laser ablation - optical cavity isotopic spectrometer (laocis) for mars rovers

Laser ablation – optical cavity isotopic spectrometer for Mars rovers Alexander A. Boľshakova, Xianglei Maob, Christopher P. McKayc, Richard E. Russoa,b

aApplied Spectra, Inc., 46661 Fremont Blvd., Fremont, CA 94538, USA bLawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

cNASA–Ames Research Center, Moffett Field, CA 94035, USA

ABSTRACT

A concept of a compact device for analyzing key isotopic composition in surface materials without sample preparation is presented. This design is based on an advanced modification of Laser Induced Breakdown Spectroscopy (LIBS). First, we developed Laser Ablation Molecular Isotopic Spectrometry (LAMIS) that involves measuring isotope-resolved molecular emission, which exhibits significantly larger isotopic spectral shifts than those in atomic transitions. Second, we used laser ablation to vaporize the sample materials into a plume in which absorption spectra can be measured using a tunable diode laser. The intrinsically high spectral resolution of the diode lasers facilitates measurements of isotopic ratios. The absorption sensitivity can be boosted using cavity enhanced spectroscopy.

Temporal behavior of species in a laser ablation plasma from solid samples with various isotopic composition was studied. Detection of key isotopes associated with signs of life (carbon, nitrogen, hydrogen) as well as strontium and boron in laser ablation plume was demonstrated; boron isotopes were quantified. Isotope-resolved spectra of many other molecular species were simulated. The experimental results demonstrate sensitivity to 86Sr, 87Sr, and 88Sr with spectrally resolved measurements for each of them. It is possible to measure strontium isotopes in rocks on Mars for radiogenic age determination. Requirements for spectral resolution of the optical measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric analysis of spectra.

Keywords: Laser ablation, optical emission spectrum, isotope, chemical analysis, molecular spectrometry, LIBS, LAMIS.

INTRODUCTION Isotopic records provide answers to fundamental questions regarding the development and evolution of stars and

planets in the universe, including the solar system. Life processes lead to distinctive isotope patterns, which give clues to the origin of life and evolution in a galactic context. Radioisotopic age dating is the primary method in which accurate geochronological ages can be established. Isotopic information in paleoclimatology plays a critical role for reconstruction of variations in past climate conditions. Consequently, isotopic data hold keys to predicting future climate changes that may influence global temperature, energy needs, availability of drinking water, and food supplies.

Currently, mass spectrometers that determine mass-to-charge ratio of ionized particles are employed to accomplish isotopic measurements. Deep vacuum is required to differentiate and analyze the ions. Only a few such instruments or prototypes have been developed for in situ measurements on surfaces of other planets.1 None of these low-resolution mass spectrometers can discriminate isobaric masses (e.g., 87Rb versus 87Sr or 12C16O versus 14N2) and even in compact form, these devices are relatively heavy. For high-resolution isotopic measurements, only large, heavy and complex mass spectrometers are found in research laboratories. Such measurements require prior separation and concentration of analytes via sophisticated physico-chemical treatment of the samples. Conventional mass spectrometers normally provide high sensitivity but measurements are generally laboratory-based and time-consuming with complex chemical dissolution routines that produce acidic waste.

Our concept is based on the integration of two analytical techniques that are both onboard of the Mars Science Laboratory “Curiosity” but presently unconnected to each other. These techniques are stand-off LIBS sensing for elemental analysis of solids and tunable semiconductor laser spectrometry for isotope analysis of gases. We intend to utilize and further develop our recently published technology: Laser Ablation Molecular Isotopic Spectrometry (LAMIS),2-4 which exploits optical spectra of molecular species produced in ablation plumes. The intrinsically high spectral resolution of the diode lasers facilitates the measurement of isotopic ratios. The absorption sensitivity can be

*[email protected] (AAB); [email protected] (XM); [email protected] (CPM); [email protected] (RER).

Invited Paper

Sensors and Systems for Space Applications V, edited by Khanh D. Pham, Joseph L. Cox,Richard T. Howard, Henry Zmuda, Proc. of SPIE Vol. 8385, 83850C · © 2012 SPIE

CCC code: 0277-786X/12/$18 · doi: 10.1117/12.919905

Proc. of SPIE Vol. 8385 83850C-1

boosted using cavity enhanced, cavity ring-down or wavelength modulation spectroscopy. In this fashion, the carbon, hydrogen, nitrogen, oxygen, chlorine and other stable isotopes can be determined in rocks and minerals. Moreover, the age at which rocks solidified can be obtained from the isotopic ratios of 87Rb/86Sr and 87Sr/86Sr following the well-known isochron method.5 The measurement of strontium and rubidium isotopic ratios can result in determination of the geological age with uncertainty less than ±500 million years that corresponds to errors within ±2% in the 87Rb/86Sr ratio and ±0.2% in the 87Sr/86Sr ratio. At present, there are no means to make direct age dating measurements on Mars but indirect estimates have uncertainties in billions of years, and validity of these age estimates for Mars geological components is unknown.6 Thus, our approach adds a major new dimension to the planetary instrumentation, enabling the high-resolution isotopic determination.

ISOTOPE-RESOLVED EMISSION FROM LASER ABLATION PLASMA Optical isotope shifts in molecular spectra are significantly larger than those in atomic spectra. This is because the

difference in isotopic masses has only a small effect on electronic transitions (as in atoms) but appreciably affects the vibrational and rotational energy levels in molecules.2 LAMIS can be implemented in a way similar to the conventional elemental LIBS analysis, however in LAMIS a few times longer delays after the ablating pulse are applied to allow formation of molecules in plasmochemical reactions during plasma plume expansion into the air. Benefits of this technique are rapid and direct isotopic characterization for real-time analysis of condensed samples that does not require sample preparation or vacuum environment and has the potential for stand-off operation. For practical applications, LAMIS is poised to speed up, to simplify and to make isotopic analysis more affordable than now, while it will remain generally less sensitive than the traditional mass spectrometry.

Examples of emission spectra of diatomic radicals SrO measured in laser ablation plasma generated from isotope-enriched SrCO3 samples are presented in Fig. 1. These emission spectra were collected using a suitable for space operations, compact echelle spectrograph EMU-65 (Catalina Scientific, Tucson, AZ) fitted with an Electron Multiplying Charge-Coupled Device (EMCCD). The results plotted in Fig. 1 clearly illustrate that isotope shifts are larger for higher vibrational quantum numbers. The isotope shift in the head of the (2,0) band is ~0.15 nm, while the shifts in the heads of the (1,0) and (0,1) bands are both approximately 0.08 nm but of the opposite sign. The shift in the head of the (0,0) band at 919.6 nm was not resolvable. All three major isotopes of strontium (88Sr, 87Sr, 86Sr) can be resolved in this manner as it was demonstrated in our previous work.4 Earlier we have also shown that partially resolved spectra can be sufficient for the quantitative isotopic detection especially if a range of multiple spectral features, such as rotational lines, is measured at once.3 Ability to measure isotope abundance with a low-resolution spectrometer is a significant attribute of LAMIS.

Fig. 1. Spectral bands of SrO A1Σ+→X1Σ+ emission measured in laser ablation of the isotope-enriched 88SrCO3 and 86SrCO3

solid samples; (a) vibrational band 2-0, (b) vibrational band 1-0, (c) vibrational band 0-1.

We also measured spectra of diatomic radicals OH, BO, CN and C2 in laser ablation plasma (Fig. 2) to demonstrate excellent spectral resolution for isotopes of B, C, H and quantitative determination of 11B. The spectra shown in Fig. 2 were measured using a Czerny-Turner spectrograph fitted with an Intensified Charge-Coupled Device (ICCD).2,3 The detected molecular radicals are either directly vaporized from a sample or formed by association of atoms or associative recombination of atomic ions in the plume. Spectra of OH and OD were obtained by ablating the ordinary water ice as well as vapors of H2O and D2O. The prominent features of the OH A2Σ+→X2Πi (0,0) transition at ~306 nm (R1, R2 branch heads) and ~309 nm (Q2 branch head) with partially resolved individual rotational lines are significantly different from the corresponding spectrum of OD (Fig. 2a). The experimental shift between the Q2 branch heads of OH and OD is

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~0.68 nm. Fig. 2b displays a tail fraction of the BO A2Πi→X2Σ+ (0,2) band, spectrum of which extends from 503.0 nm toward the red. Two enriched samples of 10B2O3 and 11B2O3 were used; the third sample was boron nitride containing the natural abundance of boron isotopes. In the latter case, BO molecules were formed in the plasma plume reacting with atmospheric oxygen. A comparison of the 10BO and 11BO spectra in Fig 2b reveals the differences in their rotational structure, both contributing to the sum spectrum of BO with natural abundance (80.2% 11B and 19.8% 10B). The spectra of C2 and CN (Fig. 2c) were measured in laser ablation of ordinary graphite (99% 12C) and isotope-enriched urea (99% 13C). For the C2 d3Πg→a3Πu and CN B2Σ+→X2Σ+ transitions, the (0,0) bands in both cases are shown. The isotopic shifts in the band heads of these radicals are approximately 0.03 nm. Spectrum of the heavier isotope in CN is shifted toward the violet, but the counterpart in C2 is shifted toward the red.

Fig. 2. Emission spectra of diatomic radicals in laser ablation plasma; (a) OH and OD vibrational band 0-0 of A2Σ+→X2Πi, (b) BO vibrational band 0-2 of A2Πi→X2Σ+, (c) CN vibrational band 0-0 of B2Σ+→X2Σ+ and C2 vibrational band 0-0 of d3Πg→a3Πu (Swan system). The data traces are vertically offset for clarity.

We simulated the 16OH, 18OH, and 16OD spectra from 307 to 314 nm as well as the spectra of 12C14N, 13C14N, and 12C15N in the region of the CN A2Πi→X2Σ+ (1,0) band from 925 to 940 nm. The simulation demonstrated that spectral resolution of ~0.03 nm is sufficient to resolve individual rotational lines of all these species. Therefore, modern compact echelle-based spectrographs can selectively measure multiple isotopomeric molecules at the same time. Multivariate statistical processing of the spectra can facilitate quantitative analysis even with only partially resolved spectra.

While working with strontium samples, we found that diatomic strontium halides are also readily formed in laser ablation.4 Logically, not only strontium but other alkaline earth metals will exhibit a similar behavior. Both CaCl and MgCl are in significant abundance on Mars, and thus they can be utilized for determination of the 35Cl/37Cl ratio. The isotopic shift for chlorine in MgCl was computed as ~0.1 nm, and in CaCl was up to 0.2 nm. These values are larger than the isotopic shifts presented in Fig. 1 for strontium, and evidently are resolvable. Chlorine isotopes reveal large fractionation in terrestrial geology and by analogy can shed light on history of past water and volatiles on Mars surfaces with large isotopic variations to be expected. Perchlorates detected by Phoenix and both Viking missions can be a source of microbial metabolism, which predictably alters isotopic ratios that can be useful in search for extinct life.

Potentially LAMIS can operate in a stand-off mode collecting emission through a telescope, similar to the LIBS instrument ChemCam currently onboard the MSL Curiosity. In general, light elements yield the largest isotopic shifts in LAMIS.2 Consequently, LAMIS is the most advantageous for isotopic analysis of life-forming elements (H, C, N, O). Carbon isotopes are indicative of primary bio-productivity and energy cycling and are most important for understanding of biochemistry. The biological enhancement of 12C over 13C can be up to 5% (any biological matter on Mars would probably also reflect an enhancement in 12C relative to the source material). Hydrogen isotopes are indicative of water and hydrologic history. Measurement of the D/H isotopic ratio is essential in paleoclimatology, material sciences, biological and medical research, among many other areas. Oxygen isotopes can characterize the exchange of CO2 and H2O reservoirs and thus this measurement can help interpret the carbon isotope fractionation.

ABSORPTION MEASUREMENTS IN LASER ABLATION PLASMA The diode laser absorption spectrometers cannot be outperformed for very high sensitivity afforded with a low-

mass, low-volume optical cavity and electronics package.7 The wavelength of diode laser emission is tunable and normally has a very narrow linewidth (<1 MHz) enabling high-resolution absorption measurements without the need for a dispersion spectrograph. The spectral resolution is determined by the diode laser linewidth, while echelle or Czerny-

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Turner spectrographs become unnecessary. A simple photodiode can be a detector. As a result, the tunable laser absorption spectrometers can be miniaturized to very small devices.

A simplified diagram of our experimental setup is sketched in Fig. 3a. A small vacuum chamber with 4 windows was used to simulate low pressure environment as on the surface of Mars (~600 Pa). The diode laser beam was steered through the laser ablation plumes at a distance of 1 to 30 mm above the surface of ablated samples. The intensity of the diode beam was measured by a fast silicon photodiode. A diaphragm of 3 mm in diameter and a shortpass dielectric optical filter were installed in front of the photodiode to restrict otherwise intense plasma emission and scattered light from the ablating laser at 1064 nm. An external cavity diode laser DL-100 (Toptica Photonics, Munich, Germany) with two interchangeable chips was continuously tunable over spectral intervals of 815–855 and 866–950 nm.

Fig. 3. Schematic diagram of the setup (a) including a vacuum chamber with four windows, a pedestal to support the ablated

samples, and a tunable diode laser, (b) emission of the atomic Cs resonant line in laser ablation plasma and the diode laser tuned at this line, (c) calculated hyperfine structure of this Cs resonant line with tabulated center at 894.347 nm.

In order to test the sensitivity of the absorption measurements, two spectral lines of Cs with different oscillator strengths were probed. The stronger line (894.347 nm) was resonant, i.e. originating at the ground state of the Cs atom. The weaker line (920.85 nm) was a transition between the excited states, which are less populated in plasma than the ground state. The sample was a tablet of pressed powder of CsNO3 that was initially ablated through open windows at atmospheric pressure in ambient air. Fig. 3b illustrates an example of the diode laser being tuned at the Cs resonant line as observed in plasma emission at the delay of 2 µs and the acquisition gate of 20 µs. This emission line exhibits the spectral width of about 1 nm due to Stark and other kind of broadening. The diode laser line appears to have a finite width owing to the limit of resolving power of our spectrograph, but in reality is significantly narrower. The resonant line of Cs has four hyperfine components (Fig. 3c) unresolved in emission. The NIST-tabulated wavelength of 894.347 nm is the value of the center of gravity for these four components.

The absorption measurements presented in Fig. 4 are the oscillograms of the photodiode output voltage. The time scale originates at the instance of the ablating pulse. The first peak at the moment t=0 is the remainder of scattered ablation pulse that passes anyway through the optical filter (cut-off wavelength at 1000 nm). Absorption manifests itself as a negative signal. When the diode laser was tuned at the wavelength 894.386 nm, close to the Cs resonant line, the absorption was near 100% during initial 20 µs of the plasma plume development. The weaker line at 920.85 nm showed

Fig. 4. Absorption of Cs atoms in laser ablation of CsNO3, (a) at atmospheric pressure ~100 kPa and 1.5 mm from the sample surface, (b) at reduced pressure ~2.5 kPa and 10 mm from the sample surface.

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respectively weaker absorption when the diode laser was tuned into proximity of it. The excited atoms represented by absorption at 920.85 nm have shorter life periods in the plasma plume relative to the ground-state atoms represented by absorption at 894.347 nm (Fig. 4a). If the diode laser is tuned out of any spectral lines, there was no significant absorption detected. These results indicate that tuning of the diode laser on the spectral features does not have to be precise because these features are fairly broad in early afterglow of the laser ablation plasma. There were some fluctuations due to acoustic and thermal shockwaves common for ablation at atmospheric pressure.8 These fluctuations significantly diminish when pressure is reduced below ~15 kPa and the plasma plume becomes visibly larger. A comparison of the absorption signal after ablation at atmospheric and reduced pressure is shown in Fig. 4 with legends indicating the wavelengths at which the diode laser was tuned.

The effect of spectral lines narrowing in time after the event of laser ablation is apparent in Fig. 4b. Detuning of the diode laser from one of the hyperfine components of the Cs resonant line by 6 pm caused the reduction of the absorption time down to ~200 µs. When the diode laser was tuned closer to this hyperfine component, the direct absorption lasted for about 300 µs in the plume. Detuning shortens the absorption time because the diode laser beam can be absorbed only until the spectral features in the plume are sufficiently broadened. As the plasma cools and spectral lines promptly narrow down, the detuned diode laser is no longer absorbed, although the absorbing free Cs atoms are still present in the plume. It is worth to note that hyperfine splittings are not resolvable in emission from laser ablation plasma, and there is no emission from the plume at delays longer than ~100 µs.

The excited states are not the most favorable for absorption testing but can anyway provide useful diagnostics. The atomic Cl line at 837.59 nm is usually one of the most intense in the chlorine emission spectrum. This is a transition between the excited states but involves the most populated (in plasma) metastable chlorine level 3p44s 4P5/2 that should result in strong absorption at this line. In order to examine pulse-to-pulse variations in absorption, the diode laser intensity was acquired through a Czerny-Turner spectrograph at the instances synchronized (and duly gated) with the ablating laser pulses in a sequence of 45 triggering events. The sample was a crystal KCl tablet ablated in open air. The first 15 acquisitions were recorded without laser ablation, then 15 acquisitions with the laser ablation, and finally another 15 acquisitions without ablation. The results measured at different delays (0.5, 1.0, and 2.0 µs) are illustrated in Fig. 5. If the diode laser is tuned at the chlorine line, absorption of the beam by chlorine atoms is recorded when the ablating laser fires and generates the plasma. The intensity of the diode laser emission remained stable and unchanged as evidenced by measurements before and after the 15 ablating pulses.

Fig. 5. Absorption of Cl atoms at 837.59 nm in laser ablation of KCl at atmospheric pressure and ~1mm from the sample

surface. Measurements were time-gated with the gate width of 1 µs and gate delay of 0.5 µs (a), 1 µs (b) and 2 µs (c) after the ablating laser pulse. Diode laser was tuned at 837.5945 nm (±0.6 pm).

The data in Fig. 5 indicate that chlorine absorption increased with the delay from 0.5 to 2.0 µs after the ablating pulses. This means that the number of the absorbing chlorine atoms was growing in a plasma afterglow. A similar effect of “recombinative population” was generally known in low-pressure plasmas but not in LIBS. Populating of the chlorine level 3p44s 4P5/2 is attributed to the population mechanism via recombination of electrons with chlorine ions. The level 3p44s 4P5/2 is metastable with a radiative lifetime of ~4 µs, and therefore its population can accumulate in a recombining plasma. When the diode laser wavelength was detuned about 2 nm from the chlorine line, no absorption (also no scattering, no beam deflection by shockwaves) was observed at delays longer than 1 µs. In comparison with the shockwave disturbances after ablation of a tablet of pressed CsNO3 powder shown in Fig. 4a, the KCl crystal lattice probably generates a fast and repeatable shock pattern that does not interfere with the absorption measurements at long

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delays. Nine hyperfine components of this Cl line (837.59 nm) have splittings on the order of 50 to 300 MHz, which are far below Doppler broadening in the ablation plasma. Precision of diode laser tuning is better than 1 pm and a short-term wavelength jitter is less than 0.4 pm (<200 MHz).

Preliminary experiments with laser ablation of a RbCl sample yielded no observable emission of any Rb-bearing molecules in ablation plumes, while the atomic Rb spectrum was intense. It is possible that rubidium molecules do not form effectively in ablation plasma, or they are very poor emitters. Therefore, the isotopic rubidium determination by absorption is required to complement the SrO measurements described above (see Fig. 1), if the radiogenic age dating of rocks is sought using this method. Fortunately, the tunable laser absorption measurements of resolved atomic Rb isotopes were already demonstrated9 in laser ablation plasma at pressure ranging from 20 to 1300 Pa, which includes ambient pressure on Mars. A detection limit of 25 µg/g for the individual Rb isotopes in calcium carbonate samples was obtained illustrating that this method is applicable for measuring 85Rb and 85Rb concentrations in geological samples.

Further work is in progress. Our general approach is to augment a ChemCam-like instrument with a miniature proximal diode laser absorption module enabling isotopic measurements in ablation plumes. This module, Laser Ablation – Optical Cavity Isotopic Spectrometer (LAOCIS) will provide high-resolution atomic and molecular vapor density measurements with the use of only a few miniature laser diodes and photodiodes. Tunable single-mode diode lasers – e.g., vertical-cavity surface-emitting lasers (VCSEL) and distributed feedback (DFB) lasers – are commercially available. Sensitivity of absorption measurements can be increased by 3 to 4 orders of magnitude using cavity enhanced, cavity ring-down or wavelength modulation spectroscopy. Neither dispersion optics nor vacuum pumps are required. Similar packages for exploration of Mars were described to include two lenses, a thermo-stabilized laser, a photodiode, a cavity and mirrors.7,10 Total mass of individual isotopic diode laser spectrometer for one element was estimated as 230 g (for H2O)7 or 360 g (for CH4).10

Absorption spectroscopy has some advantages over emission and can be complementary. In an ablation plume, the absorbing species can dwell significantly longer than the emitting species because the former can be in their ground or metastable states, while the latter must be excited and radiating, and consequently, short-lived. Molecules usually have multiple channels to dissipate energy, and their radiative transitions may be less probable than the other channels. As a result, some species can absorb but will not emit effectively. Absorption can be measured by a simple photodiode with very high resolution, while emission requires an echelle or other kind of spectrograph with a CCD camera. Contrary, emission can be measured remotely by a telescope, while absorption requires a close proximity to the sample.

CONCLUDING REMARKS A schematic picture of our conceptual design is presented in Fig. 6 as one of the possible solutions. A ChemCam-

like instrument is located on a mast and used for the primary purpose of stand-off LIBS measurements (as in MSL Curiosity). The LAOCIS module is an augmentation installed on a front arm of the rover. An ablating laser pulse hits the sample. A plume extends upward, perpendicularly to the sample surface and propagates toward the analytical zone. Light from an array of the diode lasers can be used for simultaneous multi-isotope detection. This laser light pumps a piezo-modulated cavity (or several crosswise intersecting optical cavities) with an optical axis passing through the analytical zone. The output from the cavity or cavities is detected by photodiodes. The use of ChemCam laser ablation of surface materials enables robust sampling that is nearly independent of the sample nature.

For the LAMIS mode (emission detection), if we determine that isotopic molecular spectra can be measured only from a short distance, then a robotic arm can bring an optical fiber cable close to the location of the ablated sample and collect the plasma plume emission. The cable will deliver optical emission to the ChemCam's spectrograph which is fitted inside the rover body (as in MSL). The ChemCam's laser can ablate any location within a radius of ~8 meters. In the best case scenario, isotopic molecular emission will be measured remotely using ChemCam's telescope.

The main uncertainty of measurements arises from the irreproducibility of the laser ablation. Applied Spectra’s industrial LIBS instruments routinely achieve elemental concentration precision of 2–4%.11 The reproducibility demonstrated elsewhere in the LIBS measurements of elemental abundances was significantly better: ±0.03%.12 Therefore, precision and spectral resolution of the proposed device can be sufficient for accurate age determination.

The sensitivity obtained by others in similar experiments on tunable laser absorption in laser ablation plumes is encouraging. Individual isotopes were determined at levels of 25 µg/g (87Rb)9 and 50 µg/g (235U)13 in mineral samples. However, the previous researchers did not use a cavity enhancement technique. For a comparison, basalts can have up to


600 µg/g of Sr, while in magmatic carbonates the strontium content can reach up to ~3000 µg/g. Therefore, strontium can likely be detected in direct absorption without a cavity but a cavity enhancement greatly increases the dynamic range. .

ACKNOWLEDGMENT This work was supported by the Defense Threat Reduction Administration (DTRA) of the DoD under Federal

Awards No. LB09005541 and LB09005541A, and Contract No. DE-AC02-05CH11231 awarded by the DOE through the National Nuclear Security Administration (NNSA) and NASA Contract No. NNX10CA07C awarded to Applied Spectra, Inc.

Fig. 6. Concept of a rover with the mast-based ChemCam and the LAOCIS installed on a front arm.

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laser spectroscopy,” Appl. Opt., 44, 1226-1235 (2005). [11] http://www.appliedspectra.com/products [12] B.C. Castle, K. Talabardon, B.W. Smith, J.D. Winefordner, “Variables influencing the precision of laser-induced

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ChemCam

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laser ablation - optical cavity isotopic spectrometer (laocis) for mars rovers

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