2.1

Geology & mineralogy

The current knowledge of Mars geology and mineralogy comes from two main sources, the SNC meteorites, the different remote sensing and rover-surface missions. Mars Global Surveyor, Mars Odyssey and Mars Express together with Viking, Pathfinder and Mars Exploitation Rovers Spirit and Opportunity paved the way with data from which the knowledge on mineralogy, chemical elemental composition and geological processes has been developed.

Evidence of volcanic, hydrothermal, fluvial and glacial related geological processes have been reported. The existence of water has also been proved and the scientific community is waiting for more precise details from the MARSIS-radar on-board the Mars Express orbiter.

The most important argument supporting the water activity on Mars surface comes from the Mars Exploration Rover Opportunity’s Moessbauer spectrometer. It showed the presence of an iron-bearing mineral called Jarosite in the set of rocks dubbed “El Capitan” in Mars’ Meridiani Planum. “El Capitan” is located within the rock outcrop that lines the inner edge of the small crater where Opportunity landed. The occurrence of this mineral has been interpreted as:

The Mini-Thermal Emission Spectrometer (TES) instruments on Spirit and Opportunity have studied the mineralogy and thermo-physical properties at Gusev Crater and the Meridiani Plains. At Gusev undisturbed soil spectra showed features interpreted to be due to minor carbonates and bound water. At Meridiani, the Mini-TES has confirmed the presence of coarse crystalline hematite and olivine basalt sands predicted from orbital TES spectroscopy.

In general rocks are olivine-rich basalts with varying degrees of dust and other coatings. Pyroxene, plagioclase and oxides (mainly hematite) are also present (in agreement with the data from SNC meteorites). And some sulphates (Ca, Mg, and Fe) and carbonates have been also identified.

Nevertheless, the total amount of mineral phases identified (about a dozen) compared with the phases identified on Earth, stresses the needs of developing new rovers with more instrumental capacity and new types of instruments with more sensitivity and analytical capacities.

2.2

Organics & search for life

Viking started the search for organics on Mars. Results were puzzling due to three aspects. First, was the total absence of organics as measured by the Gas Chromatography/Mass Spectrometry (GCMS) [1 and 2]. The second unexpected result was the rapid release of O₂ when soil samples were exposed to water vapour in the Gas Exchange Experiment (Gex) [3]. The third unexpected result was that organic material in the Labelled Release Experiment was consumed as would have been expected if life was present [4] in apparent contradiction with the results from the GCMS. Currently, the most widely held explanation for the reactivity of the Martian soil is the presence of one or more inorganic oxidants [5, 6, 7]. The question of the search for organics on Mars is still open, with all its implications for astrobiology.

3.1

Raman

Raman spectroscopy is well recognised as a powerful tool for the chemical and structural identification of materials in the solid, liquid or gas state. Its analytical capability, both macro- and micro-without the need to perturb a sample, have made this technique unique for many applications where the materials are scarce or very valuable and rare. In Fig. 3, the Raman Spectrum of Ferricopiapite from Rio Tinto is shown. Fig. 4 presents the Raman spectrum of Jarosite from the world-type locality Barranco del Jaroso. These figures are illustrating the vibrational features at low, medium and high wavenumber spectral ranges [9].

Fig. 3.

Raman spectrum in macro-mode of ferricopiapite mineral from Rio Tinto [9].

Fig. 4.

Raman spectrum in macro-mode of Jarosite from the Jaroso Ravine [9].

Raman spectroscopy suffers from some limitations, which are mainly those derived from the fluorescence emission induced in the samples by the laser excitation.

The choice of the Raman excitation wavelength is a very important issue to consider. Excitations with wavelength over 700 nm have in general a very limited detection capability for v(CH) and v(OH) bands, which in their turn are essential to characterise hydrated phases and biomarkers. This fact is illustrated in Fig. 3. Raman spectroscopy can easily distinguish between OH from hydroxyl groups and OH from hydration water [Fig. 3].

On the other hand, excitation at short wavelengths in some cases induces several fluorescence processes on the samples. The large experience on Raman spectroscopy of minerals, polymers and biomarkers of the scientific team involved in this project proves that most of the fluorescence effects are present in the laser excitation range from 532 to 785 nm.

Fluorescence is mostly avoided using Fourier Transform (FT)-Raman spectrometry with infrared (IR) (typically 1064 nm) excitation. Nevertheless, FT-Raman suffers from the above-mentioned limitations in identifying OH vibrations and it has important technical difficulties if combined with LIBS. Another possibility is to use gated mode with acquisition times of the order of few nanoseconds when excited with lower wavelengths, in particular with 532 nm pulsed laser.

Previous experimental research performed in the frame of the Mars Surveyer Rover (MSR) payload by Alian Wang [10] showed that the dark and red compounds emit a quite weak Raman scattering when illuminated with red to IR excitations as compared with green excitation (in fact this is a classical absorption effect). On the opposite, the light compounds emit in general more fluorescence. For these reasons, laser excitation that avoids absorption limitations as the 532 nm and avoids fluorescence as the gated mode is considered as the ideal.

This approach has however not be considered for the Mars mission, as it presents high technical risks particularly related to the lack of maturity of intensified detection systems for space applications.

Consequently for Mars application the choice of the Raman excitation wavelength should be a compromise between the minimum detection capabilities necessary to analyse important vibrations as the v(CH) for organics and the negative possible fluorescence effects.

3.2

LIBS

LIBS is based on the focusing of a high-power pulsed laser beam (>1 GW/cm²) onto a sample surface leading to the creation of a plasma composed of excited species which emit light.

Collection of the plasma light, followed by spectral dispersion and detection, permits identification of the elements present in the sample using their characteristic spectral lines and allows quantitative analysis. For space exploration, this method possesses several advantages including stand-off analysis capability, no sample preparation required, rapid analysis (few minutes), simultaneous multi-elemental detection (major, minor and trace elements), and the ability to measure the composition of weathered layers and the underlying bulk rock composition through depth profiling (repeated ablation).

Fig. 5 [11] presents the LIBS spectrum of a Basalt standard. It is worth noticing that the density of lines is higher at low wavelengths than at high wavelengths. To achieve such a spectrum typical resolution needs to be smaller than 0.1 nm in the ultra-violet (UV) and could be between 0.1 nm and 0.2 nm in the near IR.

Fig. 5.

Echelle spectrum of a basalt standard in LIBS at a distance of 3 m. Note that trace elements, such as Li, Ba, and Sr, are visible.

4.1

Spectrometer

The design is based on a concept of a cross dispersion spectrometer. As a result of the cross dispersion there are six spectra (see Fig. 7), separated in cross dispersion direction, imaged on a 2D detector array. The spectrometer covers continuously a range varying from the UV to the NIR. Its spectral resolution is varying from less than 0.1 nm in the UV to 0.2 nm in the NIR.

Fig. 7.

Dispersed lines on detector.

4.2

Optical head

The optical head consists of two optical systems: a condenser part for sample illumination and an imaging system (relay imager optics). The light path goes from two fibres (one for Raman and one for LIBS) through the condenser lens to the sample and from the sample through the relay imager to another fibre, which feeds the spectrometer.

The spot diameters for the Raman and the LIBS experiments have to be independently optimised, since the spatial resolution requirements of the two experiments differ: