Laser‐induced breakdown spectroscopy (LIBS) and Raman spectroscopy are powerful key techniques for the geoanalytical exploration of extraterrestrial bodies, especially when combined. Their data are complementary, which motivates the question of how it can be best combined to maximize the scientific output. For this study, LIBS and Raman data from pure sulfates and their mixtures as well as from other Mars‐relevant salts such as carbonates, chlorides, perchlorates, and sulfates in a basaltic matrix were measured and investigated. All measurements were performed with miniaturized setups, and LIBS experiments were done in simulated Martian atmospheric conditions. Multivariate data analysis (MVA) techniques such as principal component analysis (PCA) and partial least squares discriminant analysis (PLS‐DA) were employed to evaluate the potential for identifying the sulfates or the salts in the basalt with LIBS and Raman data alone and with their low‐level fused data. We found that low‐level data fusion, that is, combination of LIBS and Raman spectra at the data level, can improve the identification of sulfates and salts. Although the approach of low‐level data fusion aims to use all relevant information from both techniques, we observed that not all benefits from the single models are completely represented by the fused model. The computation and performance of appropriate MVA models are affected by the weighting of the single spectra in the combined one, by the dimensionality of the MVA models, and in case of PLS‐DA, by the given input data. From this study, we conclude that generally, data fusion of LIBS and Raman is an advantage for the identification of unknown samples but that more levels, especially, high‐level data fusion (decision level), should be further investigated.

It has recently become possible to grow GaAs in the wurtzite crystal phase. This ability allows interesting tests of band-structure theory. Wurtzite GaAs has two closely spaced direct conduction bands as well as three nondegenerate valence bands. The energies of the band edges are not well known, in particular not as a function of temperature. In order to improve the accuracy we have studied the temperature dependence of the conduction band minimum as well as of the second valence band maximum using resonant Raman scattering (of up to 3LO Raman lines). We find that the temperature dependence of the bandgap in wurtzite GaAs is very similar to that in zinc blende GaAs. Our results show that they have the same band gaps not only at 7 K but also at room temperature to within 5 meV. This is in some discrepancy with previous work. We find that the energy difference between the first two Γ and Γ valence bands is constant, around 100 meV, over the investigated temperature range, 7 K to 300 K. Due to a fortuitous spacing of the energy bands we find a very unexpected and strong quadruple resonance in the resonant Raman scattering.

Chlorine and fluorine play an important role in the geological history of Mars due to their high concentration in Martian magmas and their influence on the generation and evolution of Martian basalts. Chlorine-bearing salts could also facilitate the formation of eutectic brines that could be important for the fluvial history of Mars. The LIBS instruments of ChemCam and SuperCam can detect emission lines of Cl and F, but the intensity of these emission lines is comparatively low, making it difficult to quantify them correctly. A promising alternative is the quantification by molecular emission of diatomic molecules like CaCl and CaF, which can be observed as intense molecular bands in LIBS spectra if Ca is also present. However, the nonlinear dependence of the band intensity on the concentrations of both elements needs to be considered. In this study, we expand upon our previous analysis of molecular bands by investigating samples which produce CaCl bands, CaF bands, or both. We find that the highest CaCl band intensities are found in samples containing more Ca than Cl, while the strongest CaF bands are found in samples with roughly equal concentrations of Ca and F. Both observations can Preprint submitted to IcarusJune 20, 2019be described by the model that we present here. We also find that the CaCl band is significantly stronger for a sample containing CaCl 2 than it is for a sample containing the same concentrations of Ca and Cl in separate bonds.The opposite is true for the CaF band, which is significantly weaker for the sample containing CaF 2 bonds than it is for the sample that does not contain CaF 2 bonds. These matrix effects are partially attributed to fragmentation during the ablation process and differences in the dissociation energies. Furthermore, we observe that CaF formation is not affected by competing CaCl formation, while CaCl is strongly affected by competing CaF formation. All measurements are done in simulated Martian atmospheric conditions in order to assist the analysis of Martian LIBS data.

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Laser‐induced breakdown spectroscopy (LIBS) and Raman spectroscopy have a high potential for in situ geochemical and mineralogical analyses for planetary exploration, in particular in combination. The SuperCam instrument onboard NASA's Mars 2020 rover will use both techniques together on another planet for the first time. The high‐power pulsed LIBS laser ablates material, and a small luminous plasma is produced for spectral analysis. The laser–matter interaction and the plasma shock wave can alter the sample surface, and new molecules can be produced, which deposit close to the LIBS ablation crater. Subsequent Raman analysis might then not sample the original structure. Here, we investigated pure metals (Fe, Ni, and Ti), the iron‐containing oxides hematite and ilmenite, and a fragment of the Gibeon meteorite in terrestrial ambient conditions, in simulated Martian atmospheric conditions, and in vacuum. LIBS ablation craters and their close proximity were studied with subsequent Raman analysis. Our analysis shows that Earth and Mars atmosphere provide enough oxygen in the LIBS plasma to produce oxides with metals from the sample. These can then be observed in the Raman data. Also, carbon was seen in some of the Raman data from the sample after the LIBS measurement. On hematite, a reduction of the mineral, that is, the presence of magnetite, was observed inside the LIBS crater for terrestrial and Martian atmospheric conditions and in vacuum. For the analysis and correct interpretation of Raman data it is important to be aware that alteration could have occurred by a preceding LIBS measurement. Raman analysis of several positions close to the LIBS ablation crater can help to infer the original and a possibly altered structure.