Laser induced breakdown spectroscopy on meteorites

Spectrochimica Acta Part B 62 (2007) 1606 – 1611 www.elsevier.com/locate/sab

Laser induced breakdown spectroscopy on meteorites☆ A. De Giacomo a,b,⁎, M. Dell'Aglio b , O. De Pascale b , S. Longo a,b , M. Capitelli a,b a

Department of Chemistry, University of Bari, Italy b MIP-CNR sec Bari, Italy

Received 27 October 2006; accepted 2 October 2007 Available online 9 October 2007

Abstract The classification of meteorites when geological analysis is unfeasible is generally made by the spectral line emission ratio of some characteristic elements. Indeed when a meteorite impacts Earth's atmosphere, hot plasma is generated, as a consequence of the braking effect of air, with the consequent ablation of the falling body. Usually, by the plasma emission spectrum, the meteorite composition is determined, assuming the Boltzmann equilibrium. The plasma generated during Laser Induced Breakdown Spectroscopy (LIBS) experiment shows similar characteristics and allows one to verify the mentioned method with higher accuracy. On the other hand the study of Laser Induced Breakdown Spectroscopy on meteorite can be useful for both improving meteorite classification methods and developing on-flight techniques for asteroid investigation. In this paper certified meteorites belonging to different typologies have been investigated by LIBS: Dofhar 461 (lunar meteorite), Chondrite L6 (stony meteorite), Dofhar 019 (Mars meteorite) and Sikhote Alin (irony meteorite). © 2007 Elsevier B.V. All rights reserved. Keywords: LIBS; Meteorite; Mars; Moon; Chemical analysis

1. Introduction LIBS technique as an analytical tool has been successfully employed in many applications. The main advantages of LIBS with respect to conventional analytical techniques are fast response and high sensitivity (generally ppm), the extremely wide range of materials that can be investigated without the necessity of an analytical chamber, making analysis without sampling or surface treatment and the flexibility of the experimental set-up configuration which brings to compactable and automatic systems [1] and the capability of applying LIBS for remote sensing [2,3]. The above mentioned characteristics of LIBS briefly mentioned clearly show the importance of this

☆ This paper was presented at the 4th International Conference on Laser Induced Plasma Spectroscopy and Applications (LIBS 2006) held in Montreal, Canada, 5-8 September 2006, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Department of Chemistry, University of Bari, Italy. Tel.: +39 080 5929511; fax: +39 080 5449520. E-mail address: [email protected] (A. De Giacomo).

0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.10.004

technique for space exploration, planetary surface analysis and space object recognition. Indeed a great interest in LIBS for Mars rocks and soils analysis has been shown in recent years, as a consequence of many space agency programs concerning Mars exploration [4–8]. On the other hand the recognition of asteroids and meteorites is another important task, because their elemental analysis allows the calculation of their mass and as a result, the trajectory of the object as well as the effect of their impact with a gaseous atmosphere can be predicted. The analytical methods generally applied for the analysis of meteorites which can be used in the Earth are ICP-MS, EDS and XRF [9–11], while the easiest way of studying the elementary composition of meteorites in flight is based on the optical emission spectroscopy of the plasma generated when the space object crosses Earth's atmosphere [12–14]. The analysis and the consequent classification are based on the Local Thermodynamic Equilibrium (LTE) assumption and on a range of values associated to element ratios. The excitation temperature in the plasma produced by the falling meteorite is determined by the line width or Boltzmann plot and it varies between 10,000 and

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detection system and the wavelength calibration have been performed by reference lamps. 3. Methodology

Fig. 1. Experimental set-up.

5000 K with the distance from the solid body. It is obvious that the trueness of this methodology is strictly related to the selection of the investigated emission lines and to the strategy chosen for the calculation of the composition from the population of the excited levels. Whereas the system characteristics of this kind of plasma are not so simple to be exactly described, the assumptions generally accepted for their fast diagnostics are very similar to those applied to laser induced plasma [1,15]. For this reason a set of experiments on different meteorite samples have been carried out in a LIBS laboratory, with the aim of checking the feasibility of LIBS for meteorite analysis and verifying the method generally applied for on-flight object measurements, as well as selecting the better spectroscopic lines for the elemental analysis with the accuracy available in a laboratory. Four meteorite samples belonging to different classes have been investigated: Dofhar 461 (lunar meteorite), Chondrite L6 (stony meteorite), Dofhar 019 (Mars meteorite) and Sikhote Alin (irony meteorite). It is important to stress that Dofhar 461 and Dofhar 019 are meteorites generated by the impact of asteroids with Moon and with Mars respectively and so they represent a small portion of the authentic surface of the original planet [16]. 2. Experimental set-up The instrumental set-up consists of a doubled Nd:YAG laser source working at 532 nm, a spectroscopic system, a pulse generator and a target holder as shown in Fig. 1. The laser source provides 7 ns long pulses, with a repetition rate of 10 Hz and variable energy up to 400 mJ at 532 nm. The laser beam is focused on the target surface by a 250 mm focal length lens and the fluence on the target has been fixed to 5 J cm− 2. The spectroscopic system consists of a monochromator TRIAX 550 Jobin Yvon with 1800 g/mm grating and an ICCD i3000 Jobin Yvon and a pulse generator Stanford Inc. DG 535 for selecting the delay time and the gate width of the detector. The emission light of the plasma is collected by a 7.5 cm focal length biconvex lens directly to the monochromator slit. All the receiving optics are in fused silica. The ICCD can be set to acquire a single spectrum by integrating the signals of photodiode column or to detect separately the signal from each photodiode of the matrix in order to get a spectrally resolved image. In the latter case the spectral resolution depends on the slit aperture and each horizontal array provides to the spectrum of a portion of plasma slice of 26 μm [17]. Spectrally resolved imaging has been applied for the fine alignment between the plasma emission image and the slit entrance of the receiving signal. The spectral response of all the

LIBS technique has been recently applied for the elemental analysis of Mars meteorite adopting the calibration curve method [18,19]. Anyway this approach, whereas it gives a better accuracy, involves the knowledge of the investigated elements, the preparation of standards and devotes LIBS measurements for building the calibration graphs for each element. Moreover the calibration curve method is strictly correlated to the stability of the experimental configuration during the standard and the sample measurements. These requirements can be easily satisfied in laboratory measurements but not in extreme conditions as space exploration or remote observation. For this reason, in this work, we have applied a simpler method similar to that reported in Ref. [20], which requires the only LTE assumption for determining the composition directly through selected emission lines. The LTE assumption and its validity for laser induced plasma has been discussed in other works [21,22], here we justify Boltzmann equilibrium by the high electron number density (Ne N 5 × 1016 cm− 3) calculated with the Stark broadening and by the observation of the Fe I distribution in a wide energy range as shown in Fig. 2 [22]. Moreover, to be sure that this assumption is valid for other elements too, only transitions with upper energy levels in the energy range measured of Fe I Boltzmann plot have been considered. It is important to underline that the LTE assumption implies equilibrium for the electron impact processes in any specific spatial and temporal coordinates. This means that this assumption ideally requires detection gate widths as short as possible. Unfortunately for analytical tasks, the detection time needs to be large enough to show a high signal to noise ratio (SNR), in order to enhance the detection limit and to improve the reproducibility. So the excitation temperature obtained by the Boltzmann plot in the selected detection time interval can be considered as an average temperature only if the temporal profiles of the observed

Fig. 2. Boltzmann plot of Fe I in LIBS experiments on meteorite (delay time 830 ns, gate width 5000 ns).

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Table 1 Selected spectral lines and corresponding spectroscopic data of the investigated elements Species Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Fe I Ti I Ti I Ti I Ti I Ti I Al I Al I Ca I Mn I Mg I Mg I Mg I Si I Si I Si I Co I Co I Co I Co I Ni I Ni I Ni I Ni I Cr I

Wavelength (nm) 272.09 273.73 274.41 278.81 305.74 305.90 339.26 339.46 339.93 340.22 340.74 341.31 342.50 347.67 349.78 350.85 389.93 390.29 400.52 264.43 338.59 398.21 398.98 399.87 266.03 396.15 346.85 280.10 277.66 278.12 278.29 288.16 390.55 298.76 338.52 338.81 345.52 350.63 339.29 343.35 344.62 345.85 302.16

Aul (s− 1) 8

1.00 × 10 8.50 × 107 3.50 × 107 6.30 × 107 4.40 × 107 1.70 × 107 2.60 × 107 9.90 × 106 3.80 × 107 2.80 × 107 5.80 × 107 3.60 × 107 2.80 × 107 5.40 × 106 2.60 × 106 5.70 × 106 2.58 × 106 2.14 × 107 2.04 × 107 1.40 × 108 5.00 × 107 3.76 × 107 5.79 × 107 4.08 × 107 2.64 × 107 9.80 × 107 1.30 × 106 3.70 × 108 1.31 × 108 5.30 × 108 2.16 × 108 1.89 × 108 1.18 × 107 2.20 × 106 1.10 × 107 2.40 × 107 1.90 × 107 8.20 × 107 2.40 × 107 1.70 × 107 4.40 × 107 6.12 × 107 2.91 × 108

Eu (cm− 1)

gu

37,157 37,409 37,689 42,784 39,626 33,096 47,017 47,177 47,136 55,490 46,889 47,017 53,763 29,732 29,469 52,613 26,340 38,175 37,521 39,773 29,912 25,107 25,227 25,388 37,689 25,348 43,981 35,690 57,874 57,813 57,833 40,992 40,992 39,760 33,674 34,196 30,742 32,654 29,668 29,320 29,888 30,619 41,393

5 3 3 13 9 9 7 3 5 13 9 7 7 3 5 11 5 7 5 5 7 5 7 9 2 2 3 4 5 1 3 3 3 3 6 4 2 6 7 7 5 5 11

emission lines are exactly the same during all the acquisition time. In this connection the following criteria for the selection of emission lines have been chosen: 1) All the spectroscopic lines involving the ground state have been excluded. For high concentration elements, lines corresponding to transitions with lower energy level below 6000 cm− 1 have been excluded too, to minimize the selfabsorption effect on spectral line intensity. 2) All the transitions with spontaneous emission rate lower than 2 × 106 s− 1 have been excluded because the corresponding emission times could be comparable with the time associated to the plasma variations [23]. 3) For all the elements, just the atomic spectral lines have been investigated because deviations from Saha equation can take place as a consequence of the recombining character of the

expanding laser induced plasma [23]. This choice limits the number of detectable lines, for high concentration elements belonging to the first and the second group of the periodic table, but gives better results. 4) All the emission lines with high relative intensity (generally N3000) have been excluded, because, as a consequence of the spatial integration of the emission signals along the optical path, the corresponding population is over-estimated. As a matter of fact, close to the plasma border, when the emission of low intensity transition is under the detection limit, these lines are still detectable. They can be useful for the detection of the trace elements but they have high probability to deviate from the experimental Boltzmann plot. All the selected emission lines are reported in Table 1. It is important to underline that many lines fulfilling all these requirements have been anyway excluded because their fitting presents a considerable uncertainty as a consequence of peak overlapping or low SNR. To avoid the spectral continuum due to the mechanisms involving free electrons, and the deviations in the temporal profile of the emission lines due to the difference in the spontaneous emission coefficients (particularly effective for short delay time), a delay time of 830 ns and a gate width of 5000 ns have been selected for the spectroscopic measurements. The chosen detection times allow obtaining a good spectral resolution and do not require intensification of the ICCD. Moreover at these experimental detection times, the measured temperature is around 6000 K and the electron number density is about 1 × 1017 cm− 3 (see Table 2) for all the examined samples, so the contribution of ionized fraction of the elements, as calculated by Saha equation, is generally 2 orders of magnitude less than the neutral fraction, with the exception of few elements, as for example Ca. This observation justifies the choice of measuring directly the neutral lines by emission spectroscopy and then adopting the correction for the ionization degree of the species by Saha equation. In this way, if some deviations from the Saha relation occur, because of the recombining character of the laser induced plasma, they will not affect considerably the quantitative analysis. The following steps have been developed for the quantitative analysis: For all the selected lines the number density of atoms N0,a has been calculated by the following equation:   Iul Eu N0;a ¼ Z ðT Þ exp 4pGhmul Aul gul kT Where T is the experimental temperature obtained by the Fe I Boltzmann plot and all other terms have the usual meanings Table 2 Excitation temperature and electron number density during LIBS experiments on meteorites (delay time 830 ns, gate width 5000 ns) Sample

T ± 350 K

Ne × 1017 cm− 3

Dhofar 019 Dhofar 461 Sikhote Alin Chondrite L6

6000 6300 6250 5950

2.2 2.4 0.9 2.2

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as in Refs. [21] and [22] and their values can be found in Ref. [24]. All the spectroscopic windows have been normalized imposing the N0,a values of the Boltzmann plot lines to all the Fe I selected transitions. After scaling of investigated spectral windows, all the N0,a calculated by different emission lines for each element have been averaged. The obtained average values of the atom number density of each element has been inserted in the Saha equation [22]:     N0;i Zi ðT Þ 1 me kT 3=2 EIon Ne ¼2 exp  Z a ðT Þ 2ph2 N0;a kT Where T is the excitation temperature obtained by Fe I Boltzmann plot, Ne is the electron number density obtained by the Stark effect, and all other terms have the usual meaning [21,22]. In this way the fraction of ionized elements has been evaluated and added to N0,a to determine the total relative element number density, Nj of the species j. Finally, the composition of the species has been determined as suggested in Ref. [20]: all the determined values of Nj have been multiplied by their atomic weight and the corresponding sum has been forced to 100 in order to get the weight percentage of each element. Concerning the oxygen content, not measurable in air experiments, it has been calculated by the stoicheiometric relation in the mineral oxide of the detected metals. Regarding the trace elements detection, they have been searched in the spectral window between 250 and 600 nm, and only elements with concentration higher than tens ppm have been detected unambiguously. Anyway their quantification is not feasible by LIBS, because their distribution between the major elements can vary in the target, so that the ablated portion of the sample could not be representative of the mean distribution in the sample. Moreover for such a low concentration, a calibration curve and a double pulse technique would be required for a correct quantification. 4. Analytical results 4.1. Dofhar 019 The meteorite Dofhar 019 has been found in Oman in 2000 and it has been classified as a Martian basalt. It is composed mainly of the following oxides: SiO2 (49.2%), TiO2 (0.7%), Al2O3 (6.4%), FeO (18.4%), MnO (0.5%), CaO (7.28%), MgO (14.6%), Na2O (0.9%), K2O (0.1%), P2O5 (0.4%). Unfortunately no suitable lines for the quantitative analysis have been found for Na, K and P, as the observed transitions do not fulfill the selection criteria mentioned in the previous paragraph, or they are strongly overlapped with other emission lines. The LIBS analyses, as well as the certified composition [25] have been reported in Table 3. By the inspection of Table 3 it is possible to note that for most elements the correlation between LIBS results and certified composition is fairly good. The difference between the composition determined in this work and the reported one varies between 5 and 20% with the exception

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Table 3 Concentration of the main elements in the analyzed meteorites as obtained by LIBS and as reported in literature Element

NJ (cm− 3)

wt.% LIBS

wt.% literature

Dhofar 19 Al Ti Mg Mn Cr Ca Fe Si O

9.800 × 1016 1.700 × 1016 7.240 × 1017 1.180 × 1016 2.043 × 1016 1.610 × 1017 5.760 × 1017 1.540 × 1018 4.761 × 1018

1.45 0.45 9.53 0.35 0.58 3.63 17.60 23.66 41.67

3.40 0.37 8.80 0.38 0.40 5.20 14.70 22.62 44.20

Dhofar 461 Fe Mg Mn Ti Al Ca Si O

3.240 × 1015 2.290 × 1015 3.380 × 1013 1.900 × 1014 1.650 × 1016 1.330 × 1016 2.880 × 1016 1.016 × 1017

4.94 1.52 0.05 0.25 14.56 12.16 22.10 44.41

3.17 2.40 0.05 0.13 15.50 11.90 21.00 45.00

Sikhote Alin Fe Ni Co

4.244 × 1017 2.469 × 1016 2.515 × 1015

93.69 5.73 0.59

93.00 5.90 0.42

Chondrite L6 Fe Ca Ni Mn Mg Si Ti O

7.030 × 1017 1.570 × 1017 1.700 × 1016 1.176 × 1016 1.777 × 1018 1.786 × 1018 1.400 × 1015 6.240 × 1018

16.33 2.62 0.41 0.27 17.96 20.86 0.03 41.52

– – – – – – – –

The relative experimental error has been estimated b10%.

of Al and Ca those are underestimated probably as a consequence of their high ionization degree. Anyway it is important to underline that some discrepancy should be attended because LIBS technique samples a portion of surface with a diameter of 100 μm (about 50–100 ng), but the composition reported in Ref. [25] is obtained analyzing 1 g of sample and so it is not affected by the element differential distribution in the matrix. Moreover in the LIBS methodology here applied, not quantified elements such as Na, K and P, have not been added in the composition normalization. In the investigated spectral window, the following not quantified trace elements have been detected: Sc (286.38 nm), W (290.18, 391.85 nm), Zr (298.54 nm), Zn (302.58 nm), Cu (282.42 nm), Na (297.50, 589.00, 589.59 nm), Ni (300.24, 300.36 nm) and Sr (687.84, 689.26 nm). 4.2. Dofhar 461 The meteorite Dofhar 461 has been found in Oman in 2001 and it has been classified as lunar, anorthositic crystalline melt

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Table 4 Element–silicon ratio of basalt like meteorite determined by LIBS

Dhofar 019 Dhofar 461 Chondrite

Fe/Si

Ca/Si

Mg/Si

Mn/Si

0.74 (0.65) 0.22 (0.16) 0.78 (n.q.)

0.15 (0.23) 0.66 (0.57) 0.125 (0.11)

0.40 (0.39) 0.70 (1.00) 0.86 (0.86)

0.015 (0.017) 0.023 (0.022) 0.013 (0.016)

The certified values are reported in brackets (n.q. means not quantified).

breccia. The lunar meteorite is composed of the same set of oxides found in Dofhar 019. Anyway the content of Fe and Mg in lunar breccia is appreciably less than in Dofhar 019, while the quantity of Ca and Al is considerably higher. The LIBS analysis and the literature composition [26] are reported in Table 3. Slight improvements in the percentage of discrepancy with respect to Dhofar 019 are observed. Other trace elements detected are: Cr (299.66 nm), Sc (296.96 nm), Ni (301.75 nm), Co (338.38 nm), Na (589.00, 589.59 nm) and Sr (687.84, 689.26 nm). 4.3. Sikhote Alin This is a piece of the meteorite fallen in eastern Siberia in 1947. The meteorite type is iron, coarse octahedrite class II B. This meteorite can be considered as a ternary alloy from the analytical point of view and it is made mainly of iron with a small percentage of Co and Ni, together with other trace elements [27]. As a consequence of the high percentage of iron, only transitions between high excited levels have been considered for the determination of the excitation temperature and that is why the corresponding Boltzmann plot shown in Fig. 2 misses the energy range below 35,000 cm− 1. LIBS results shown in Table 3 are in good agreement with values reported in literature, as long as Sikote Alin is made of few elements, which in turn gives a minimal normalization error. Minor trace elements have been found too: Ge (265.16, 422.66 nm), Ir (263.97, 266.20, 293.93 nm), S (264.17, 264.76, 293.11 nm), Ga (265.98, 270.05 nm).

4.5. Discussion The analysis carried on by LIBS shows promising results for all the different meteorite samples. One of the most important sources of error is the determination of the oxygen content, calculated by the stoicheiometric relation in the mineral oxide of the detected metals. As an example, in Dhofar 019, if the normalization of the concentration summation is done only for the measured element, excluding the oxygen content, the discrepancy between LIBS results and literature data is considerably smaller (3–10% instead of 5–20%). If the LIBS experiment is carried on in vacuum or in an atmosphere without oxygen, the oxygen content in the sample could be measured by the O I triplet around 777 nm, improving the analytical results of LIBS. The analytical method proposed in this work, is the simplest one and, as a main advantage, allows determining the composition with an acceptable accuracy and trace element identification, in a single experiment without any need of standard measurements and no constrain on sample positioning. Fig. 3 reports the correlation between measured composition and literature ones. By the inspection of Fig. 3 it is observable that this correlation is linear with a slope equal to one, demonstrating the trueness of the proposed data processing method. In atmosphere crossing meteors studies, the element/silicon ratio is really important because it is not always possible to determine all the components. For this reason the most useful element/silicon ratios Fe/Si, Ca/Si, Mg/Si and Mn/Si are listed in Table 4. The cases of Dofhar 019, Dofhar 461 and Chondrite L6 can be examined to prove the possibility to discern different basaltlike extraterrestrial samples by specific element–silicon ratio values. The Lunar breccia can be easily distinguished from the Martian basalt and the chondrite by the Fe/Si ratio, while the Chondrite L6 and Dhofar 019 have a similar ratio of Fe/Si and Ca/Si, but they differ for what concerns the Mg/Si values. This criterion, together with trace elements, when they are detectable, can be applied to the emission spectroscopy of falling meteors selecting adequate spectroscopic lines.

4.4. Chondrite L6 This class of meteorite is the most common. It is basalt-like characterized by the inclusion of fused SiO2. This meteorite has been found in Sahara desert in 2000. The analysis has been performed in bulk by a previous sectioning. The LIBS analysis has been reported in Table 3 and shows a similar composition to Dhofar 19. We did not find a specific elemental analysis of L6 in literature and so an EDS analysis has been performed to quantify some elements as Mg, Si, Mn and Ca to compare their ratios to silicon, as is generally done for the classification of this kind of meteorites. The comparison of the ratios obtained by LIBS and certified values is shown in Table 4 and is in agreement with the range of values reported in literature for chondrites [27]. The trace elements detected in such type are: Cr (289.92, 272.65, 299.66, 301.49 nm), Sc (282.83, 267.67, 286.38, 296.96 nm), W (267.31, 276.55, 286.64, 294.81, 391.85 nm etc.), Co (345.35, 350.22 nm etc.), Zn (302.58 nm), Zr (347.12 nm), V (390.98 nm).

Fig. 3. Correlation plot between the element composition determined by LIBS and that reported in literature (certified).

A. De Giacomo et al. / Spectrochimica Acta Part B 62 (2007) 1606–1611

5. Conclusion In this paper, the feasibility of a simple and fast LIBS application for meteorite analysis has been reported. Four different meteorite samples have been analyzed Dofhar 019 (Mars meteorite), Dofhar 461 (lunar meteorite), Chondrite L6 (stony meteorite) and Sikhote Alin (irony meteorite). It is important to underline that the experiments on Dofhar 019 and 461 also represent a LIBS measurement of the real surface of Mars and of Moon respectively and show the possibility of recognizing different kinds of extraterrestrial basalts. The analysis has been performed by specific spectral lines in the LTE assumption, without any calibration measurements. The results of this simple approach are really encouraging and could be applied for a fast classification of meteorites, while the selection criteria of the investigated spectral lines can be useful for the optical emission diagnostics of meteors crossing the atmosphere. Acknowledgment This research is partially supported by M.I.U.R. under the project FIRB RBAUO1H8FW_003 “Dinamica microscopica della reattività chimica”. References [1] A.W. Miziolek, V. Palleschi, I. Schechter (Eds.), Laser Induced Breakdown Spectroscopy, Cambridge University Press, 2006. [2] S. Palanco, J.M. Baena, J.J. Laserna, Open-path laser-induced plasma spectrometry for remote analytical measurements on solid surface, Spectrochim. Acta Part B 57 (2002) 591–599. [3] D.A. Cremers, The analysis of metals at a distance using laser-induced breakdown spectroscopy, Appl. Spectrosc. 41 (1987) 572–579. [4] A.K. Knight, N.L. Scherbarth, D.A. Cremers, M.J. Ferris, Characterization of laser induced breakdown spectroscopy (LIBS) for application to space exploration, Appl. Spectrosc. 54 (3) (2000) 331–340. [5] F. Colao, R. Fantoni, V. Lazic, A. Paolini, F. Fabbri, G.G. Ori, L. Marinangeli, A. Baliva, Investigation of LIBS feasibility for in situ planetary exploration: an analysis on Martian rock analogues, Planet. Space Sci. 52 (2004) 117–123. [6] L. Radziemski, D.A. Cremers, K. Benelli, C. Khoo, R.D. Harris, Use of the vacuum ultraviolet spectral region for laser-induced breakdown spectroscopy-based Martian geology and exploration, Spectrochim. Acta Part B 60 (2005) 237–248. [7] B. Salle, D.A. Cremers, S. Maurice, R. Wiens, P. Fichet, Evaluation of a compact spectrograph for in-situ and stand-off Laser-Induced Breakdown Spectroscopy analyses of geological samples on Mars missions, Spectrochim. Acta Part B 60 (2005) 805–815. [8] S.K. Sharma, P.G. Lucey, M. Ghosh, H.W. Hubble, K.A. Horton, Stand-off Raman spectroscopic detection of minerals on planetary surfaces, Spectrochim. Acta Part B 59 (2003) 2391–2407.

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