. Wentworth e, W.T. Wallace g, T.S. Lee b

a Astromaterials Research and Exploration Science, KA, NASA Johnson Space Center, Houston, TX 77058, USAb International Space Exploration Research Institute, Hanyang University, Ansan, Gyeonggi-do 426-791, Republic of Koreac Planetary Geosciences Institute, University of Tennessee , Knoxville, TN 37996-1410, USAd Biomedical Research and Environmental Sciences Division, NASA Johnson Space Center, Houston, TX 77058, USAe Astromaterials Research Group, Jacobs Technology ESCG, Houston, TX 77258-8447, USAf Department of Geological Sciences, Brown University, Providence, RI 02912, USAg Wyle Science, Technology, and Engineering Group, Houston, TX 77058, USA

a r t i c l e i n f o

Article history:Received 21 April 2014Received in revised form20 October 2014Accepted 25 October 2014Available online 7 November 2014

Keywords:Lunar dustDust toxicityNanophase ironSpace weatheringLunar gardeningLunar samples

x.doi.org/10.1016/j.actaastro.2014.10.03265/& 2014 IAA. Published by Elsevier Ltd. A

viations: npFe0, nano-phase iron; IS/FeO, mae ratio of np-Fe0 normalized to the total ironed as FeO; PSD, Particle size distribution. Thismean, area mean, volumetric mean, or masiameter, depending on the needs of the resesponding author.ail address: bcooper108@gmail.com (B.L. Cooceased.

a b s t r a c t

We separated the respirable dust and other size fractions from Apollo 14 bulk sample14003,96 in a dry nitrogen environment. While our toxicology team performed in vivoand in vitro experiments with the respirable fraction, we studied the size distribution andshape, chemistry, mineralogy, spectroscopy, iron content and magnetic resonance ofvarious size fractions. These represent the finest-grained lunar samples ever measured foreither FMR np-Fe0 index or precise bulk chemistry, and are the first instance we know ofin which SEM/TEM samples have been obtained without using liquids.

The concentration of single-domain, nanophase metallic iron (np-Fe0) increases asparticle size diminishes to 2 mm, confirming previous extrapolations. Size-distributionstudies disclosed that the most frequent particle size was in the 0.1–0.2 mm rangesuggesting a relatively high surface area and therefore higher potential toxicity.

Lunar dust particles are insoluble in isopropanol but slightly soluble in distilled water(�0.2 wt%/3 days). The interaction between water and lunar fines, which results in bothagglomeration and partial dissolution, is observable on a macro scale over time periods ofless than an hour.

Most of the respirable grains were smooth amorphous glass. This suggests less toxicitythan if the grains were irregular, porous, or jagged, and may account for the fact that lunardust is less toxic than ground quartz.

& 2014 IAA. Published by Elsevier Ltd. All rights reserved.

1. Introduction

The nature of the finest fraction of lunar soil is of interestto lunar scientists because of its genesis in an airless

ll rights reserved.

turity index of lunarin the soil fraction,may be measured ass median aerody-earcher.

per).

environment and its influence on the spectral properties ofbulk soils. Inhalation toxicologists are concerned with lunardust because it can be respired into the lungs of humanexplorers. Environmental-control engineers are also inter-ested in the properties of the dust because they must createa system capable of removing it from lunar habitats androvers. The magnetic properties of the finest dust may offer away to capture the dust when it enters a lunar habitat [1].

The Apollo astronauts were exposed to lunar dust thatwas inadvertently brought into the lunar modules aftereach surface field trip. Some of the astronauts reportedthat the dust was irritating when inhaled [2], but crew

Fig. 1. Nanophase iron particles (np-Fe0) in lunar soil 14003,96 seen in abright-field transmission electron micrograph (TEM-BF). The iron grains,which appear dark or black in this image, are all smaller than 30 nm indiameter.

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176164

exposures to dust were limited to only a few days. Whilenone of the astronauts reported any serious detrimentaleffects from these short exposures, future crews may beexposed to lunar dust for months, so it is important todetermine if longer exposures could be hazardous to theirhealth.

The depth of penetration of dust and percent deposi-tion in the human lung depend on the size of the dustgrains [3–5]. In addition to grain-size-driven lung deposi-tion, the toxicity of a mineral dust is also affected by itscrystalline structure, chemical composition, and reactivesurface areas. For example, crystalline silica is significantlymore toxic to the lung than amorphous silica of the sameparticle size [6]. Quartz (SiO2) is much more toxic than thenuisance mineral-dust TiO2 [6]. Freshly fractured quartzsurfaces are more reactive than aged quartz, due to anabundance of reactive unsatisfied bonds, and this propertyis known to produce detrimental physiological effects inhuman lungs [7].

Although lunar soils usually contain little quartz (butsee [8] p. 133), their surface reactivity may be substantial.Physical breaking (micrometeorite comminution), solar-wind proton bombardment, and UV radiation can all resultin broken chemical bonds. These broken bonds are rapidlyneutralized on Earth by water chemisorption but maypersist indefinitely on the Moon [9].

This report focuses on the chemistry, mineralogy, size,morphology, maturity and spectral properties of lunar dust,in particular the particles of less than 2 μm volumetricmedian diameter. Particles of this size were separated frombulk lunar soil 14003,96 for use in both in vitro and in vivo(animal) testing.

Two related papers demonstrate how these dusts wereused to estimate a safe exposure level for astronauts con-ducting sustained operations on the lunar surface [10,11].The physical and chemical properties of these particles maysuggest ways that engineers might remove respirable dust tomaintain safe conditions within a lunar habitat or vehicle.

1.1. Lunar dust characterization, soil maturity,and abundance of nanophase metallic iron

When micrometeorites (o1 mm) impact the lunar soilat typical velocities of �15 km/s [12], they transfer so muchenergy that some of the impacted soil reaches temperatureshigh enough to vaporize oxide, silicate and glassy compo-nents of the soil, and the FeO (among other species)dissociates into Fe and O [13–15]. The vaporized materialpenetrates the soil and condenses to form amorphous rinds,in some cases containing nano-phase iron (np-Fe0) spheresthat range in size from 10 nm up [14,16,17] (Fig. 1).

The maturity of a lunar soil refers to the length of timethat the soil has been exposed on the lunar surface. Spaceweathering that occurs during this time can be measuredby comparing the amount of np-Fe0 to the total amount ofiron (including iron oxides) present in the soil. The ratioused is the ferromagmatic resonance (FMR) index, which isthe ratio of the intensity of np-Fe0 (expressed as Is) signalrelative to the total iron content of the soil (expressed asFeO). The resulting value is usually written as Is/FeO [18,19].This ratio is a good measure of soil maturity because over

time a larger portion of iron is reduced to elemental np-Fe0.The maturity index is typically measured on the fraction ofa soil r250 mm, and has been determined for virtually allreturned lunar soils and many core samples. The indexvaries from �1 for the most immature soil to 106 for themost mature soils [18].

The Is/FeO value has been measured for size-separatedsoil down to �7 μm (the 0–10 μm fraction) [20]. Smallergrains are more ‘mature’ (have a larger value for Is/FeO)because the np-Fe0-deposited layer is surface-area depen-dent [21]. Although the o10 μm fraction has been mea-sured, the relative amount of np-Fe0 in lunar soil grains ofsmaller size fractions has not previously been studied.

1.2. Chemical reactivity of lunar soil

Bogard [9] described the processes on the lunar surfaceby which regolith grains become chemically reactive andremain so. In addition to physical breaking, these processesinclude irradiation by charged particles and high-energyphotons. Solar-wind protons at 1–2 keV have enoughenergy to break Si–O bonds [9], and thus solar wind playsan important role in surface erosion. The chemically-activated grain surfaces are probably strongly reactivetoward other atoms/ions with which they come into con-tact, but in the lunar environment there are few volatilespecies that can quickly neutralize their reactivity—thus thechemically reactive surfaces persist. The environmentwhich creates these conditions is very difficult to duplicatein the laboratory and even when the chemically reactivesurfaces are created, it is difficult to preserve them longenough to measure their properties.

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176 165

Although micrometeorite-produced impact-melting of thesoil may reduce the surface reactivity, the ever-present UVand solar-wind bombardment of the soil continuously re-activates these newly neutralized grain surfaces. The lunar soil(dust) would remain chemically reactive until exposed to anatmosphere in a habitat. Even then, its rate of deactivationwould be determined by the kinetics of gas and solid-statereactions near grain surfaces, and this rate has yet to be fullycharacterized. Thus fresh lunar dust may still be physiologi-cally reactive when it enters human lungs.

1.3. The lunar airborne dust toxicity assessment group(LADTAG)

Habitats for human exploration on other solar systembodies must be designed to limit exposure to lunar dust tosafe levels. The Lunar Airborne Dust Toxicity AssessmentGroup (LADTAG) was mandated by the ConstellationProgram to determine safe exposure levels for humansduring extended stays of up to six months on the lunarsurface.

LADTAG was organized into four teams: Geology; Chem-istry; Respiratory Toxicology; and Cutaneous and OcularToxicology. The team's products contributed to our under-standing of the health effects of lunar dust. There was someoverlap of work between the teams to independently verifykey data and to ensure consistency among the labs forcritical data. The geology team was responsible for provid-ing lunar dust to the other three teams, with variousamounts and particle size distributions provided accordingto each team's research needs. The size separation of lunarsoil into pristine fractions of much smaller diameter thanwas possible with established techniques gave us an oppor-tunity to study unique aspects of the soil in these very smallsize ranges.

1.4. Previous studies of the finest fraction of lunar soil

Most early studies of the o10 mm fraction of lunar soilswere conducted using beam instruments to study individualgrains because there was usually not enough material forother types of analysis (e.g. [22,23]). Instrumental NeutronActivation Analysis (INAA) was used on these smallest grains[24], from which it was found that the chemistry of theo10 mm fraction of a representative suite of Apollo sampleswas significantly different from that of the other size fractions.The o10 mm fraction is more feldspathic and enriched inhighland material and KREEP, relative to the coarser fractionsin all of the soils that were studied. The relative proportions ofanorthositic and KREEP components vary widely in bothcoarse and fine fractions of all soils studied.

The trend toward increasing plagioclase and KREEPbegins at approximately 25 μm. Down to 25 μm, all thedata show a continuous decrease in lithic fragments and anincrease in the mafic (olivine plus pyroxene) and plagio-clase components [25]. Below 25 mm there is a break in thistrend with feldspar concentrating relative to mafic grains inthe finest fraction [22].

1.5. Sample selection

Lunar soil simulants available at the time this study wasperformed did not include nanophase iron, nor did theparticles possess surface chemistry reflecting the effectsinduced by space weathering. Meanwhile, it was shown by[26] that nanophase iron results in a unique chemicalreactivity for lunar soil that could affect its toxicity. It hadalso been shown by [27] that the chemical reactivity oflunar soil was quickly lost upon exposure to terrestrialambient atmosphere. Thus we needed actual lunar soil forour experiments, and we required a sample that had notbeen exposed to terrestrial atmosphere to any significantextent. Moreover, we needed a fairly large amount of lunarsoil in order to obtain sufficient respirable dust for ourstudy. Respirable dust accounts for only about 2% of atypical lunar soil, and 23 g of respirable dust were neededto in order to perform a properly controlled inhalationstudy. If we were able to extract all the respirable dust froma given sample, we would have needed 1.15 kg of material.By modifying our protocol to include lunar dust that hadbeen finely ground we were able to reduce the amount oflunar soil required from 1.15 kg to 200 g. The ground lunarsoil would not contain as much np-Fe0 as does natural lunardust, having been derived from larger grains. However thiswas the only method deemed feasible for obtaining theamount of respirable dust required.

Mature soils have the greatest amount of nanophaseiron. A mature soil would have been preferred for our studybecause it would represent a worst-case toxicity potential.However, it was impossible to obtain a pristine sample ofthe quantity needed from the pristine lunar collection, thuswe searched the Returned Sample collection for a samplethat would meet our needs. It was essential that thesamples had not previously been subjected to fluids or toother contamination. We identified one sample in theReturned Sample Vault that was of sufficient quantity forour purposes. This sample was from the Apollo 14 mission.With a maturity (Is/FeO) of 66 [18], this was deemed anacceptable compromise between a mature soil and theother available samples, which were of insufficient quantity.The Returned Samples are somewhat easier to obtain thanpristine lunar material. Thus the Curation and AnalysisPlanning Team for Extraterrestrial Materials (CAPTEM)provided a returned lunar sample, 14003,96, for the presentinvestigation. Although this sample had been previouslyallocated, it had been returned unopened and consisted of200 g of lunar fines. No previous studies had been per-formed on this specific sample, but studies of other daugh-ter samples of 14003 have been conducted by our groupand others (e.g. [28–35]).

Had we been able to obtain a similar quantity of maturesoil we would have expected more np-Fe0 in the bulk soil,which would have resulted in an increased amount of np-Fe0 in the ground dust. In either case however, the grounddust would still have less np-Fe0 than did the naturally-formed dust. The respirable fraction (2.68 g) was separa-ted from sample 14003,96 for a pulmonary toxicity studyadministered by intratracheal instillation route (ITI) in rats,which established a preliminary dose for the inhalationstudy [36].

Fig. 2. (a) Dust separation system inside a nitrogen glove box at NASAJohnson Space Center's lunar sample curatorial facility. (b) Dust separa-tion system components and flow path.

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2. Methods

2.1. Measurements of particle size distributions of lunar soilsamples

For grain sizes down to about 1 μm we use a laser lightscattering instrument (Microtrac™, Largo, FL). A proprietarymodified Mie scattering algorithm addresses the irregularshapes of the particles and their non-transparent nature.This instrument was chosen because of the demonstratedreproducibility and accuracy of the method, the relativeease of use, and the ability to analyze a large number ofsamples in a relatively short time. It is feasible to analyzemultiple splits from a lunar soil allocation of 0.25 g orsmaller to increase the level of confidence in the results.

The method consists of adding a small amount of soil ordust to a working fluid, usually isopropyl alcohol [37], whichis circulated through the Microtrac. This instrument includesan analysis cell illuminated by lasers and surrounded bydetectors at known geometries. The Microtrac is periodicallychecked against National Institute of Standards and Technol-ogy (NIST)-traceable standard powders [38]. Care is taken todisperse the sample and eliminate clumping. The data areaccumulated in multiple measurements over a few minuteswhich are averaged. Between runs, the system is flushed anda blank is evaluated to ensure that the next measurement isnot affected by grains remaining in the instrument from theprevious measurement.

For sizes below �1 μm, and well down into the nan-ometer scale, sizes are determined using the methodreported in [39], which includes a magnetic preparation ofthe grain mount. The SEM technique was also used tocompare the shape properties of natural and ground dustswith each other and with previously-studied lunar dusts.

2.2. SEM/TEM examination of physicochemical propertiesof lunar dust

Selected grains obtained by the dry-pneumatic techni-que [38] were analyzed to document both typical andatypical morphology and mineralogy. Using the pneumaticseparation system it was possible to collect individualrespirable-size dust grains for SEM/TEM studies by placinga glass slide or TEM stub inside the filter holder and flowingnitrogen through the system for two minutes. The amountof dust collected during that time was so small that it couldnot be seen with the unaided eye but was sufficient forelectron beam analysis. Our method dispersed the dustgrains so that they did not usually touch each other. This isthe first instance we know of in which SEM/TEM samples oflunar soils had been obtained without subjecting thesample to liquid separation techniques [37].

High resolution imaging and chemical analyses wereperformed to evaluate the chemistry and morphology of therespirable grains. We used a JEOL 7600 field emissionscanning electron microscope coupled with a Noran System6 energy dispersive X-ray spectrometer. A Pt surface coat-ing�1 nm thick was applied to enable imaging and chemi-cal characterization of elements at 15 kV. TEM analyseswere performed using a JEOL 2500SE field emission scan-ning transmission electron microscope (FESTEM) featuring

a large area, thin window Noran System 6 EDX detector forimaging and element mapping down to a resolution of10 nm. Imaging resolution in STEM and TEM modes are0.2 nm and 0.14 nm, respectively.

2.3. Extraction of pristine respirable dust from lunar soil

For this study the dust separation system had to meet astringent requirement. The soil had to be kept dry andcould not be exposed to terrestrial atmosphere. Everyattempt was made to preserve the original near-pristinegrain surfaces so that toxicity testing results would resem-ble the effects of exposure to dust on the lunar surface.

A new dust separation system was developed for thistask (Fig. 2), because sieving is not practical for sizes below10 μm, and other available methods of grain separationrequire the use of fluids. All separation, grinding and re-packaging steps were performed in a nitrogen glovebox(0.5 ppm H2O; 20.6 ppm O2) at NASA Johnson Space Center.

The fluidized bed was originally developed at JSC for useas an in-situ resource utilization (ISRU) demonstration unit,and later loaned to our team. Modifications included theaddition of a cup-shaped filter holder at the bottom, whichprevents material from being caught in the corner spacebetween the bed's wall and floor, and improved dust seals.Following the fluidized bed is a settling flask, in which theinput is directed via a tube to the bottom of the flask. The

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sudden expansion of size at the flask reduces the speed ofthe dust-filled airstream causing heavier particles to settleout. Smaller particles remain in the air stream and arecarried to the next component—the cyclone (CH Technolo-gies Inc, Westwood, NJ). A membrane filter is the finalcomponent in the system. A small amount (10 mg) of thedust collected from the filter was measured in the Microtracto ensure that the particles collected were of respirable size.The coarser particles retained in the other componentswere saved in nitrogen, and aliquots of this coarser materialwere ground to obtain additional respirable dust, asdescribed below.

Using the method described in [38], we extracted 2.68 gof natural respirable dust with a volumetric mean diameterof 2.1 μm from the 200 g bulk soil sample 14003,96. If weinclude the �0.8 g of respirable material that remainedunrecovered in the larger size fractions, the respirable dustaccounts for 1.7% of the mass of the sample. This isconsistent with previous studies showing that respirable-size particles comprise 1–3 wt% of other lunar soils [40].Table 1 gives a breakdown of the masses, mean diametersand median diameters of the separates that resulted fromthis process.

2.4. Grinding and aerosolization

The starting materials for grinding were the coarserparticles retained after the extraction of natural respirabledust as described above and in [38]. These consisted of65.3 g of material of volumetric mean diameter 22.35 mmand 12.1 g of material with mean diameter 4.15 mm. Twomethods of grinding were used to determine if eithermethod would result in (a) higher chemical reactivity or(b) increased evidence of toxicity. Further details of thisprocedure are given in [38].

2.4.1. Jet-mill grindingA jet-mill grinder uses high-pressure gas to collide streams

of particle-laden gas jets against each other. Contamination isminimized with a jet mill because grinding is accomplished bycolliding particles into each other, rather than using a metal ormineral crusher. Ball-mill grinders use metal or ceramic jarliners and grinding balls, which wear out over time, thusadding some contamination to the sample.

Table 1Separated components of 14003,96.

Component Mass(g)

Vol. mean(μm)

Median(μm)

Vol. mean,Φ units

Fluidized bed 116.19 54.36 53.56 4.20Connector #1 0.26 8.77 7.26 6.83Settling flask 65.93 22.35 16.74 5.48Connector #2 0.86 8.52 6.69 6.87Cyclone body 12.06 4.15 3.89 7.91Cyclone cup 0.99 3.76 3.83 8.06Connector #3 1.13 2.7 2.66 8.59Membrane

filters4.14 2.1 1.85 8.90

[Total] oraverage

[200.1] 39.3 31.9 4.97

The jet-mill was operated inside a nitrogen glove box.We produced �55 g of jet-mill-ground dust with a volu-metric mean diameter of �3.2 mm. While some of thismaterial was separated further (described below), it wasmainly used for the inhalation study [41] which followedthe intratracheal instillation study [10] and was performedon a larger group of animals.

2.4.2. Ball-mill grindingA ball-mill grinder consists of a sealable container into

which the material to be ground is placed along with one ormore grinding balls, which in this case are made of thesame material as the jar liner (alumina). The sealed jar isplaced on a mechanical sample holder that vibrates at aselectable frequency for a desired length of time. Weoperated the ball-mill within the nitrogen environment,as had been done for all previous operations. The materialthat was placed in the ball-mill grinder had a volumetricmean diameter of 4.15 mm, yet we obtained a finishedproduct with a mean diameter of 20.02 μm and mediandiameter of 4.81 mm, indicating that clumping was affectingthe result. Aerosolization, the final step, dis-aggregated theclumped material.

2.4.3. Cycloning and aerosolization of ground materialTo produce a respirable dust from the jet-mill-ground and

ball-mill-ground material, we separated some of each mate-rial using an aerosolizer and cyclone system. We placed�0.5-g quantities of the material into a Vilnius aerosolizer(CH Technologies Inc, Westwood, NJ) with nitrogen flowingthrough the system at �10 l/min. The fine dust stream, afterpassing a cyclone, was collected in a Teflon membrane filter.The respirable dust collected from the jet-milled and theball-milled lunar dust samples had volumetric mean dia-meters of �2.5 μm and 1.8 μm, respectively.

The jet-mill-ground material showed less tendency toagglomerate than did the ball-mill-ground material. Thesetwo dusts were used only for intratracheal instillationstudies [36], because of the limited amount obtained(0.25 g ball-mill-ground and 0.85 g jet-mill-ground).

2.5. Dissolution study of lunar soil

From the late 1960s until now, lunar soil particle sizedistributions have typically been determined by sieving—sometimes dry, and at other times with fluids such as wateror Freon [42–44]. When we began our analyses of lunar soilsusing laser diffraction our first measurements were madewith distilled water [45]. Although the particle size mediansthat we measured were comparable to earlier sieve data, themeans tended to be significantly larger than expected.

A dispersion test as described in [46] showed that the use ofdistilled water resulted in clumping of lunar soil [37]. However,the use of isopropyl alcohol resulted in little or no clumping,making it a more useful carrier fluid for the Microtrac. Never-theless, we were concerned that the isopropyl alcohol mightpartially dissolve and disaggregate the agglutinates because oftheir high proportion of glass, and thus create smaller particlesthan would have occurred naturally.

To address this concern, we performed dissolution tests onlunar soil 14003,96 using isopropanol and distilled water. All

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176168

lunar dust dissolution studies were performed at concentra-tions of 0.5 mg/mL for 3 days. Sterile, 50 mL polystyrenecentrifuge tubes were rinsed with distilled water (Milli-Q)prior to testing. Twenty milliliters of the test solution wereadded to the tubes followed by the addition of 10mg of lunardust (maintaining a single tube with only solution to serve as acontrol). The tubes were capped and wrapped with Parafilm to

Fig. 3. Natural lunar dust, measured after a four-hour separation run. Ahistogram of each bin and a cumulative curve are shown. Volumetricmean of this sample is 2.1 þ0.6/�0.9 mm.

Fig. 4. Size distributions of the respirable fractions of 14003; (a) natural dust; (b)Liu et al. [40] was used to determine the number of particles in each size rangsample is very similar to the jet-milled sample, whereas the ball-milled samplethe other two. Number mode: unground: 0.5 mm; jet-mill-ground: 0.5 mm; ball-

Fig. 5. (A) Aspect ratio—the length of the long axis of a grain divided by the lendetermine the values. (B) The shape parameters are similar for all three sample

exclude gas exchange to the extent possible. The tubes werethen shaken to disperse the dust within the test solution. Thisagitation procedure was repeated regularly during the timethat the dust was in solution to ensure that no chemicaldiffusion layer was present at the solution/dust interface,thereby guaranteeing a surface-controlled diffusion mechan-ism [47]. At the conclusion of the testing, the mixtures (as wellas the control solution) were filtered through a 0.22 μmsyringe filter (Corning) to remove the dust. These filters hadbeen flushed with distilled water prior to use. Chemicalanalysis of the filtered solutions was then performed usinginductively-coupled plasma mass spectrometry (ICP-MS).

2.6. Consortium studies of size-separated fractions of lunarsoil

The dust separation system provided us with severaladditional size-separated fractions of lunar soil that had notbeen exposed to air and were not needed for the toxicitystudy. We took the opportunity of using these size fractions todo three additional studies. The fractions we used hadvolumetric mean diameters of 22.4 mm, 8.5 mm, 2.7 mm, and

jet-mill-ground dust; (c) ball-mill-ground dust. The method described bye (as opposed to the volumetric measure used for Fig. 3). The ungroundcontains particles with a numeric mean that is significantly smaller thanmill-ground: 0.3 mm.

gth of the short axis. The method described by Liu et al. [40] was used tos.

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176 169

2.1 mm. We examined each fraction to determine its chem-istry, maturity, and spectral properties.

2.6.1. Dust chemical compositionApproximately 10 mg of each size-separated fraction

was melted on a molybdenum-strip heater in the presenceof nitrogen. The resulting glass beads were mounted inepoxy, polished with diamond paste, and subjected to10–20 electron microprobe analyses, using a defocusedbeam of 10 mm.

2.6.2. MaturityThe size-sorted fractions of Apollo 14 soil 14003 were

measured to obtain the IS/FeO values using the methoddescribed by Morris [48]. These represent the finest-grained lunar samples ever measured for either FMRnp-Fe0 index or precise bulk chemistry.

2.6.3. Spectral reflectanceSize-sorted fractions of Apollo 14 soil 14003 were

analyzed using the Reflectance Experiment Laboratory(RELAB) bidirectional spectrometer [49] at 301 incidenceangle and 01 (normal) emergence angle. The Nicolet 740FT-IR spectrometer operates in reflectance mode from 0.9to 25 μm. Infrared (off-axis) biconical reflectance spectraare normally produced relative to a gold standard. Infraredspectra may be scaled to and merged with bidirectionalreflectance spectra to produce continuous spectra from 0.3to 26 mm.

3. Results and discussion

3.1. Particle size and shape distribution

The natural respirable dust extracted from 14003,96(measured by the laser diffraction technique) had a meandiameter of 2.1þ0.6/�0.9 μm (Fig. 3) and median diameter of1.85 mm. Values given here and in Table 1 were calculatedfrom the geometric mean and standard deviation using

Fig. 6. SEM images of individual grains in the respirable size fraction of natural lpair, the top image is made by secondary electrons and the bottom image is mdeeper into the particles and show differences in average atomic number as diffeas bright areas.

Φ units, then converted into micrometers. (For furtherinformation the Φ measurement system, refer to [50]).

Number modes, aspect ratios and complexity factors forthe three types of dust were determined by SEM measure-ments (described in [39]) in addition to the laser diffractionmethod. The SEM technique gives a number mode of0.5 mm for the natural (unground) respirable dust (Fig. 4a),which is significantly smaller than the value determined bylaser diffraction of 1.3 mm. The SEM results were obtainedvia measurement of the individual particles from digitalmaps, whereas the laser technique uses statistical model-ing. It is not unusual for results to vary significantly whendifferent methods are used for particle size distributionanalysis. An interesting result from the SEM measurements(Fig. 4a) is the smooth decrease in the particle size down toa few 10s of nanometers diameter, with almost no particlessmaller than about 20–30 nm.

The SEM results for the two artificially-created respir-able dusts (ball-milled and jet-milled) give number modesof 0.3 mm and 0.5 mm respectively (Fig. 4b and c), close tothe same diameter as the natural dust. Laser-diffraction-based number modes were 0.8 mm and 1.5 mm respectively.The three dusts are similar to each other with respect toaspect ratio and complexity factor (Fig. 5).

Dust samples contain individual grains as small as �20 nm,although grains this small are relatively few. The respirablefraction of lunar soil contains more than half of the individualparticles in each aliquot of lunar soil and dominates the surfacearea of the soil. This dominance of extremely small particles,coupled with the near-ubiquity of np-Fe0 spheres inside them,make lunar dust unique among any dusts breathed by humansduring their evolution—even during their present-day expo-sure to industrial minerals.

We were unable to measure the particle size distributionof the bulk sample prior to the separation of the compo-nents. However, the bulk particle size distribution afterseparation was calculated using the mass fractions andmean diameters of the material contained in each compo-nent of the system (Table 1). The calculated volumetric

unar dust. Each grain consists of more than a single phase. For each imageade by back-scattered electrons (BSE). Back-scattered electrons penetraterences in brightness. Nanophase iron spherules appear in the BSE images

Front Face

Edge

Fig. 7. TEM images and EDX spectra of typical sub-micron grains within the respirable fraction of natural lunar dust. (A) Bright-field TEM and spectra of aHASP (High Alumina Silica Poor) glass droplet with minor Fe. (B) An ilmenite grain partially coated with a silica-rich glass. Top: Dark-field image andspectra of the area shown by the center circle which indicates the ilmenite composition. Bottom: bright-field image and spectra of the area near the edge ofthe grain showing the composition of the glass coating. (C) TEM images and EDX spectra of a silica-rich glass droplet with inclusions of nanophase Fe. Top:dark field image and spectra of inclusion; bottom: bright-field image and spectra of the entire droplet. The Cu peak is an artefact of the electronmicroscope grid.

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176170

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mean diameter of 39 μm is much smaller than theMicrotrac-measured mean for sample 14003,28 of108 μm. Another previous measurement of 14003,28resulted in a grain size of 104 μm [51]. It is likely thatfluidization and passage through the dust separation sys-tem caused a significant number of grains to break, thusreducing the overall size distribution. This is consistent withthe high percentage of agglutinates in the sample (50% byweight [1]), which are known to be friable [52].

Fig. 8. A respirable-size compound grain of natural lunar dust shows multipleweathering processes.

3.2. SEM/TEM assessment of physicochemical propertiesof lunar dust

A comparison of secondary electron images and back-scattered electron images of the same micrometer-size grainsreveals the abundance of np-Fe0 on their surfaces (Fig. 6). Anilmenite grain (Fig. 7b) is mostly covered by an amorphoussilica-rich layer. Such layers can be extremely complex,reflecting multiple events of vapor deposition and sputter

phases of glass and minerals, all apparently welded together by space

Fig. 10. Major-element chemistry of the size fractions of lunar soil14003,96 determined by the fused-bead technique. FeO and MgOdecrease while Al2O3 and CaO increase with decreasing grain size.

Wei

ght P

erce

nt

Unground Jet-Mill-Ground

Ball-Mill-Ground

Bulk Soil

Fig. 11. Major-element chemistry for the three types of respirable lunarsoil (natural, jet-mill-ground, and ball-mill-ground) as determined byelectron microprobe analysis of fused bead aliquots. A previously-published bulk analysis of another aliquot of soil 14003 [70] is includedfor comparison. Note the decreasing ratios of FeO and MgO and increas-ing ratios of Al2O3 and CaO with decreasing grain size, similar to thepattern found in the size separates of the natural soil.

1

10

100

1000

10000

Con

cent

ratio

n, u

g/L

1

10

100

1000

Con

cent

ratio

n, u

g/L

Fig. 9. Dissolution data for (a) isopropanol and (b) distilled water. Theisopropanol shows no significant change in dissolved minerals afterexposure to lunar soil for three days. For the water test, increased soluteis seen for silicon, calcium, aluminum and magnesium in comparison tothe control.

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176172

deposition [13,15,53]. Some glass droplets contain abundantnp-Fe0 in the interior (Fig. 7c).

Approximately 70% of the respirable grains are predomi-nantly amorphous glass, as was predicted by previous studies[19,54]. Other particle types include crystalline minerals(plagioclase, pyroxene, ilmenite, and apatite) and metalliciron. Compound particles consisting of smaller grains tightlyaggregated and welded together (Fig. 8) are much morecommon than single-phase grains. Most grains have well-rounded surfaces—sharp or jagged grains are uncommon. Thissurface smoothing tends to reduce the overall surface area ofthe dust, which may reduce its toxicity [55].

The mineral fragments are often partially covered by asilicate coating that is interpreted as a vapor-condensate(Fig. 7). They occur as single grains and compound grainsmade of multiple smaller grains, tightly attached together(Fig. 8). Some chemically homogeneous glasses are pre-sent, but in the nanometer-size range (and in accord withthe finding of [54,56]), glass particles are typically hetero-geneous in composition. This is probably a result of theheterogeneity of the agglutinates from which many of thefiner particles are derived. Nanophase iron is a commoncomponent of such amorphous grains [54].

3.3. Dissolution in water

During our studies of this and other lunar soils, it wasfound that distilled water tends to cause clumping in thesoil particles that cannot be de-agglomerated by sonica-tion or by dispersant [37]. We also observed that distilledwater would dissolve the silicon, calcium, and aluminumin lunar soil, whereas isopropanol dissolved a negligible

amount of these elements when corrected for the blank-control (Fig. 9a and b).

Water dissolves more lunar soil material than doesisopropanol, preferentially removing plagioclase elements(Si, Ca, and Mg). The finest fraction of lunar soil is knownto concentrate both glass particles (which are usually Si-rich) [57] and plagioclase-rich particles [58]. Either (a) thesmallest particles are dissolving more rapidly than thelarger ones; or (b) the glass rinds on all lunar soil grainsare dissolving, reducing the overall diameter of the entiregrain population, and increasing the porosity of the grainsby preferentially dissolving plagioclase.

3.4. Consortium study results

3.4.1. Dust chemical compositionBulk chemistry by fused-bead analysis was performed

on the natural, ball-mill-ground, and jet-mill-ground dusts.The results are shown in Figs. 10 and 11.

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176 173

3.4.1.1. Natural dusts. Fig. 10 summarizes the chemistryof the less-than-25 µm fractions of lunar soil 14003,93.FeO and MgO decrease with decreasing grain size, afunction of decreasing ferromagnesian minerals. Al2O3

increases with decreasing grain size, which is a functionof both decreasing ferromagnesian minerals and anincrease in the plagioclase feldspar content. Theseobservations are consistent with [19,22–24,58] down to2 mm.

3.4.1.2. Ground respirable dusts. The chemical compositionsof the ground dusts are shown in Fig. 11. The ground dustswere obtained from material that was already size-fractionated and of a smaller diameter than the bulk soil. Itmakes sense that their chemistries lie between those of bulksoil and natural dust. For example, the amount of Al2O3 ishighest in the unground dust, lower in the ground dusts, andlower still in the bulk soil. The opposite trend is seen for FeO,MgO, and P2O5. These results are consistent with previousfindings, e.g. [8,24,59,60] down to 2 µm.

3.4.2. MaturityThe amount of np-Fe0 in the respirable dust (volumetric

mean diameter 2 mm) is far greater than in the averagelunar soil. Because the accumulation of npFe0 is a surface-correlated phenomenon [61], Is/FeO increases withdecreasing grain size, extending the trend noted by [19]and [62] down to 2mm diameter.

A decrease in total iron content with decreasing grainsize [63,64] adds to the higher ratio of nano-phase iron tototal iron (Is/FeO) as grain size decreases. Our data supportthe general pattern e.g. [8,65] that the finer the soilfraction, the greater is the ratio Is/FeO, even down to2.1 mm. Fig. 12 and Table 2 show the Is/FeO values for thefinest-grained lunar soil fractions ever measured. The

Wei

ght P

erce

ntIs

Val

ue (u

nitle

ss)

Is/F

eO v

alue

(uni

tless

)

FeO (wt%) IS (Gauss) IS/FeO

Fig. 12. IS/FeO values for size-fractionated and bulk samples of lunar soil14003. Although the amount of iron decreases with decreasing particlesize, the amount of metallic np-Fe0 (expressed by IS) increases. Theincrease in np-Fe0 results in increasing ‘maturity’ as particle sizedecreases.

Table 2IS/FeO values for size-fractionated and bulk samples of lunar soil 14003.

Sample fraction, 14003 22.4ammþ19.1/�8.4 mm 8.5ammþ7/�3.4 mm 2.7am

FeO (wt%) 9.77 8.94 8Is (Gauss) 770 923 1020Is/FeO 79 103 119

a Volumetric mean values.b Calculated from given values of IS/FeO and total iron as FeO wt%.

values range from 79 (for the 22.35 mm volumetric meansize sample) to 133 (for the 2.1mm volumetric meansample), whereas the bulk soil has a value of 66.

3.4.3. Spectral reflectanceThe high abundance of np-Fe0 in fine-grained dusts has

important effects on the ‘maturing’ of lunar soils, causingvisual darkening and spectral reddening [62,66]. Keyproperties such as the slope of the curve across visiblewavelengths and a flattening into the near-infrared (NIR)are greatly influenced by the relative abundance of metal-lic Fe, both nanophase and larger [53].

Individual spectra from 600 to 5000 nm wavelength areshown for four size fractions in Fig. 13. Characteristicmineral absorption bands are totally absent from the finestfraction, but an absorption band appears in the spectrum ofthe 2.7 mmvolumetric mean size fraction near 1000 nm andbecomes more pronounced with each larger fraction. Thedata shown in Fig. 13 are possibly the smoothest spectrumacross the �1 mm absorption band ever recorded fornatural lunar soil fine-grained fractions. The influence ofthe finest fraction on the total spectrum of a bulk soildepends on how much of the reflectance signal comes fromthe dust versus how much comes from the interior ofcoarser grains with a less abundant supply of np-Fe0.

For the infrared part of the spectra a �3 um (2800 nm)water-absorption band appears for all measured size frac-tions. The 3-μm band has also been observed in spectra ofpristine lunar samples [67]. While preparing these sam-ples, we were careful to minimize exposure to air and allprocessing was done in a dry-nitrogen chamber. However,the samples were exposed to a dry-air environment duringspectral measurements.

In experiments reported by Schaible and Dukes [68],exposure of lunar soils to terrestrial atmosphere for up tofour hours and exposure to H2 gas for 24 h caused nosignificant change in the appearance of the 3-um band.Nevertheless, when facilities are available to performspectroscopic measurements without air exposure, mea-surements should be made on some of the special samples(unopened cores, etc.) in the Apollo collection to verifythat this water band exists in the pristine soil [69].

4. Conclusion

The Lunar Airborne Dust Toxicity Assessment Group(LADTAG) studied the particle shape and size distribution,chemistry, spectral properties and toxicity of respirabledust from lunar soil 14003,96. The major components ofthe respirable fraction are fine-grain-size glasses, many

mþ1.1/�0.8 mm 2.1ammþ1.0/�0.7 mm Bulk soil: 101.9 mmþ76.1/�35.2 mm

.59 7.49 7.61995 502b

133 66[18]

Fig. 13. Spectral reflectance curves are shown for dry-separated dustfractions of Apollo 14 lunar soil 14003,96. (a) 0.4 to 2.4 μm wave-length; (b) 1 to 5 μm wavelength; and (c) 8 to 20 μm wavelength. Theincreased reflectance in the 2.1 μm dust is a direct function of higherrelative amounts of np-Fe0 in the glass coatings on each grain. Thedownward curvature at 2800 nm signals the presence of OH–HOH onthe surfaces of the particles. The strongest dip at 2800 nm is seen inthe 2.1 μm spectrum, which is a function of its relatively highsurface area.

D.S. McKay et al. / Acta Astronautica 107 (2015) 163–176174

containing np-Fe0. Other common particle types includecrystalline minerals, mainly plagioclase and pyroxene, butalso ilmenite, apatite, and metallic iron. Most grains in therespirable size range were smooth, amorphous glass,which would result in less toxicity than if the grains wereirregular, porous, or jagged. This may be one reason whylunar dust is less toxic than ground quartz [36].

We separated the soil into size fractions in a dry nitrogenenvironment and examined each fraction to understand howthey vary as a function of grain size down to 2 μm. Therespirable fraction of lunar soil contains more than half of theindividual particles, and accounts for most of the surface areaof the soil. The IS/FeO values for the size-fractionated samplesrange from 79 for the 22.4 mm sample to 133 for the 2.1 mmsample. Although the total iron content (FeO) systematicallydecreases with smaller grain sizes, the actual concentration ofnp-Fe0 systematically increases.

Water can both dissolve and agglomerate lunar soilparticles, therefore water should be avoided when prepar-ing lunar samples for geochemical or geotechnical studies.Although the amount dissolved is small, the observedagglomeration of lunar particles after exposure to waterfor a brief period shows that significant physico-chemicalchanges can occur rapidly.

This dominance of extremely small particles, coupled withtheir abundant np-Fe0 spheres, make lunar dust uniqueamong any dusts breathed by humans during their evolution.Our results provide the foundation for understanding thebasic physical and chemical aspects of respirable lunar dust,which are critical to the design of dust monitoring andremoval hardware, and therefore, to future long-term habita-tion on the Moon. Uncertainty remains as to how well wehave simulated the physical and chemical state of fresh lunardust, which cannot be addressed without fresh lunar soiltested immediately after collection. Such studies must awaitfuture lunar sample return missions or in-situ measurementson the lunar surface.

Acknowledgments

This work was supported in part by the research fundof Hanyang University Korea (HY-2013-N) via the Interna-tional Space Exploration Research Institute. The researchwas conducted under the auspices of NASA Lyndon B.Johnson Space Center's Human Research Program.

Many thanks to the anonymous reviewers who helpedto improve this manuscript.

Appendix A. Supporting information

Supplementary data associated with this article canbe found in the online version at http://dx.doi.org/10.1016/j.actaastro.2014.10.032.

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