Spectroscopic study of terrestrial analogues to support rover missions to Mars - A Raman-centred review.

The 2020s could be called, with little doubt, the "Mars decade". No other period in space exploration history has experienced such interest in placing orbiters, rovers and landers on the Red Planet. In 2021 alone, the Emirates' first Mars Mission (the Hope orbiter), the Chinese Tianwen-1 mission (orbiter, lander and rover), and NASA's Mars 2020 Perseverance rover reached Mars. The ExoMars mission Rosalind Franklin rover is scheduled for launch in 2022. Beyond that, several other missions are proposed or under development. Among these, MMX to Phobos and the very important Mars Sample Return can be cited. One of the key mission objectives of the Mars 2020 and ExoMars 2022 missions is the detection of traces of potential past or present life. This detection relies to a great extent on the analytical results provided by complementary spectroscopic techniques. The development of these novel instruments has been carried out in step with the analytical study of terrestrial analogue sites and materials, which serve to test the scientific capabilities of spectroscopic prototypes while providing crucial information to better understand the geological processes that could have occurred on Mars. Being directly involved in the development of three of the first Raman spectrometers to be validated for space exploration missions (Mars 2020/SuperCam, ExoMars/RLS and RAX/MMX), the present review summarizes some of the most relevant spectroscopy-based analyses of terrestrial analogues carried out over the past two decades. Therefore, the present work describes the analytical results gathered from the study of some of the most distinctive terrestrial analogues of Martian geological contexts, as well as the lessons learned mainly from ExoMars mission simulations conducted at representative analogue sites. Learning from the experience gained in the described studies, a general overview of the scientific outcome expected from the spectroscopic system developed for current and forthcoming planetary missions is provided.

ExoFiT trial at the Atacama Desert (Chile): Raman detection of biomarkers by representative prototypes of the ExoMars/Raman Laser Spectrometer.

In this work, the analytical research performed by the Raman Laser Spectrometer (RLS) team during the ExoFiT trial is presented. During this test, an emulator of the Rosalind Franklin rover was remotely operated at the Atacama Desert in a Mars-like sequence of scientific operations that ended with the collection and the analysis of two drilled cores. The in-situ Raman characterization of the samples was performed through a portable technology demonstrator of RLS (RAD1 system). The results were later complemented in the laboratory using a bench top RLS operation simulator and a X-Ray diffractometer (XRD). By simulating the operational and analytical constraints of the ExoMars mission, the two RLS representative instruments effectively disclosed the mineralogical composition of the drilled cores (k-feldspar, plagioclase, quartz, muscovite and rutile as main components), reaching the detection of minor phases (e.g., additional phyllosilicate and calcite) whose concentration was below the detection limit of XRD. Furthermore, Raman systems detected many organic functional groups (-C≡N, -NH2 and C-(NO2)), suggesting the presence of nitrogen-fixing microorganisms in the samples. The Raman detection of organic material in the subsurface of a Martian analogue site presenting representative environmental conditions (high UV radiation, extreme aridity), supports the idea that the RLS could play a key role in the fulfilment of the ExoMars main mission objective: to search for signs of life on Mars.

ExoMars Raman Laser Spectrometer: A Tool to Semiquantify the Serpentinization Degree of Olivine-Rich Rocks on Mars.

We evaluated the effectiveness of the ExoMars Raman laser spectrometer (RLS) to determine the degree of serpentinization of olivine-rich units on Mars. We selected terrestrial analogs of martian ultramafic rocks from the Leka Ophiolite Complex (LOC) and analyzed them with both laboratory and flight-like analytical instruments. We first studied the mineralogical composition of the samples (mostly olivine and serpentine) with state-of-the-art diffractometric (X-ray diffractometry [XRD]) and spectroscopic (Raman, near-infrared spectroscopy [NIR]) laboratory systems. We compared these results with those obtained using our RLS ExoMars Simulator. Our work shows that the RLS ExoMars Simulator successfully identified all major phases. Moreover, when emulating the automatic operating mode of the flight instrument, the RLS ExoMars Simulator also detected several minor compounds (pyroxene and brucite), some of which were not observed by NIR and XRD (e.g., calcite). Thereafter, we produced RLS-dedicated calibration curves (R2 between 0.9993 and 0.9995 with an uncertainty between ±3.0% and ±5.2% with a confidence interval of 95%) to estimate the relative content of olivine and serpentine in the samples. Our results show that RLS can be very effective in identifying serpentine, a scientific target of primary importance for the potential detection of biosignatures on Mars-the main objective of the ExoMars rover mission.

ExoMars Raman Laser Spectrometer (RLS): development of chemometric tools to classify ultramafic igneous rocks on Mars.

This work aims to evaluate whether the multi-point analysis the ExoMars Raman Laser Spectrometer (RLS) will perform on powdered samples could serve to classify ultramafic rocks on Mars. To do so, the RLS ExoMars Simulator was used to study terrestrial analogues of Martian peridotites and pyroxenites by applying the operational constraints of the Raman spectrometer onboard the Rosalind Franklin rover. Besides qualitative analysis, RLS-dedicated calibration curves have been built to estimate the relative content of olivine and pyroxenes in the samples. These semi-quantitative results, combined with a rough estimate of the concentration ratio between clino- and ortho-pyroxene mineral phases, were used to classify the terrestrial analogues. XRD data were finally employed as reference to validate Raman results. As this preliminary work suggests, ultramafic rocks on Mars could be effectively classified through the chemometric analysis of RLS data sets. After optimization, the proposed chemometric tools could be applied to the study of the volcanic geological areas detected at the ExoMars landing site (Oxia Planum), whose mineralogical composition and geological evolution have not been fully understood.

Spectroscopic study of olivine-bearing rocks and its relevance to the ExoMars rover mission.

We present the compositional analysis of three terrestrial analogues of Martian olivine-bearing rocks derived from both laboratory and flight-derived analytical instruments. In the first step, state-of-the-art spectroscopic (XRF, NIR and Raman) and diffractometric (XRD) laboratory systems were complementary used. Besides providing a detailed mineralogical and geochemical characterization of the samples, results comparison shed light on the advantages ensured by the combined use of Raman and NIR techniques, being these the spectroscopic instruments that will soon deploy (2021) on Mars as part of the ExoMars/ESA rover payload. In order to extrapolate valuable indicators of the mineralogical data that could derive from the ExoMars/Raman Laser Spectrometer (RLS), laboratory results were then compared with the molecular data gathered through the RLS ExoMars Simulator. Beside correctly identifying all major phases (feldspar, pyroxene and olivine), the RLS ExoMars Simulator confirmed the presence of additional minor compounds (i.e. hematite and apatite) that were not detected by complementary techniques. Furthermore, concerning the in-depth study of olivine grains, the RLS ExoMars simulator was able to effectively detect the shifting of the characteristic double peak around 820 and 850 cm-1, from which the FeMg content of the analyzed crystals can be extrapolated. Considering that olivine is one of the main mineral phases of the ExoMars landing site (Oxia Planum), this study suggests that the ExoMars/RLS system has the potential to provide detailed information about the elemental composition of olivine on Mars.