analysis of water ice and water ice/soil mixtures using laser-induced breakdown spectroscopy:...

Volume 58, Number 8, 2004 APPLIED SPECTROSCOPY 8970003-7028 / 04 / 5808-0897$2.00 / 0q 2004 Society for Applied Spectroscopy

Analysis of Water Ice and Water Ice/Soil Mixtures UsingLaser-Induced Breakdown Spectroscopy: Application toMars Polar Exploration

ZANE A. ARP, DAVID A. CREMERS,* ROGER C. WIENS, DAVID M. WAYNE,BEATRICE SALLE, and SYLVESTRE MAURICEGroup-NMT-15 (Z.A.A., D.M.W.), Group C-ADI (D.A.C.), Group ISR-1 (R.C.W.), Los Alamos National Laboratory, Los Alamos,New Mexico 87545; CEA Saclay, DEN/DPC/SCP/LRSI, 91191 Gif sur Yvette Cedex, France (B.S.); and Laboratoired’Astrophysique, Observatoire Midi-Pyrenees, 14 avenue Edouard Belin, 31400 Toulouse, France (S.M.)

Recently, laser-induced breakdown spectroscopy (LIBS) has beendeveloped for the elemental analysis of geological samples for ap-plication to space exploration. There is also interest in using thetechnique for the analysis of water ice and ice/dust mixtures locatedat the Mars polar regions. The application is a compact instrumentfor a lander or rover to the Martian poles to interrogate stratifiedlayers of ice and dusts that contain a record of past geologic history,believed to date back several million years. Here we present resultsof a study of the use of LIBS for the analysis of water ice and ice/dust mixtures in situ and at short stand-off distances (,6.5 m) usingexperimental parameters appropriate for a compact instrument.Characteristics of LIBS spectra of water ice, ice/soil mixtures, ele-ment detection limits, and the ability to ablate through ice samplesto monitor subsurface dust deposits are discussed.

Index Headings: Laser-induced breakdown spectroscopy; LIBS; Wa-ter ice; Ice/dust mixtures; Mars.

INTRODUCTION

The use of laser-induced breakdown spectroscopy(LIBS) for the analysis of geological samples has beenwell documented.1–3 Recently, the method has been pro-posed for the analysis of planetary surfaces, the Moon,and asteroids, and preliminary work has been reported inthe literature.4–7 Current and past methods of elementalanalysis used for instruments to the Martian surface arein situ or contact detection techniques. These require ei-ther the retrieval of a local sample, which is then intro-duced into an on-board analyzer (X-ray fluorescence onthe Viking lander,8 1976), or require positioning a rovercontaining the detector adjacent to the sample (e.g.,APXS on the Pathfinder rover,9 1997). Using a fixed land-er platform limits the area that can be sampled to theshort length of a sampling arm, and although a rover canbe positioned adjacent to a remotely located sample, driv-ing to the sample is a time consuming process. Using thestand-off analysis capability of LIBS,5 however, only op-tical acquisition of the sample is required, followed by arelatively rapid analysis. Stand-off LIBS capability offersto greatly increase the scientific return from missionsthrough an increase in the number of discrete targets thatcan be accessed during the limited mission lifetime. Otheradvantages of LIBS for this application include (1) usefulsensitivity for many minor and trace elements of interest(e.g., C, Mn, Ti, Li, Sr, Ba, Cr, Ni, Cu, Zn, Sn, and Pb),(2) simplicity of the method, (3) feasibility of developing

Received 11 December 2003; accepted 20 April 2004.* Author to whom correspondence should be sent.

compact instrumentation, and (4) the ability of repetitivelaser plasmas to ablate surface material, thereby remov-ing dusts and weathered layers to permit analysis of theunderlying bulk rock.

Prior successful missions to the Martian surface havebeen directed at locations away from the polar caps (22.5N, 48.0 N, and 19.3 N for Vikings 1, 2, and Pathfinder,respectively). The polar regions of Mars are of interest,however, because of the presence of water ice and be-cause the polar regions are likely to provide informationon the climate history of the planet. Through repeatedcycles of precipitating dusts and H2O combined with sea-sonal condensation of CO2, layers of these geologicalsamples appear to have been built up in the polar regions.These so-called polar-layered deposits (PLD) may in-clude volcanic ash, fallout from surface impacts, evapo-rates from subliming lakes and seas, and even wind-blown ancient microbial life.10 Estimates indicate thatthese PLD may preserve a stratified record ranging in agefrom months to millions of years.11

Because of significant differences in elevation, thenorth and south poles display different behavior in regardto seasonal changes in ice cap composition. While thesouth polar cap displays a year-round layer of frozenCO2, the north polar region is 6 km lower in elevation,12

resulting in warmer temperatures and no permanent CO2.In both the southern polar region surrounding the per-manent residual CO2 cap and in the north, CO2 frost con-denses and sublimes seasonally. The underlying materialin these regions has been shown to have a high water-icecontent.13–15 Dusts deposited on the CO2 frosts by duststorms will be eventually deposited on the water ice dur-ing the summer thaw. Through repeated seasonal cycling,therefore, the PLD should consist of mainly water ice andsoil layers. The ability to analyze compositional differ-ences between layers of the PLD will be important forunderstanding the climate and geologic history of Marsand perhaps even to determine the current or past pres-ence of life.

For these reasons, surface rovers and landers to thepolar regions will be outfitted with instrumentation toprovide an analysis of the ice fields in some way. Theupcoming Phoenix mission, to be launched in 2007, con-sists of a lander equipped with a sampling arm to feedsamples to a thermal evolved gas analyzer (TEGA) anda mass spectrometer. This instrumentation should confirmthe presence of water and may identify constituents inthe water ice at the Phoenix landing site.16 However, char-

898 Volume 58, Number 8, 2004

acterization of the PLD would be very desirable on botha larger scale, such as measurements during a rover tra-verse over kilometers of terrain, and potentially also ona smaller scale, such as point measurements, likely to berequired to resolve successive layers.

Using LIBS, several different analysis scenarios can beenvisioned to provide such measurements. First, throughdevelopment of a compact sensor head mounted on a panand tilt mechanism, measurements may be conducted byfiring the laser at a remotely located target and collectingthe plasma light at a distance. This was the method em-ployed to analyze rock samples during a recent field testof LIBS on board a prototype rover.6 Repetitive laserpulses may be useful to ablate away the water ice layerand so bore through the layered materials and obtain arecord of the elemental composition of each layer. Alter-natively, a subsurface probe may be developed to samplethe layers in situ with instrumentation mounted directlyin the probe. Such penetrometer devices employing othertypes of sensors have been reported in the literature andare under development for future Mars and Europa mis-sions.17,18 LIBS-based cone penetrometer systems for ter-restrial soil analysis have been developed previously.19 Inanother scenario, a core sample may be removed fromthe ice sheet and then analyzed by forming the laser puls-es along the length of the core surface. This method hasalready been deployed for LIBS analysis of mineral drillcores20 and for the analysis of terrestrial ice cores usinglaser ablation inductively coupled plasma mass spectros-copy (ICP-MS).21,22

Although there is extensive literature on the use ofLIBS to analyze liquids,23–26 the analysis of ice by LIBShas not been studied extensively. Prior reported work hasonly dealt with LIBS analysis of trace metal ions in ice.27

Detection limits on the order of ppm were determined forAl and Na. To the best of our knowledge, no work hasbeen reported on the use of LIBS to analyze water iceand ice/soil mixtures at close and stand-off distances. Theobjective of this paper is to report the main results ofrecent work by us in this area. As a guide to this work,we considered the severe constraints that would be placedon a LIBS instrument developed for planetary explora-tion in the area of size, mass, and power consumption.Typical values mentioned are 1000 cm3, 3 kg, and 5 Wmaximum, respectively. Developments in compact spec-trographs and detectors are occurring rapidly and spe-cialized devices would be required for actual missions.Based on current diode laser technology and modest re-quirements for laser pulse rate and a reasonable stand-offdistance of 10 meters maximum, a laser pulse energy of100 mJ appears feasible and is the benchmark value usedhere. This pulse energy is somewhat greater than thatused in a previous study of the stand-off analysis of soilsamples at Mars atmospheric pressures (35–80 mJ),5 butreflects the greater difficulty of forming an analyticallyuseful plasma on ice samples at a distance.

EXPERIMENTAL

The experimental setup used to acquire LIBS spectraof water ice and ice/soil mixtures is shown in Fig. 1. Forall experiments, a Nd–YAG laser (Spectra Physics GCR-130, 12 ns pulse length) operating at 1064 nm was used

for plasma formation. This laser is equipped with a dualpulse option that permits the generation of two closelyspaced laser pulses (separated by 25–150 ms) at the laserrepetition rate characteristic of single pulse operation(typically 10 Hz). For some experiments it was necessaryto reduce the rate of pulses interrogating the sample tobelow 10 Hz. For stable laser performance, the laser flashlamps were operated at 10 Hz and a divide-by-N11 gen-erator was used to reduce the Q-switch repetition ratefrom 10 Hz to 2 Hz. Spectra were obtained at distancesof 1, 4, or 6.5 m, defined as the distance from the sampleto the collection lens labeled CL in Fig. 1 with a laserpulse energy of 100 mJ/pulse as measured at the sample.For experiments performed at 4 and 6.5 m, a 103 laserbeam expander (BE in Fig. 1) was used to focus the puls-es. For experiments performed at 1 m, a 1-m focal lens(FL in Fig. 1) was used because the beam expanderwould not permit focusing at distances less than 2 meters.Laser power was monitored continuously (Molectron 3Sigma power meter with a J-25 pyroelectric head) to in-sure constant laser pulse energy at the sample. In eachexperimental setup, the focus at the sample was adjustedto give the maximum LIBS signal. For all experiments,a 100-mm-diameter quartz lens (CL in Fig. 1) with afocal length of 1 m was used to collect and focus thespark light onto a fused silica fiber optic (FOC in Fig. 1)connected to the spectrograph. Since chromatic aberra-tion existed in this setup, the fiber was positioned to givethe most intense signal across the spectral range of thespectrograph.

Two spectrographs and detectors were used here de-pending on the experimental requirements. The majorityof data were gathered using an echellette spectrograph(Catalina Scientific SE 200), equipped with a high-orderdispersion module (spectral range of 200–1100 nm andl/Dl 5 1800). The spectrally dispersed light was record-ed by an intensified charge-coupled device (CCD) camera(Andor Model DH534-18F). Due to the fixed resolutionand significant amount of light lost using the echellette(f/10) with a fiber optic, a grating spectrograph (Chromex250IS, f /4) was used for some measurements to obtainhigher spectral resolution and to monitor weaker elementemissions. This 0.25-m-focal-length spectrograph (grat-ings 600, 1200, and 3600 lines/mm) was coupled to anintensified CCD camera (Oriel Instaspec V ModelDH500). Resolution of this spectrograph ranges from l/Dl 5 5000 (3600 lines/mm grating) to l/Dl 5 800 (600lines/mm grating). Spectral coverage ranged from 20 nmto 125 nm per ICCD image, depending on the gratingused. The gating parameters used here were tb 5 10 msand 300 ns , td , 1 ms, where tb is the width of theintensifier gate pulse and td is the delay time betweenlaser pulse and leading edge of the gate pulse. Dependingon the sample and the spectrograph and detector config-uration used, td was chosen to produce the highest emis-sion signal-to-background ratio.

Combined data from the two Viking lander sites in-dicate that the Mars atmosphere consists mainly of CO2

(95.3%), N2 (2.7%), and Ar (1.6%), with surface pres-sures in the range of 6.8 to 8.4 torr, and temperatureranges from 185 to 240 K.28 To duplicate the conditionsimportant for a LIBS analysis, samples were placed in anevacuable chamber (30.5 3 30.5 3 30.5 cm) with large

APPLIED SPECTROSCOPY 899

FIG. 1. Experimental setup for analysis of ice/soil mixtures. (Sh) shutter; (M) 1064 nm mirror; (MD) power meter; (BS) beam splitter; (FL) focallens; (BE) beam expander; (S) sample; (CL) 100-mm-diameter collection lens; (FOC) fiber-optic cable; and (GE) gating electronics. Inset is apicture of the Lexant sample holder used for repetitive ablation experiments.

quartz windows for optical access. The side of the cham-ber was fitted with a cold tip to cool the samples. Thecold tip consisted of a hollowed-out copper block feed-through through the side of the chamber with the tip ma-chined to hold the water ice samples (Fig. 1). A smallreservoir located outside the chamber was filled with thecooling liquid, either liquid nitrogen (LN2) or a dry ice/ethanol eutectic mixture. The eutectic mixture was orig-inally used to keep the CO2 gas used in some experimentsfrom condensing on the cold tip (Tsubl 5 194.5 K) buthad the added advantage of allowing the cold tip to beheld at a higher temperature (T ø 246 K) than achievableusing LN2 cooling (T ø 165 K). This permitted the eval-uation of ice ablation characteristics at the different tem-perature extremes found on Mars. The temperature of theice sample was measured by placing a thermocouple tipinto a small hole (2–3 mm deep) drilled into the surfaceof the ice sample. The chamber was evacuated to ,1 torrand then back-filled with CO2 at 7 torr to duplicate theMars pressure. In a few measurements, 7 torr air wasused. Exclusion of the minor gases Ar and N2 from thesimulated atmosphere was not deemed important as thesewill not affect the measurement results. During all mea-surements the chamber was continually pumped and aflow of CO2 gas or air was maintained through the cham-ber at the desired pressure to prevent build-up of ablatedsample material along the optical path to the target.

Experiments were performed to determine the depth towhich a series of repetitive laser pulses could drill into

the ice and to correlate spectra to this depth. For theseexperiments, ice samples were contained in a clear Lex-ant sample container (shown in Fig. 1). This containerwas cylindrical in shape with an inner radius and depthof 25 mm and a wall thickness of about 4.8 mm. Thenumber of pulses required to reach a certain depth wascounted by using a Lecroy 500 MHz Oscilloscope (inpulse counting mode) triggered by a fast photo-diode po-sitioned to monitor the laser pulses directed at the sample.The depth reached was measured visually using a rulerpositioned to the side of the Lexant container and bynoting the position of the sparks below the surface.

Pure water ice samples were prepared by freezing de-ionized water in Al sample cups (31 mm diameter) madefor pelletizing powdered solid samples. Synthetic silicatecertified reference materials (GBW07703 through 07708,Brammer Standard Company) were used to simulate soilsin most experiments. In this paper, the term soil will rep-resent these synthetic silicate samples. These sampleshave a wide range of concentrations of elements of in-terest and have a constant bulk matrix composition (SiO2,Al2O3, Fe2O3, CaMg(CO3)2, Na2SO4, and K2SO4). Sam-ples used to construct element calibration curves wereprepared by mixing 10% (by weight) of the syntheticsilicate with deionized water. Here, all references to %soil concentrations refer to weight percents. For experi-ments that did not require calibration curves (i.e., exper-iments to determine ablation rates into ice/dust mixtures),reference material GBW07703 was used as the sample.


FIG. 2. Comparison of LIBS ice spectra at (a) 585 torr air, (b) 7 torrair, and (c) 7 torr CO2. The insets show the OH bandhead used toidentify water and the 656.3 nm H(I) line. The maximum value of thescale for each OH spectrum is the same.

Homogeneous ice/soil mixtures were prepared by mixingthe constituents in a blender and then pouring in LN2 torapidly freeze the mixture in situ as a slush. When auniform frozen slush of soil/ice was achieved by mixing,the mixture was poured into a sample cup where it wasquickly flattened and dropped into a dewar of LN2 torapidly freeze solid the sample before melting could oc-cur and the soil settled out. Visually, the samples ap-peared to be highly homogeneous in soil distribution.

A simulated ice core was assembled by stacking in-dividual disks of ice (21 mm diameter 3 5–7 mm high)with certified rock powders deposited between the disks.The ice disks were prepared by putting deionized waterinto small plastic vials and freezing to form ice. To beginassembly, the first rock powder was poured onto the icedisk at the bottom of the first vial. Then a few drops ofdeionized water were added to form a thick slurry. Thena second ice disk was removed from a second vial andinserted into the first vial on top of the slurry. This two-disk assembly was then bonded together by immersingin LN2 to freeze the slurry between the two ice disks.Then a second rock powder was added on top of thesecond disk, a few drops of water were added to producea slurry, and then a third ice disk was put on the top ofthe second slurry and the three-disk assembly was cooled.This process was repeated until five rock powder layerswere layered between six ice disks. The certifiedrock powders used (Brammer Standard Company) inorder were: basalt (GBW07105); synthetic silicate(GBW07703); clay (GBW03102); obsidian (NIST 278);and basalt (GBW07105). In each case, about 0.25 g ofrock powder was used for each layer. When completelysolidified, the core was slid out of the plastic containerand then placed horizontally on a LN2 cooled Al surfaceand then interrogated by repetitive laser pulses.

RESULTS AND DISCUSSION

Effects of Atmospheric Pressure and AtmosphericComposition. The effects of atmospheric pressure andcomposition on the LIBS spectra were examined usingpure water ice samples. Figure 2 shows echellette spectraof ice taken at 585 (Los Alamos atmospheric pressure)and at 7 torr in air and in 7 torr CO2 at a distance of 1m. The spectra in Fig. 2 show a relatively low density ofemission lines with strong lines due to H(I) (656.3, 486.1,and 434.0 nm) and O(I) (700.19/700.22, 777.2, 777.4,777.5, and 844.6 nm) prominent. Also visible in the spec-tra taken in air are several N(I) lines (742.4, 744.2, 746.8,822.3 nm, etc.) and in 7 torr CO2, C(I) (247.9 nm) canbe seen. The echellette detection system as configuredhere was not sensitive to the strong C(I) line at 193.1 nm.Nitrogen lines can readily be identified by their loss ofintensity between the spectra taken at atmospheric pres-sure and in 7 torr air. Also, these lines do not appear inany of the spectra taken in the CO2 environment. Thesespectra indicate that water ice should not produce signif-icant spectral interferences in the detection of elementsin soils in ice/soil mixtures in a Mars atmosphere. Thecarbon signal due to CO2, however, will complicate thedetermination of carbon in soil, an element of interest atlevels usually less than a few percent, although for targets

such as carbonate minerals it may be possible to distin-guish the source of the carbon signal.

The behavior of H and O signals from the plasmaformed on pure water ice as the air pressure is reducedreveals information about the origin of these emissions(i.e., originating from ice or from air). In these experi-ments, the observed H signals all showed a significantdecrease in integrated intensity as the pressure was re-duced from 585 to 7 torr air. Although previous studieshave shown an increase in the intensity of emissions fromelements in a solid target with reduced pressure,5 with anintensity maximum occurring between 10 and 100 torr,in the present case, the reduction in air pressure also re-duces the amount of H2, O2, and H2O present in the air.Even though lower pressures may increase the laser pulseenergy coupled into the surface and hence increase themass of material ablated through decreased plasmashielding, evidently, in the analysis of ice, this is a small-er effect than the loss of H and O species through reducedair pressure. Figure 3 shows an increase in the H/O ratioas the pressure is reduced. Here H and O were monitoredat 656.3 and 777.4 nm, respectively. The ratio increaseoccurs because O2 constitutes 20.9% of the atmospherethat at 7 torr is present at about 1% of the concentrationas compared to atmospheric pressure. These results in-dicate that a significant amount of the observed O(I) sig-nal at 585 torr results from air, with the majority of Hemissions arising from the ice.


FIG. 3. H/O intensity ratio as a function of the air pressure for the laser plasma formed on pure water ice.

All emission lines from LIBS spectra of pure water iceshowed significant narrowing as the pressure was de-creased. This is easily seen in Fig. 2 by comparing thewidth of the 656.3 nm H(I) line at 585 torr to that re-corded at 7 torr. This narrowing (a factor of ;3.4) canbe attributed to both a reduction in the electron densityas the pressure is reduced (the H(I) line is particularlysensitive to electron density29) and a reduction in pressurebroadening.

OH Spectrum. Molecular OH emission is an impor-tant diagnostic tool for detecting water under Mars con-ditions. Shown in Fig. 2 are LIBS spectra of the strongOH emissions near 306.4 nm taken at 585 and 7 torr inair and in a 7 torr CO2 environment using the Chromexspectrograph. The appearance of the OH system in thespark is expected as the emissions are readily observedin many emission sources containing water vapor, evenflames.30 Many rotational transitions are observed in thisregion both blue- and red-shifted from the band head at306.4 nm (A2S1, n 5 0 → X2P, n9 5 0). The rotationalstructure of OH has been described elsewhere.31,32 A de-crease in pressure led to a significant decrease in back-ground continuum light from the spark. This can be seenin Fig. 2 by comparing the spectral region to the right ofthe 306.4 nm band head in each spectrum. In this region,all spectra show a very broad observable signal with verylow intensity rotational structure super-imposed on top.At 585 torr, much of the structure observed in the emis-sion band is degraded to the red and appears to be muchless intense and less resolved than that observed at thelower pressure due to this background emission and pres-sure broadening. Spectra taken in the CO2 environmentat different pressures showed trends similar to those ob-served in air. We believe the OH is arising from the mo-lecular fragment formed directly from ice by the actionof the laser pulse rather than recombination of H and O

because OH is not observed from other H and O con-taining materials such as a plastic.

Ice/Soil Mixtures. Figure 4a shows spectra of two ice/soil mixtures taken in 7 torr air at soil concentrations of9% and 100%. Air was used to prevent condensation ofCO2 on the samples that were cooled by LN2. Many el-ement emission lines are observed originating from boththe soil and the ice. Table I lists some of the strongerelement emissions observed, most of which are com-monly found in LIBS spectra of dry soils (i.e., 100%soil). At soil concentrations above 50%, all of the ele-ments listed in Table I are easily identified. At lower con-centrations, many element lines are reduced in intensityto the point of either being barely visible above the back-ground noise in the spectra or are not observed at all.This decrease in intensity can easily be seen in Fig. 4awhen comparing the number of lines observed at the 9%and 100% soil concentrations. Some elements, though,have very intense lines that are visible in the most dilutesamples (4.1% soil) used here. Of these elements, themost prominent are Ca, Al, and Na, which are still readilyobserved in the spectra of the 9% soil sample shown inFig. 4a.

Interestingly, in the 100% soil spectra, some lines dueto Si(II) are readily observed but the addition of waterice strongly quenches these transitions. This quenchingeffect is seen in Fig. 4a by noting the Si(II) lines thatappear at 504.1 and 505.6 nm. In the 100% soil spectrumthese lines are quite intense, but in 69% soil in ice (notshown here) they have decreased in intensity substantial-ly, especially when compared to other strong lines in the100% soil spectrum that are still intense in the 69% soilspectrum. In the 9% soil in ice spectrum these lines areabsent. A comparison of the dependence on soil concen-tration of emission signals for a few selected elements isshown in Fig. 4b. The lines used here are 281.6 nm


FIG. 4. (a) Spectra of water ice/soil mixtures containing 9% and 100% soil. (b) Dependence of element emission signals on the wt. % soil in ice.The signals recorded for each element at the different ice/soil concentrations were normalized to the signal obtained for that element using 100%soil. Each data point is the average of six replicate measurements. The sample distance was 1 m.

TABLE I. List of some stronger element emission lines observedin ice/soil spectra.

Species Lines (nm)

Al(I)Al(II)Ba(II)C(I)Ca(I)Ca(II)Fe(I)H(I)K(I)

396.2,a 394.4,a 308.2,a 309.3a

281.6, 559.3455.4b

247.9c

422.7,a 452.7393.4,a 396.8,a 315.9,a 317.9,a 373.7, 370.6278.8, 404.6,b 406.4,b 407.2b

656.3,a 486.1,a 434.0a

766.5,b 769.9b

Li(I)Mg(I)Mg(II)Mn(I)Na(I)Si(I)Si(II)Sr(I)Sr(II)Ti(I)

670.8b

285.2a

279.6,a 280.3a

403.0,b 403.3,b 403.4b

589.0,a 589.6a

390.6,a 288.2a

504.1, 505.6, 634.7, 637.1460.7407.8,b 421.6b

398.2,b 399.0,b 399.9b

a Lines that are still easily seen in the lowest ice/soil concentration stud-ied.

b Lines that are seen on the grating spectrograph but do not show upusing the echellette spectrograph.

c Line only seen here in the CO2 environment.

Al(II), 393.4 nm Ca(II), 504.1 nm Si(II), 396.2 nm Al(I),288.1 nm Si(I), and 589.0 nm Na(I). The precision bars(one standard deviation from six replicate measurementson each mixture) for the Na data are representative of theprecision for other element data in the figure. Here wesee that an element typically easy to excite such as Na(I),with an upper energy level (Eu) of the monitored transi-tion of 2.1 eV, shows a largely linear dependence on in-creased ice concentration. Other species, however, thatare more difficult to excite, such as Si(II) and Al(II),show a large reduction in intensity as the percent ice inthe sample increases. The neutral species, Si(I), also ex-hibits strong quenching as the ice concentration increases.The most likely factors that may affect the behavior ofthese lines are an ice-matrix-induced effect, a change inthe electron density, or a large change in the plasma tem-perature.

Table II lists the measured plasma temperatures forvarious ice/soil mixtures. These temperatures were deter-mined using a Boltzmann plot of selected Fe(I) lines inthe 370–390 nm region. These data show that the tem-peratures of the ice/soil samples do not change substan-tially from that measured for a 100% soil sample, indi-cating that temperature change is an unlikely factor in thestrong decreased intensity of Si(II) and the lesser, but stillsignificant, decrease in Si(I) and Al(II) intensities with


TABLE II. Plasma temperatures for ice/soil mixtures.

Soil concentration(% by wt)

Temperature(K)

RSD(%)

8.817.927.636.345.757.469.380.6

100

12 30011 40011 30012 00011 00011 80010 80010 700

9 100

716

81314

965

13

FIG. 5. H/Na intensity ratio as a function of the soil concentration in an ice/soil mixture.

increased ice content. A change in the electron densityshould affect all ionized species similarly. The line ofanother ionized element, Ca(II), though, does not showthe strong quenching found for Si(II). Emission from thisionized species behaves the same as emission from thenon-ionized element Al(I) that appears in the spectra.Therefore, an ice- (or water-) induced change in the elec-tron density is probably not a large contributor to thestrong quenching observed for Si(II) and Al(II). The firstionization potential of Si is somewhat greater than thatof Ca and Al (8.15 versus 6.11 and 5.97 eV, respectively),and Eu for Si(I) is 5.08 eV, somewhat greater than Eu forthe monitored transitions of either Al(I) or Ca(II) (3.14and 3.15 eV, respectively). Therefore, the explanation forall the trends shown in Fig. 4b is not obvious but maybe due to some water ice related matrix effect. Such ef-fects, if present, were not studied in the present investi-gation.

As expected, H and O emissions observed from ice/soil samples decrease as the soil concentration increases.At 100% soil virtually no H or O bands can be observed(Fig. 4a). Although soil contains many element oxides,we have found the oxygen signals from these to be, ingeneral, of low intensity. Figure 5 shows a graph of theH/Na ratio (Na from soil) as a function of soil concen-

tration. It should be noted that although the Na intensitywas used in Fig. 5, this graph is similar to the behaviorobserved for other ratios (e.g., H/Ca, H/Al, and H/Si).Figure 5 shows that at low soil concentrations the Naband is weak compared to the H band, which is expectedsince the Na (and other elements) originate from the soilalone. The H/Na intensity ratio, though, decreases veryquickly such that at 20% soil the intensity ratio reaches1.8 and slowly decreases to near zero at 100% soil. Thesignificant loss of sensitivity in the H/Na ratio above a20% soil concentration indicates that the H emission sig-nal is not a useful measure of water content at moderateand low ice concentrations. What is interesting is thatemission lines due to some elements in soil remain sig-nificant even at very low soil concentrations and ap-proach the intensity of H and O, which are the dominat-ing elements in these ice/soil mixtures at low soil con-centrations. This observation, along with the marked de-crease in H signal as the soil concentration increases from0 to 20%, suggests stronger coupling of the laser pulseto the soil particles than to the water ice, leading to pref-erential ablation of soil. The precision bars (equal to onestandard deviation of the ratio from six replicate mea-surements) for the H/Na ratio at the two lowest soil con-centrations in Fig. 5 are extremely large and are attributedto local nonhomogeneity of the samples and the low Naintensity at the lower soil concentrations. Homogeneitywas indicated visually, but this is only an estimate ofsample homogeneity. LIBS spectra, originating from asmall area of the sample due to the small focused spotdiameter (,1 mm), would be sensitive to small spatialvariations.

Laser Ablation of Ice. Experiments were performedto determine the depth to which repetitive laser pulseswould drill into ice and ice/soil samples and at whatdepths useful spectral information could be recorded.This is important because a key requirement for stand-


FIG. 6. Comparison of spectra of ice/soil mixtures at different ablationdepths in the mixture and for different cooling in a 7 torr atmosphere.Shown are (top) surface spectrum of the ice/soil mixture in 7 torr CO2

using eutectic cooling, (middle) 16 mm deep into the sample in 7 torrCO2 using eutectic cooling, and (bottom) 11 mm deep in 7 torr air withLN2 cooling. Sampling distance was 1 m.

FIG. 7. Comparison of ablation depth versus number of laser pulsesusing (a) a eutectic cooled cold tip [for ice (n) and ice/10% soil (C)]and (b) a LN2 cooled cold tip [for ice (m) and ice/10% soil (●)]. Ab-lation data for ice/10% soil is also shown in (b) using the double pulsemode option (n). Sample distance was 1 m.

off analysis is the ability to drill into ice to sample un-derlying soil/dust layers. Samples for this experimentwere pure water ice and 10% soil in ice. Both sampleswere held in the 25-mm-diameter Lexant sample holdershown in Fig. 1.

Shown in Fig. 6 are spectra of the ice/10% soil mixturetaken at the surface and 16 mm below the surface witheutectic cooling in a CO2 environment and 11 mm belowthe surface with LN2 cooling in an air environment.Again, air was used to prevent condensation of CO2 onthe sample when using LN2 cooling. All of these spectrawere taken at 7 torr to emulate Mars atmospheric pres-sure. The spectrum taken at the surface of the ice/soilmixture is typical of all surface spectra taken at 7 torrregardless of environment except for the 247.8 nm C(I)line from the CO2 gas. A noticeable difference that tem-perature does effect is the maximum depth below thesurface at which useful spectra can be recorded. For theeutectic cooled ice/soil mixture, it was possible to obtainspectra in which H(I) (656.3), O(I) (700.19/700.22,777.2, 777.4, 777.5, and 844.6 nm), Na(I) (589.0, 589.6nm), Ca(II) (393.4, 396.8 nm), and Al(I) (394.4, 396.2nm) can be clearly seen above the noise in the spectra atdepths up to 16 mm, as shown in Fig. 6. For the LN2

cooled tip in which the ice was held at a lower temper-

ature, the deepest point at which a similar spectrum couldbe obtained was 11 mm.

The spectra in Fig. 6 are representative of the deepestpoints at which spectra were obtained with a signal-to-noise ratio large enough to monitor at least some of thestrongest emissions lines due to elements in soil. Typicaldepths at which reproducible spectra could be obtainedand other element emissions observed were 5 mm for thesamples at ;165 K (LN2) and 10 mm for samples at;246 K (eutectic cooling).

The temperature of the cold tip also had large effectson other aspects of the laser drilling properties. Figure 7ashows the depth drilled into samples of pure ice and ice/10% soil as a function of the number of laser pulses usingeutectic cooling. Figure 7b shows corresponding data forLN2 cooling. Except for the ice/10% soil sample in Fig.7a for depths greater than 24 mm, these graphs appear toclosely approximate a linear relationship. This linear re-lationship is somewhat surprising as the drilling processwould be expected to be a complex process at the greaterdepths due to the narrow channel that is formed in thesample and the formation of liquid water in the channelthat may hinder laser pulse propagation. Due to voids andslight changes in the visual appearance of the ice matrix,the laser exhibited slightly different drilling properties atdifferent points on the same sample. These measurementswere repeated several times and slightly different curves


FIG. 8. Comparison of holes drilled into two water ice samples held at different temperatures on the cold tip: (a) uncooled cold tip, (b) eutecticcooled cold tip, and (c) LN2 cooled cold tip. Sample distance was 1 m.

were obtained each time, but the general trends observedare shown in Fig. 7 for each measurement condition.

A comparison of the data in Fig. 7 shows some inter-esting behavior. First, it appears that the ice and ice/10%soil mixtures drill at about the same rate and to about thesame depth under similar conditions. Visually, the ice/10% soil mixtures appear opaque and would be expectedto couple into the laser pulse strongly. On the other hand,the pure water ice sample, although somewhat opaquebecause of bubbles in the ice, is colorless and ice has avery small absorption coefficient at 1064 nm.33 At highfocused power densities used here to form the laser spark,however, differences in absorption become less crucial toforming the spark. It might be expected that the ablationrate will be dependent on the energy required to raise thetemperature of the sample from 165 K (LN2 cooling) tothe vaporization temperature. For ice this is about 3.4 kJ/g. The thermodynamic constants of rocks are not as wellknown, but using values from the literature34,35 we com-pute that the energy needed to vaporize basalt starting at165 K is 18 kJ/g, much greater than for ice. For the ice/10% soil mixture, the corresponding energy should beabout 4.9 kJ/g. Although this is about 50% greater thanthe energy required for pure water ice, given the uncer-tainties in the thermodynamic values for basalt, the ob-served similarity in the ablation rates appears to agreewith this simple analysis.

The second observation is that the slopes of the ap-proximately linear curves for drilling using eutectic andLN2 cooling are significantly different. For the eutecticcooled sample, the drill rate is about 62 mm/pulse, where-as for the colder LN2 cooled samples the rate is about 8.7mm/pulse. Again, the rates appear to be the same for purewater ice and for ice/10% soil mixtures for the same typeof cooling. Because of the greater energy needed to meltice in the case of LN2 cooling compared to eutectic cool-ing, a reduced drilling efficiency would be expected atthe lower temperature, which is observed. The observedefficiency difference (ratio of line slopes) is 7.1. Giventhe differences between the sample temperatures (165 K

for LN2 and 246 K for eutectic cooling), however, wecompute an expected efficiency difference of 4 basedonly on the difference in energy required to raise the icetemperature to the melting point. Other factors may beinvolved, however, such as the rate at which the meltedice refreezes between laser pulses and the differences inthe sizes of the holes formed during ablation. Regardingthe former point, no information was obtained during thisstudy, but it may be a reasonable assumption to concludethat at a laser repetition rate of 10 Hz, refreezing of anywater produced during ablation may be a negligible effecteven at the lowest ice temperature used here. As regardsthe sizes of holes ablated in ice, refer to Fig. 8, whichshows the difference in appearance of holes ablated inice for three different cooling regimes. For Figs. 8b and8c, approximately 600 laser pulses were used to form theholes. In Fig. 8a a large hole is observed in ice repeti-tively ablated (,600 pulses) but not cooled. In Fig. 8b,a much smaller hole is observed in a sample eutecticallycooled, with the smallest hole produced in ice cooled byLN2 (Fig. 8c). The ratio of the areas of hole sizes betweenthe two actively cooled samples is (0.4/0.2)2 5 4. In thecase of LN2 cooling, producing the smallest aperturethrough which the laser beam is transmitted, a loss ofenergy directed to the bottom of the hole would be ex-pected, thereby decreasing the efficiency of ablatingthrough the ice. We can estimate the relative efficienciesby first estimating the spot size of the laser by measuringthe diameter of the area damaged on an Al plate by thefocused laser pulse. This was done using the same ex-perimental parameters (pulse energy, focal distance, etc.)used to ablate through the ice samples. A diameter of 0.6mm was determined, which we take as an estimate of thefocused spot diameter. Assuming a Gaussian-like laserbeam profile, the energy transmitted through a hole ofdiameter d can be determined using Gaussian beam pro-file analysis.36 Assume that for an aperture of 0.6 mm,about 100% of the energy will be transmitted. For d 50.4 mm (eutectic cooling) the transmission factor is about95% and drops to about 58% for d 5 0.2 mm (LN2 cool-


FIG. 9. Na intensity as a function of the depth drilled into the sample (ice/10% soil) using LN2 cooling (m) and eutectic cooling (●). Sampledistance was 1 m.

ing). Therefore, the pulse energy reaching the bottom ofthe hole in the case of eutectic cooling will be about 95/58 5 1.63 times greater compared to LN2 cooling. Mul-tiplying this factor by 4 (i.e., the factor determined aboverelating to the greater amount of ice that could be broughtto the melting temperature for a given pulse energy usingeutectic cooling compared to LN2 cooling), we get 6.5,which is close to the ratio of 7.1 found for the ablationefficiencies for the two types of cooling.

A third observation regarding the data of Fig. 7 is thatusing the single pulse mode of the laser and LN2 cooling,the laser stopped drilling into the sample at ;11 mm. Forsamples eutectically cooled, the laser could drill to thebottom of the 25-mm-deep sample cup. Again these dif-ferences are probably related to the much narrower chan-nel and the lower temperature of the sample that resultedfrom LN2 cooling.

Also shown in Fig. 7b is the drilling rate and depthsachieved by using the double pulse mode of the laser andLN2 cooling. Double pulse operation was adjusted so thetwo pulses, separated by 100 ms, each had the same pulseenergy of 40 mJ. The drill rate (line slope) using thismode was about four times faster than for the single pulsemode (100 mJ/pulse). In addition, use of this mode al-lowed ablation to the bottom of the 25-mm-deep sampleholder, which was not achievable with the single pulsemode and LN2 cooling. Although these are advantages ofusing the dual pulse mode of the laser, the depth at whichLIBS spectra could still be observed (;11 mm) was un-changed as compared to spectra obtained with the singlepulse mode.

In some cases it may be useful to merely monitor thepresence of a soil layer. The element Na, a strong emitterand ubiquitous in soil, is a good candidate. Shown in Fig.9 is a comparison of the Na intensity versus ablationdepth for eutectic cooling and LN2 cooling. Here the Nasignal decreases in intensity somewhat faster as a func-

tion of depth at the lower LN2 cooling temperature com-pared to eutectic cooling. For the LN2 cooled tip, thesignal is negligible at ;11 mm, whereas for the eutecticcooled tip the signal is still seen as deep as ;20 mm.This ablation depth (20 mm) is slightly deeper than themaximum depth at which multi-elemental data was ob-tained in Fig. 6 (16 mm) in a separate measurement andis probably due to slight differences in the samples. Nais one of the easiest elements to observe with LIBS, al-lowing it to consistently show up at depths at which linesdue to other elements are no longer observable. The dif-ference in maximum depths for the two types of coolingis probably due to the difference in the size of the abla-tion hole shown in Fig. 8. The smaller the aperture, theless laser pulse energy that reaches the bottom of the holeto ablate material and form a plasma and the less lightwill be acquired by the detection system.

Laser-Induced Breakdown Spectroscopy of Ice/SoilSamples at a Stand-Off Distance. One important aspectof LIBS over other techniques for elemental analysis isthe unique ability to analyze materials at stand-off dis-tances using point detection. In order to demonstrate thisimportant aspect for ice/soil mixtures, several samples ofvarying concentrations of analytes were studied at stand-off distances of 4 and 6.5 m. Analytes were chosen forthis study based on their signal strengths at the distancesstudied and interest in these elements for Mars geology.These distances were selected as being representative ofthose that may be achievable using a miniaturized LIBSsystem on a Mars lander or rover using 100 mJ pulseenergy. In order to focus the laser pulses at these dis-tances, the 1-m-focal-length lens (FL) in Fig. 1 was re-placed with a laser beam expander (BE). Through over-compensation with the laser beam expander, the laserbeam can be focused at varying stand-off distances.

Calibration curves for each of the analytes were madeand detection limits were calculated based on the formula


TABLE III. Detection limits as a function of distance in an at-mosphere of 7 torr CO2.

Distance (m)

4 6.5 19a

Element Ice/10% soil mixtures Dry soil

Detection limits (ppm)BaLiMnSrTi

126

151

111

663

1012

520

2120···2

···

a Ref. 5.

FIG. 10. LIBS spectrum of ice/10% soil recorded at a distance of 6.5 m obtained using the Chromex spectrograph.

CL 5 3s/S37 where s is the standard deviation of sixmeasurements taken with a low analyte concentration andS is the slope of the linear calibration curve at lowerconcentrations. Detection limits, obtained with the sam-ples in 7 torr CO2, are listed in Table III. Figure 10 showsspectra of ice/10% soil taken with the Chromex spectro-graph centered at 400 nm and at a stand-off distance of6.5 m. In this region Sr, Mn, and Ti have relatively strongemissions that were used to form calibration curves forthese species at 4 and 6.5 m. By moving the sample clos-er to the light collection lens by 2.5 m, the emissionsignals increased significantly. This increase in intensityproduced lower detection limits (by a factor of 2 to 6)for 4 of the 5 analytes studied. Note that for the elementsmonitored here, the detection limits obtained at 4 m forice/10% soil mixtures are comparable to those obtainedfor dry soil samples at 19 m in a previous study.5

Ice Core Sampling. The ice core prepared as de-scribed in the Experimental section is shown in Fig. 11.The core was analyzed by forming a series of repetitivelaser sparks along the long axis, perpendicular to theplane of the rock powder layers. The core was movedslowly under the laser pulses (lens focal length 5 50 mm,

90 mJ/pulse, 10 Hz) at a rate of 1 mm/s. Ten spectrawere averaged over a period of 1 s to produce a singlemeasurement. Between measurements, translation of thecore was stopped to permit data readout before the nextmeasurement was conducted. In this way, the core wassampled with a resolution of 1 mm. The ice layers werebetween 5–7 mm thick with rock powders between thelayers (layer thickness ,1 mm). In all cases, spectral dif-ferentiation between the different rock powder layers wasobserved and the resulting spectra resembled the spectraobtained by analyzing the dry powders. Representativespectra, obtained using the echelle spectrograph, areshown in Fig. 11 for each rock powder layer along withthe pure ice spectrum. Detailed comparison of the spectrashows that they are distinct and they resemble the spectraobtained from the dry rock powders. There was no no-ticeable contamination of adjacent layers for the spacingbetween soil layers used here.

CONCLUSION

Laser-induced breakdown spectroscopy can be used toanalyze water ice and ice/soil mixtures to record usefulcompositional information. Detection limits for elementsin ice/soil mixtures, at least at the 10% soil level, are notsignificantly changed from those obtained by analyzing adry soil. LIBS can be used for the stand-off analysis ofice but significant penetration into the ice layers to accessdeep-lying PLD is not possible with the basic experi-mental arrangement used here. In addition, the stand-offdistances at which useful compositional data can be ob-tained are much less than those attainable using dry soilsamples. This can be attributed, in part, to the greaterdifficulty of forming an analytically useful laser plasmaon an ice or ice/soil mixture compared to dry soil. Theuse of LIBS on a penetrometer system or for the analysisof a retrieved ice core, in which only the surface or near


FIG. 11. Photo of simulated ice core and representative spectra obtained from ice and the four rock powders used as layered geological material.The repetitive laser pulses were directed onto the surface of the side of the core. The core was moved under the pulses to trace out the path shown.Complete echellette spectra are shown for ice and obsidian.

surface of the sample (,10 mm deep) must be interro-gated and the sample distance is ,1 m, appears a morefeasible approach for LIBS deployment for Mars PLDanalysis.

ACKNOWLEDGMENTS

This work was partially funded by the Nuclear Materials Manage-ment (Group NMT-15) and Chemistry (Group C-ADI) Divisions at LosAlamos National Laboratory. Los Alamos National Laboratory is op-


erated by the University of California for the U.S. Department of En-ergy.

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