Planetary Geochemical Investigations Using Raman and Laser-Induced Breakdown Spectroscopy Samuel M. Clegg,a,* Roger Wiens,a Anupam K. Misra,b Shiv K. Sharma,b James Lambert,c Steven Bender,a Raymond Newell,a Kristy Nowak-Lovato,a Sue Smrekar,c M. Darby Dyar,d Sylvestre Mauricee a

Los Alamos National Laboratory, Los Alamos, NM 87545 USA University of Hawaii at Manoa, Hawai‘i Institute of Geophysics and Planetology, 2525 Correa Road, Honolulu, HI 96822 USA c California Institute of Technology, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109 USA d Mount Holyoke College, Department of Astronomy, 50 College Street, South Hadley, MA 01075 USA e Universite´ de Toulouse, Institut de Recherche en Astrophysique et Plane´tologie, 9 Avenue du Colonel Roche, BP 44346-31028 Toulouse Cedex 04, France b

An integrated Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) instrument is a valuable geoanalytical tool for future planetary missions to Mars, Venus, and elsewhere. The ChemCam instrument operating on the Mars Curiosity rover includes a remote LIBS instrument. An integrated Raman-LIBS spectrometer (RLS) based on the ChemCam architecture could be used as a reconnaissance tool for other contact instruments as well as a primary science instrument capable of quantitative mineralogical and geochemical analyses. Replacing one of the ChemCam spectrometers with a miniature transmission spectrometer enables a Raman spectroscopy mineralogical analysis to be performed, complementing the LIBS chemical analysis while retaining an overall architecture resembling ChemCam. A prototype transmission spectrometer was used to record Raman spectra under both Martian and Venus conditions. Two different high-pressure and high-temperature cells were used to collect the Raman and LIBS spectra to simulate surface conditions on Venus. The resulting LIBS spectra were used to generate a limited partial least squares Venus calibration model for the major elements. These experiments demonstrate the utility and feasibility of a combined RLS instrument. Index Headings: Raman spectroscopy; Laser-induced breakdown spectroscopy; LIBS; Mars geology; Venus geology; Remote sensing.

INTRODUCTION Robotic planetary geological investigations are capable of changing the way we view the Earth relative to our solar system. The National Aeronautics and Space Administration (NASA) has successfully landed three generations of rovers onto the Martian surface, including the Pathfinder rover Sojourner, the Mars exploration rovers (MERs) Spirit and Opportunity, and the Mars Science Laboratory (MSL) rover Curiosity. The NASA robotic platforms continue to evolve and enable scientific opportunities that rival some of the best laboratory capabilities. The desiderata for planetary geological investigations are to rapidly acquire quantitative elemental (geochemical) and mineralogical compositions, interpreted within a morphological context, with stable Received 18 November 2013; accepted 16 April 2014. * Author to whom correspondence should be sent. E-mail: sclegg@ lanl.gov. DOI: 10.1366/13-07386

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isotope resolution. An integrated laser-induced breakdown spectrometer (LIBS) and Raman spectrometer is capable of realizing many of these goals. The Curiosity rover’s CheMin instrument has come the closest to achieving the goal of acquiring geochemistry and mineralogy from the same sample. The CheMin instrument was originally designed to perform mineralogical investigations through X-ray diffraction and geochemical detection using X-ray fluorescence (XRF).1 The rover’s drill is used to grind rocks to provide finely powdered tailings,2 and a scoop on the arm is used to collect material and deliver the sample to the CheMin instrument located in the rover body. These activities require careful planning as well as significant time and energy. A remote instrument capable of providing both mineralogy and chemistry without the use of the arm will ultimately be able to analyze far more samples. ChemCam is the first LIBS instrument to operate on another planet and is integral to the scientific mission of the Curiosity rover.3,4 ChemCam is the integration of a remote LIBS instrument capable of analyzing samples up to 7 m from the rover mast and a remote microimager (RMI) that records high-resolution context images. As a remote-sensing instrument, ChemCam is a reconnaissance tool for identifying targets that should be considered for more detailed investigations using the other rover instruments as well as a scientific tool with many unique capabilities to contribute to the overall mission goals. Raman spectroscopy and LIBS are highly synergistic analytical techniques. Raman spectroscopy is sensitive to the molecular structure of the sample, from which researchers can definitively determine the mineralogy and infer the elemental composition. Laser-induced breakdown spectrometry is sensitive to the elemental composition, from which researchers can definitively determine the geochemistry and infer the mineralogy. Remote LIBS instruments capable of planetary geochemical analysis have been under development for more than a decade.5–9 Initial experiments involved standard laboratory lasers and spectrometers to probe samples under the 0.01 atm Martian surface pressure, ideal for LIBS analysis. Arp et al.10 later explored using LIBS to probe geological samples under the more challenging 92 atm Venus surface pressure. Recently, Lasue et al.11 completed an initial study designed to

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investigate LIBS geochemical analysis under lunar surface conditions, where ChemCam laboratory spectrometers were used to record spectra from samples at ,10 5 Torr. Now ChemCam is operating on Mars, collecting micrometer-scale geochemical data along the traverse from the Bradbury landing site to Mount Sharp. In recent years, some studies have also explored using LIBS to obtain geochronological data, independently or integrated with a mass spectrometer.12 In situ and remote Raman spectroscopy instruments have also been developed for planetary mineralogical analysis.13–32 The development of these instruments started with a green 532 nm frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser; more recently researchers have started to use ultraviolet (UV) lasers to excite the Raman active vibrational modes. Raman spectroscopy has many advantages as a mineralogical analysis tool because a single laser is capable of exciting nearly any important mineral and the signal can be recorded using a single spectrometer. Raman spectroscopy is also completely insensitive to the ambient pressure and is insignificantly sensitive to the atmospheric temperature.24,28,30 Raman spectroscopy and LIBS can be integrated into a single Raman-LIBS (RLS) instrument that is the same size as the ChemCam, and this concept has been under development for more than a decade.29,33–38 Investigating the elemental and molecular composition of artwork has been the most popular use of RLS.39–46 More recently, RLS has started to gain acceptance as a geological analysis tool47–49 and as a standoff-sensing instrument for future planetary missions.16,34,35,36,50–53 Wiens et al.34 explored the use of a single laser beam containing two different wavelengths to obtain both LIBS and Raman spectra from relevant environments. In this case, commercial spectrometers and a commercial laser were used. Misra et al.29 and Sharma et al.36 explored increasingly sophisticated prototype spectrometer designs with off-the-shelf detector systems, aimed at optimizing Raman spectroscopy but also capable of LIBS. These led to the development of the system proposed for the Surface and Atmosphere Geochemical Explorer (SAGE) mission to Venus, developed through a NASA New Frontiers Phase A Program in 2010, and to the spectrometer used in some of the current studies. In this article, we discuss some of the challenges and capabilities of a remote RLS instrument for meeting the desired goals of geochemical and mineralogical investigations on Mars and Venus as the future of remote laser-based observation is realized. The Martian surface environment is absolutely ideal for LIBS in that the relatively low 0.01 atm atmospheric pressure is enough to contain the evolution of the plasma over the , 20 ls lifetime while not perturbing or quenching the excited species. Relatively minor changes to the ChemCam architecture would allow researchers to also record Raman spectra using the same laser (frequency-doubled for Raman spectroscopy), telescope, spectrometers, and detectors. A similar RLS instrument could also be developed for future missions to the surface of Venus. The 92 atm, 740 K Venus surface conditions pose an exceptional challenge for planetary investigations that require optically, me-

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chanically, and electronically efficient instruments such as the RLS instrument discussed here. The surface of Venus was investigated using nine Venera landers between 1965 and 1981, as well as VEGA 1 and 2 in 1984. The Venera 13, Venera 14, VEGA 1, and VEGA 2 landers were capable of collecting samples and transferring them into the lander body for XRF geochemical analysis. In contrast, the proposed SAGE mission to Venus includes a remote RLS instrument that will record hundreds of LIBS elemental spectra as well as the first direct mineralogical compositions using Raman spectroscopy within 2 m of the RLS window.

EXPERIMENTS AND RESULTS Raman–Laser-Induced Breakdown Spectroscopy Instrumentation Concept. An instrument capable of planetary investigations must be designed to be lightweight, to be low volume, to be low power, and to survive the specific planetary environmental conditions. The ChemCam instrument that landed on Mars on August 6, 2012, embodies these requirements and is the basis for the development of an integrated RLS instrument. ChemCam consists of a mast unit (MU)4 and a body unit (BU)3 connected by a 6 m long optical fiber and electrical cables. The MU consists of a neodymium-doped potassiumgadolinium tungstate (Nd:KGW) laser, an autofocus laser, a telescope, a RMI, and the electronics required to power and control these systems. The LIBS spark is generated using a diode-pumped Nd:KGW laser that produces a 1067 nm pulsed beam operating at 3 Hz.4 The laser is directed through a Galilean telescope to enlarge the laser beam before it is directed through the 110 mm diameter telescope. The telescope focuses the laser beam, imparting up to 14 mJ into a 350 to 550 lm diameter spot; the spot size increases slightly with distance from 2 to 7 m. The MU also includes an integrated RMI that produces high-resolution context images. Although context imaging is an exceedingly valuable geological contribution to the ChemCam architecture, it is outside the scope of this paper (detailed descriptions regarding the RMI can be found in Maurice et al.4). The LIBS emission is collected using the MU telescope and directed into a 300 lm core optical fiber connected to the ChemCam BU. The optical elements of the BU consist of a demultiplexer, three reflective spectrometers, and three charge-coupled detectors (CCDs).3 The collected LIBS emission is first sent through a custom demultiplexer that consists of two dichroic mirrors and an aluminum mirror to direct specific spectral bands into the ultraviolet (UV; 240.1–342.2 nm), violet (VIO; 382.1– 469.3 nm), and visible near-infrared (Vis-NIR, 474.0–906.5 nm) spectrometers. The demultiplexer output channels are connected to the three spectrometers with fiber bundles. As described by Wiens et al.,3 the demultiplexer end of the fiber bundle is arranged in a circular orientation and the spectrometer end is in a linear orientation to maximize the amount of light transmitted through the 21, 25, and 21 lm slits affixed to the ends of the fibers connected to the UV, VIO, and Vis-NIR spectrometers, respectively.

Modifying the ChemCam architecture so that it is capable of generating and recording a Raman spectrum can be accomplished with minimal changes. The key is to identify the driving requirements for both Raman spectroscopy and LIBS. First, the MU must be upgraded to generate light capable of exciting the Raman active vibrational modes that can be observed within the spectral range of the detectors. This can be accomplished by adding a frequency-doubling crystal to the 1067 nm laser to produce a green 533.5 nm laser beam. Although producing a LIBS spark using a focused 533.5 nm laser beam is also possible, the doubling crystal also transmits the fundamental 1067 nm laser beam, which in this concept is still used as the LIBS source.34,35 Consequently, a relatively minor change to the ChemCam MU enables the excitation of detectable Raman active modes. Collecting and recording the Raman and LIBS signals using a ChemCam-like BU requires additional changes. The Stokes Raman spectra excited by a 533.5 nm laser must be observed using a spectrometer capable of recording the full 4200 cm 1 (up to 687.5 nm) Raman spectrum with an approximately 10–20 cm 1 spectral resolution, equivalent to 0.25–0.50 nm at 500 nm. In contrast, the ChemCam LIBS Vis-NIR spectrometer requires collecting light to at least 840 nm with a 0.65 nm spectral resolution. Consequently, the spectral range requirement is driven by LIBS, whereas the resolution requirement is driven by Raman spectroscopy. The LIBS spectral-range requirement on the long wavelength side derives from our desire to observe the Cl (837.59 nm) emission line. The Raman signal is significantly weaker than the LIBS signals, and an optical throughput higher than the ChemCam f/4 spectrometers is necessary. An optimized design consists of a miniature f/2 transmission spectrometer similar to those designed at the University of Hawaii29,36 and Los Alamos National Laboratory (LANL), using two volume-phase holographic transmission gratings: a dual-plex grating (500–745 nm) and a singleplex grating (745–900 nm). These spectrometers produce three spectral windows on the detector that meet the spectral resolution and range requirements. Finally, the detector must have a dynamic range capable of recording the relatively weak Raman signal without saturating the bright LIBS signals. Given that the Raman signal lifetime is equal to the pulse width of the laser, 4.5 ns for the ChemCam laser,4 and most of the geological samples that we would probe also produce fluorescence, the detector also needs to be capable of a dynamic gate width to meet both the Raman and LIBS requirements. A carefully designed intensified CCD (ICCD) is capable of meeting all these requirements and was used in these experiments. With this arrangement, assuming that the gating window can be adjusted relative to the timing of the laser pulse, studies of fluorescence can also be carried out (e.g., Sharma22). Experimental Setup and Results. The Raman spectroscopy and LIBS experiments involve four different and independent instrument configurations. Two separate Raman spectroscopy and two separate LIBS instruments were used under Martian and Venus planetary conditions. All four of these experimental configurations

produced different sets of experimental results from samples that are relevant to the planetary geology discussed here. We describe each data set in turn, providing both the relevant experimental details and results in each section. Martian Elemental Analysis Using Laser-Induced Breakdown Spectroscopy. The Martian LIBS analyses discussed in this article were collected using the ChemCam instrument on the Curiosity rover operating on Mars. The Martian surface pressure is approximately 0.01 atm, consisting of 95% carbon dioxide (CO2) with the rest comprising nitrogen gas (N2), Ar, and oxygen gas (O2). Figures 1a–1f show the LIBS spectrum from the first ChemCam analysis on a rock named ‘‘Coronation’’, located 2.7 m from the rover mast. The analysis consisted of 30 laser shots, at 14 mJ/pulse, for which the LIBS spectrum from each laser shot was recorded. A dark spectrum was also collected under identical conditions but without the laser, depicted in Fig. 1g. Note that dark spectra contain many structural features that represent the reflected ambient sunlight such as the H Fraunhofer line at 656 nm. Wiens et al.54 provides a detailed description of the analysis steps used to extract the elemental compositions from the ChemCam spectra using a partial least squares (PLS) calibration model. The preprocessing steps include subtracting the dark spectrum, denoising the spectrum, removing the Bremsstrahlung continuum, and multiplying by the instrument function, resulting in an instrument-independent spectrum. All the geochemical standards used to generate the PLS calibration model, as well as the spectrum from Coronation depicted in Fig. 1, followed these preprocessing steps. To account for intensity changes due to distance, the spectra were normalized to the total integrated intensity, where each pixel is divided by the sum of all 6144 pixels in the spectrum. The ChemCam team used a calibrated multivariate analysis technique, as well as univariate analysis, to extract the quantitative elemental compositions from each spectrum. The normalized calibration standards were used to generate a PLS1 model that was used to extract the elemental compositions from the normalized Coronation spectrum. Partial least squares models simplify the 6144-channel spectra into eigenvectors, or components, which can be used to model a training set of standards. These component correlations are then used to yield the chemical abundances from the spectra of unknown samples.54 The ChemCam analysis used the PLS1 algorithm to generate independent models for every element; this contrasts with the PLS2 algorithm, which generates one model for all elements.55 Geochemical samples are elementally complex systems, and independent PLS1 models can be customized to generate the most accurate results. Table I lists the major element compositions from Coronation. Specific results from the ChemCam instrument have already been published.56–59 Martian Mineral Analysis Using Raman Spectroscopy. The Raman spectra from many natural rock samples were illuminated using a spare ChemCam laser and recorded using a breadboard-quality custom Raman spectrometer. Pulses from the spare ChemCam laser were directed through a potassium titanyl phos-

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FIG. 1. The first LIBS spectrum collected using the ChemCam instrument on the Mars Curiosity rover of the rock named Coronation. This spectrum represents the average of 30 laser shots. (a) Entire UV spectral region. (b) Entire VIO spectral region. (c) Entire Vis-NIR spectral region. (d) Spectrally enhanced view of the UV region. (e) Spectrally enhanced view of the VIO region. (f) Spectrally enhanced view of the Vis-NIR region. (g) Nonlaser dark spectrum from Coronation. The LIBS spectrum has been annotated to identify all the major elements and some of the minor elements. The black-and-white mosaic image was taken using the ChemCam RMI, and the color image was taken using the MastCam instrument.

phate (KTP) doubling crystal to create the green 533.5 nm Raman illumination laser. The laser was operated at a relatively conservative 20 mJ/pulse (the full capability of the laser is 35 mJ/pulse) and a conservative repetition rate of 1 Hz. The doubling crystal was quite efficient and created 13 mJ/pulse at 533.5 nm (65% efficiency). The 1067 nm fundamental beam was separated from the 533.5 nm beam using a pair of dichroic mirrors that reflected the 533.5 nm light and transmitted the 1067 nm TABLE I. Coronation major oxide compositions.a Major oxide SiO2 TiO2 Al2O3 FeOT MgO CaO Na2O K2O

Oxide (wt %)

PLS model RMSE

54.3 1.2 8.7 15.7 2.1 7.4 3.1 1.0

7.1 0.6 3.7 4.0 3.0 3.3 0.7 0.9

a Al2O3, aluminum oxide; CaO, calcium oxide; FeOT, total iron; K2O, potassium oxide; MgO, magnesium oxide; Na2O, sodium oxide; TiO2, titanium dioxide.

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beam into beam dumps. The laser was directed into a 103 beam expander (Newport), which was used to focus the laser down to 2 mm on the sample, which was located 9 m from the laser. A 89 mm telescope (Questar) was used to collect some of the Raman emissions, directed through a 532 (68.5) nm notch filter (Semrock) and into a spare ChemCam fiber bundle, as previously described. The output of this fiber, with the 21 lm slit affixed to the end,3 served as the slit for the transmission spectrometer developed at LANL. The light transmitted through the slit is directed through a pair of collimating lenses into a 50:50 dichroic beam splitter cube assembly and toward two holographic transmission gratings (Kaiser Optical). One of the transmission gratings is a dual-plex grating that disperses the signal into two spectral windows (500–655 and 655–740 nm) and a single-plex grating (740–900 nm) primarily used when collecting LIBS spectra. All three spectral windows were recorded using a miniature ICCD (Syntronics) with a readout-limited detector (Sony). To maximize the spectral signal-to-noise ratio, the intensifier was triggered by the laser using a 40 ns gate while the CCD detector was held open for 10 s to record the Raman signals from 10 laser shots with each exposure.

FIG. 2. Raman spectra collected 12 m away from the laboratory-based instrument illuminated using a frequency-doubled spare ChemCam laser and recorded using a miniature transmission spectrometer. The spectra of BaSO4 and CaMg(CO3)2 depict some of the molecular signatures used to determine mineralogy.30

This was repeated to collect a spectrum that represents 500 total co-added laser shots. Note that the notch filter was intentionally placed close to the focal point at the output of the telescope. Placing the filter so close to the focal point resulted in the transmission of some of the laser light without overwhelming the Raman signal. The transmission of some of the laser beam served as a convenient diagnostic because it indicates where 533.5 nm is in the CCD image and indicates that the intensifier and CCD detector triggers are properly set to record as the laser illuminates the sample. The notch filter can be placed at other locations within the spectrometer so that all the laser light is eliminated, if desired. The samples were placed under 0.01 atm CO2 to simulate the Mars atmospheric pressure and at room temperature. Raman spectroscopy is sensitive to the molecular structure of the samples, from which the mineralogical identity can be determined. Figure 2 contains the Raman spectra from barite (BaSO4) and dolomite (CaMg(CO3)2) with the molecular vibrational modes identified. Venus Elemental Analysis Using Laser-Induced Breakdown Spectroscopy. Laboratory investigations of samples under conditions relevant for the surface of Venus were designed to define and demonstrate LIBS elemental analysis using instruments and facilities at NASA’s Jet Propulsion Laboratory (JPL) and at LANL. To fully investigate LIBS under Venus temperature and pressure, JPL constructed a 2 m long, 110 mm diameter chamber capable of containing at least 92 atm CO2 at 740 K. One end of the chamber was capped with a sapphire window (Specac) through which the LIBS (and Raman spectroscopy) investigations were completed. The LIBS spark was generated with a Nd : YAG laser, producing a 80 mJ/pulse at 1064 nm and focused onto pressed-powder geological standards mounted at the end of the Venus chamber. Some of the LIBS emissions were collected using a 89 mm telescope (Questar), directed into one of several dispersive spectrometers

using an optical fiber, and were recorded using an ICCD (Syntronics). The Bremsstrahlung continuum was removed, and the spectra were spectrally calibrated to produce the spectra depicted in Fig. 3. A smaller high-pressure cell was used at LANL to initially develop the functional requirements and elemental analysis models that were ultimately demonstrated using the fully capable JPL cell. The LANL cell could be filled up to 680 atm, but the temperature was limited to 423 K. The LANL cell is 30 cm long and 1 cm in diameter, and consequently, the laser was co-aligned with the Questar 89 mm telescope by placing a mirror in the telescope obscuration. The resulting spectra were collected using a spectrometer (Princeton Instruments) and recorded using an ICCD (Syntronics). Venus Mineral Analysis Using Raman Spectroscopy. The Raman analyses were completed in the same two pressure cells used to complete the LIBS experiments under the same temperature and pressure conditions. All these experiments were run using the same 2 m standoff distance used in the LIBS experiments. Figure 4 contains some of the Raman spectra collected under the two Venus simulations. The Raman experiments using the JPL Venus chamber involved making minimal changes to the laser and the spectrometer. The Nd : YAG laser used in the LIBS experiments was frequency-doubled to generate the up to 25 mJ/pulse, 20 Hz 532 nm light. The Raman emission was collected using the same 89 mm telescope and directed into the same LANL-developed transmission spectrometer and Syntronics ICCD used in the Martian Raman experiments. These experiments used the same 40 ns intensifier gate and 10 s integration time. The experiments using the same small pressure cell (92 atm and 423 K) were completed at the University of Hawaii using a commercial transmission spectrometer (Kaiser) and ICCD camera (Princeton Instruments). These experiments also involved a doubled Nd:YAG laser to generate a 20 mJ/pulse, 20 Hz, 532 nm light to excite the Raman active vibrational modes. The laser

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FIG. 3.

The LIBS spectra collected using the JPL Venus chamber under 92 atm CO2 at 740 K. All the major elements are identified in the spectra.

was co-aligned with an f/2 camera lens (Nikon) used to collect the Raman emission from the small pressurechamber opening. The output of the f/2 camera lens was directly coupled to the transmission spectrometer without the use of an optical fiber.28,30

DISCUSSION Raman–Laser-Induced Breakdown Spectroscopy for Martian Geochemical Investigations. The spectrum of Coronation is representative of most of the ChemCam observations on Mars in terms of overall dynamic range. This experiment was conducted using the full ChemCam laser power, which resulted in saturating the 777 nm oxygen emission line. The ChemCam signal-to-noise ratio is high enough to resolve and identify many weak emission lines associated with major and minor elements; Figs. 1d–1f highlight some of these regions. Figure 1d contains an enhanced view of the Al (308 and 310 nm) and Ca (315 and 317 nm) emission lines. Figure 1e contains the very weak Ti and Mn lines in the 398– 404 nm spectral region. Finally, Fig. 1f depicts the part of the first five single-shot spectra from Coronation that contains the H (656 nm), C (567 nm), and Li (671 nm) emission lines. The first spectrum from most ChemCam rock analyses contains the H emission line, suggesting that there is some hydrated dust on the surface of the rocks. Although the H signal dissipates on subsequent laser shots, the C emission from the breakdown of atmospheric CO2 and Li embedded in Coronation remains constant. The ChemCam LIBS instrument sensitivity is high enough to collect quantitative results from a single laser shot. This provides a highly desirable geological tool to probe under the immediate surface of the geological sample. Experiments conducted prior to launch3 indicated that each laser shot removes approximately 0.34– 0.5 lm, leading to depths of 10–15 lm for rocks of

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moderate hardness with a normal ChemCam observation consisting of 30 laser shots in one location. Figure 5 contains the single-shot silicon dioxide (SiO2) compositions from Coronation that are representative of many ChemCam elemental observations. The surface of most Martian rocks is covered with a dust deposited by either a local wind or periodic global dust storms. The SiO2 abundance in the dust is lower than inside Coronation itself. This dust layer is blown away from the surface of the rock as laser shots are accumulated, and the SiO2 composition of Coronation increases as contributions from the dust decrease until it ultimately reaches a steady state of 54.3 (6 7.1) oxide wt %. Similar trends are observed for other elements, and the dust composition could be higher or lower than the composition found in the underlying rock. The Martian surface pressure and temperature produce an equally ideal environment for Raman spectroscopy. A Raman spectrometer was originally meant to be part of the Athena payload package carried by the MERs but the package was descoped. There have been many Raman laboratory investigations of geological materials over the last decades that demonstrate the utility of the technique. Raman spectra usually contain several distinct diagnostic Raman emission peaks corresponding to different molecular vibration modes, resulting in more descriptive spectra than if they simply showed a single peak. Figure 2 contains the Raman spectrum of BaSO4, in which four different sulfate (SO4) vibrational modes are excited and spectrally identified. The spectrum includes the strong 988 cm 1 Raman line associated with the m1(SO4) symmetric stretching vibration mode, which is common among all sulfates while the absolute vibrational frequency shifts relative to the actual mineralogical material, such as anhydrite (1018 cm 1) and gypsum (1008 cm 1).30 The weak spectral feature at 462 cm 1 is produced from a doubly degenerate m2(E)

FIG. 4. Raman spectra collected under the two Venus simulations. Raman spectra collected under 92 atm up to 423 K from pressed powder of mixed minerals in a basaltic matrix (top). Raman spectra collected under 92 atm at 740 K from natural mineralogical samples (bottom). The spectral regions highlighted in gray are from either the sapphire window or the CO2 atmosphere, as noted in the figure.

sulfate ion (SO42 ) vibrational mode. The weak spectral features 612 and 648 cm 1 are produced from the triply degenerate m4(F) SO42 vibrational mode, while the 1141 and 1166 cm 1 features are produced by the triply degenerate m3(F) SO42 vibrational mode.30 Figure 2 also contains a spectrum of CaMg(CO3)2, in which several carbonate and lattice vibrational modes are observed. The 723 cm 1 and bright 1094 cm 1 spectral features are from the m4 and m1 carbonate vibrational modes, respectively, while the 174 and 297 cm 1 features are lattice vibrational modes. We can definitively use this Raman spectrum to identify the presence of a carbonate far better than we can using inferences from LIBS spectra, and it can distinguish CaMg(CO3)2 from calcite (CaCO3) or siderite (FeCO3), largely from the frequency of the lattice vibrational modes.35 Analysis using LIBS on Mars always produces a carbon emission from the breakdown of the CO2 atmosphere, which complicates the identification of carbonates relative to routinely observed oxides. Whereas LIBS spectra can be used only to infer whether a sample is actually a carbonate

from the elemental composition, an integrated RLS instrument could definitively distinguish CaMg(CO3)2 from CaCO3 and FeCO3 from the combination of Raman and LIBS spectra. This would be most useful in a mixed-phase material, for example, if carbonates and silicates are intimately mixed; in this case, LIBS by itself may not easily distinguish whether the Mg or Fe is part of the silicates or the carbonates. The Martian dust has been a topic of many investigations as NASA has operated three generations of rovers on Mars. This dust can have a profound affect on the results from various instruments. Both the MERs, Spirit and Opportunity, as well as MSL rover, Curiosity, include brushes to clear the dust and enable other scientific investigations such as Mossbauer and alpha proton Xray spectroscopy of the dust-free surface. This dust is also effectively removed from the rock surface by the ChemCam laser. Sharma et al.35 conducted a similar experiment years before Curiosity landed on Mars in which the LIBS laser was used to remove basaltic dust from a strikingly white anhydrite surface. The Raman

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FIG. 5. The change of the SiO2 composition of Coronation is plotted against the number of laser shots and distinguishes the composition of the rock from the dust layer. Many Martian rocks are covered with dust that can geochemically obscure the analysis of the underlying rock; the ChemCam laser effectively blows away this dust layer to expose the rock.

spectrum from this anhydrite sample was obscured by the dust layer, and the LIBS laser effectively removed the dust, enabling both Raman spectroscopy and LIBS investigation of the anhydrite sample. The ability to remove this dust layer is equally critical for other remote analytical techniques, such as infrared spectroscopy on Mars, Venus, and the Moon or other planetary remote geochemical investigations. The only realistic ways to remove the dust for such analyses is with a brush, requiring arm deployment, or by using the shock wave produced by the laser plasma, which can be done rapidly at any distances at which LIBS can be measured. Any instruments that are co-boresighted with the LIBS laser would benefit substantially from the remote dust removal. Raman–Laser-Induced Breakdown Spectroscopy for Venus Geological Investigations. Venus is a much more challenging environment in which to study planetary geology and atmospheric composition. The relatively high surface pressure tends to squeeze the expansion of the LIBS plasma, as was depicted by Arp et al.10 This high-pressure plasma compression results in collisional quenching of the excited species and an overall reduction of the signal. The high pressure can also result in pressure broadening of the otherwise spectrally narrow LIBS emission lines and could result in relaxing the spectrometer resolution requirements for any instrument intended for the Venus surface. The LIBS spectra depicted in Fig. 3, which were collected under Venus temperature and pressure, demonstrate that we can resolve all of the major element emission lines. Although the spectrometers and ICCD used in these experiments were the best available commercially, the relative intensity and throughput

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suggests that we would acquire far more sensitive LIBS data using a fully customizable system, based on the improvements realized in ChemCam compared to commercial systems. We can highlight some of the improvements that can be achieved using a customized system by comparing the LIBS spectra collected under the Venus conditions in Fig. 3 to the LIBS spectra collected using ChemCam in Fig. 1. The spectral features depicted in violet-blue region in Fig. 3 under Venus conditions are clearly more intense than the emission lines in the UV or Vis-NIR channels. The UV and Vis-NIR channels include several equally bright emission lines, such as Mg at 279 nm and Na at 589 nm, which should be more pronounced in these Venus experiments because they are in the Martian experiments. If these same data were taken with higher-quality spectrometers, equivalent to those in ChemCam, the emission lines in all three spectral ranges could be optimized. Given that a LIBS instrument designed to work on Venus would employ ICCDs rather than the CCDs used on ChemCam, we would be able to optimize the intensifier gate time and eliminate some of the noise that was collected by this system after the plasma dissipated. These experiments indicate that the LIBS plasma dissipates within 1 ls under Venus conditions, shorter than the up to 20 ls lifetime for LIBS plasma generated under terrestrial and Martian conditions. This is identical to the results presented by Lawrence-Snyder et al.60,61 for LIBS spectra collected under .200 atm of ocean water. The data collected using the small Venus chamber were used to develop a preliminary PLS2 calibration model using the samples and compositions listed in Table II. Figure 6 contains the PLS validation plots from

TABLE II. Some of the accepted (not predicted) elemental compositions of the six standards used in the PLS2 model generated with data collected under Venus pressure (92 atm) and at 423 K with the LANL pressure cell.a Sample GBW07105 JA1 BHV02 KV0425 TAP04 GUWBM a

Si

Ti

Fe

Mg

Ca

K

0.15 0.23 0.18 0.16 0.16 0.18

0.0060 0.0023 0.0063 0.0072 0.0028 0.0075

0.034 0.018 0.034 0.029 0.023 0.034

0.039 0.008 0.060 0.044 0.036 0.039

0.032 0.021 0.038 0.025 0.022 0.045

0.010 0.003 0.001 0.029 0.001 0.002

In atomic fractions. Note that oxygen accounts for most of the balance of the atomic fractions.

this limited sample set, and the plots show a good correlation between the known and expected values. We emphasize, however, that this limited set of samples is not nearly large enough to develop a broadly applicable model for samples outside this set.54 To compare, the first ChemCam model employed a calibration model using 66 geochemical standards, and a more expansive database is under development in response to the observations from Curiosity at Gale Crater. We used the results of this preliminary model to plot a comparison between the observations from Venera and these data (Fig. 7). The error bars certainly suggest that this limited LIBS investigation is comparable with the Venera accuracy and should significantly improve as the calibration database increases. Raman spectroscopy under the Venus atmosphere is no more challenging than under the Martian surface conditions. The green Raman laser illuminates the sample surface to excite vibrational modes in the samples; high pressure has no affect on these modes. The high sample temperature, however, does have a slight impact on the excited vibrational modes. Sharma

and colleagues28,30 indicated that the Raman molecular peaks tend to shift by 10 cm 1, generally within the resolution of the spectrometer, relative to spectra collected at room temperature. Otherwise, the library spectra collected for Mars or terrestrial investigations are equally applicable under Venus conditions. Finally, the thick Venus atmosphere, consisting of 96.5% CO2, 3.5% N2, and trace amounts of other gases also produces bright Raman CO2 bands, as depicted in Fig. 4. The spectra in Fig. 4 (top) were collected from synthetic mixtures of various minerals in a basaltic matrix. Olivine produces the weakest of the Raman signatures in this set, and it was observed down to 1 vol%, the lowest concentration tested in these experiments. The Raman spectra show part of the intense CO2 Raman bands relative to the molecular signatures of the rock samples. Fortunately, the location of these CO2 lines is in a relatively isolated part of the Raman spectrum and do not interfere with the identification of the molecular and mineralogical signatures required for a Venus geological investigation. Furthermore, the sapphire windows also produce four Raman lines (at

FIG. 6. The PLS2 validation models for six of the major elements extracted from the LIBS spectra collected under Venus surface pressure.

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FIG. 7. Plot of the SiO2 versus potassium oxide (K2O) compositions using data collected from Venera (blue points) on the surface of Venus as well as the LIBS observed (black points) and accepted values (red points).

380, 418, 660, and 750 cm 1) that do not interfere with any of the diagnostic mineralogical signatures.30

CONCLUSION An integrated Raman spectroscopy and LIBS instrument is capable of meeting many of the core geological requirements for planetary investigations, including geochemical and mineralogical compositions. Raman spectroscopy is sensitive to the sample molecular structure from which mineralogy is directly determined, and LIBS is an elemental analysis technique that can detect all elements above the detection limit independent of the elemental mass. These two analytical techniques can be efficiently integrated into a single instrument that is the same size as the ChemCam LIBS instrument operating on the Mars Curiosity rover. In such an RLS instrument, the same laser can produce both the 1067 nm beam used for LIBS analysis and the frequency-doubled 533.5 nm illumination required for Raman spectroscopy. These beams can be directed through the same telescope so that researchers can probe samples far beyond the reach of any arm on the rover or lander. The Raman and LIBS signals can be collected using the telescope and recorded using a common set of spectrometers and detectors. The result will significantly enhance scientific planetary investigations on Mars, Venus, the Moon, and any other planets that will eventually be visited by rovers or landers. This integrated RLS instrument will be capable of probing specific geological features and generating a quantitative elemental and mineralogical composition without the need for an arm to prepare or collect the sample. This RLS instrument will be remarkably efficient and will be able to acquire hundreds of spectra within minutes, allowing the mission to select many diverse

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targets within the targetable region. Finally, RLS will serve as both a primary instrument capable of independent scientific investigations and a reconnaissance tool for identifying targets for further investigations, such as sample return missions. ACKNOWLEDGMENTS We gratefully acknowledge the LANL Laboratory Directed Research and Development (LDRD) program, the NASA New Frontiers program, and the NASA Mars Science Laboratory program for funding various aspects of the study presented here. We also gratefully acknowledge David J. Cremers and Amy J. Bauer for the gracious invitation to present at the SCIX conference as well as the SCIX organizers for the opportunity to contribute to this special issue. 1. D. Blake, D.D. Vaniman, C. Achilles, R. Anderson, D. Bish, T. Bristow, C. Chen, S. Chipera, J. Crisp, D. Des Marais, R.T. Downs, J. Farmer, S. Feldman, M. Fonda, M. Gailhanou, H. Ma, D.W. Ming, R.V. Morris, P. Sarrazin, E. Stolper, A. Treiman, A. Yen. ‘‘Characterization and Calibration of the CheMin Mineralogical Instrument on Mars Science Laboratory’’. Space Sci. Rev. 2012. 170(1-4): 341-399. 2. R. Anderson, L. Jandura, A. Okon, D. Sunshine, C. Roumeliotis, L. Beegle, J. Hurowitz, B. Kennedy, D. Limonadi, S. McCloskey, M. Robinson, C. Seybold, K. Brown. ‘‘Collecting Samples in Gale Crater, Mars; An Overview of the Mars Science Laboratory Sample Acquisition, Sample Processing and Handling System’’. Space Sci. Rev. 2012. 170(1-4): 57-75. 3. R.C. Wiens, S. Maurice, B. Barraclough, M. Saccoccio, W.C. Barkley, J.F. Bell III, S. Bender, J. Bernardin, D. Blaney, J. Blank, M. Bouye, N. Bridges, P. Cais, R.C. Clanton, B. Clark, S. Clegg, A. Cousin, D. Cremers, A. Cros, L. DeFlores, D. Delapp, R. Dingler, C. D’Uston, M.D. Dyar, T. Elliott, D. Enemark, C. Fabre, M. Flores, O. Forni, O. Gasnault, T. Hale, C. Hays, K. Herkenhoff, E. Kan, L. Kirkland, D. Kouach, D. Landis, Y. Langevin, N. Lanza, F. LaRocca, J. Lasue, J. Latino, D. Limonadi, C. Lindensmith, C. Little, N. Mangold, G. Manhes, P. Mauchien, C. McKay, E. Miller, J. Mooney, R.V. Morris, L. Morrison, T. Nelson, H. Newsom, A. Ollila, M. Ott, L. Pares, R. Perez, F. Poitrasson, C. Provost, J.W. Reiter, T. Roberts, F. Romero, V. Sautter, S. Salazar, J.J. Simmonds, R. Stiglich, S. Storms, N. Streibig, J.-J. Thocaven, T. Trujillo, M. Ulibarri, D.

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