Challenges Analyzing Gypsum on Mars by Raman Spectroscopy

Design and development of a stand-off Raman brassboard (SDU-RRS) for the spectroscopic study of planetary materials

Raman spectroscopy has emerged as a crucial mineral analysis technique in planetary surface exploration missions. Nonetheless, the inherently low Raman scattering efficiency of planetary silicate materials makes it challenging to extract enough Raman information. Theoretical and experimental studies of the remote Raman scattering properties of planetary materials are also urgent requirements for future lunar and planetary explorations. Here, Shandong University Remote Raman Spectrometer (SDU-RRS) was developed to demonstrate the feasibility of lunar remote Raman technology and conduct preliminary research on remote Raman scattering properties. SDU-RRS utilizes a pulsed 532 nm laser, a non-focal Cassegrain telescope, a volume phase holographic grating, an intensified charge-coupled device, and the time-gating technique to detect weak-signal silicate minerals. The spectral resolution obtained with atomic emission lamps was <4.91 cm-1, and the wavelength accuracy was <1 cm-1, across the spectral range of 241-2430 cm-1. SDU-RRS can detect natural augite within a feldspar-olivine-augite matrix at a concentration of 20 % at ∼1 m under ambient lighting conditions. A series of experiments were conducted to evaluate the influence of measurement conditions and physical matrix effects on acquired Raman signals, either qualitatively or quantitatively, on geological materials. The study indicates that the transmission of Raman-scattered light conforms to Lambert's cosine law, and a linear correlation exists between Raman intensity and laser power. The study also evaluated the impact of grain size, surface roughness, porosity, and shadow-hiding effects. Reducing grain size decreases Raman intensity and broadens Raman spectra. These characteristics are essential for achieving definitive mineralogical information from granular materials by remote Raman spectroscopy in lunar and planetary explorations.

Quantitative analysis of binary and ternary organo-mineral solid dispersions by Raman spectroscopy for robotic planetary exploration missions on Mars

The Mars 2020 and ExoMars 2022, rover-based missions are specifically dedicated to the search for evidence of life and will both utilise Raman spectrometers on the surface of Mars. Raman spectroscopy is indeed a valuable analytical technique for planetary exploration that enables in situ characterisation of rocks and soils collected directly from the surface or retrieved as cores and subsequently crushed when extracted from the subsurface with a drill. On Mars, the miniaturised spectrometers will interrogate ancient geological deposits, in order to try and identify hydrated or aqueously altered minerals and organic matter to assess the habitability of Mars. While the identification of relevant hydrous minerals and organic components is the primary analytical objective of the missions, quantifying their abundances would be of particular significance for interpreting past geological conditions (e.g. formation or alteration processes) and for ascertaining the putative presence of biosignatures. Therefore, we have developed quantitative models that enable the quantification of both mineral proportions from crushed mixtures of geological components and spiked mixtures containing organic analytes dispersed in mineral matrices. Based on data normalisation with appropriate standards (internal and external), we demonstrate that robust quantitative models can be (1) applied for solid dispersions of various complexities relevant to planetary exploration; and (2) applied to different Raman set-ups, including an instrument representative of the ExoMars Raman Laser Spectrometer. With important Raman-active minerals (calcite, gypsum, baryte, quartz), we demonstrate that using a correction factor F_ϕ2/ϕ1, based on the ratio of apparent Raman scattering coefficients, the relative proportion of minerals in binary mixtures can be accurately determined. Regarding the organics, evaluated in clay-rich sediments (Fe-smectite) and crushed rocks of coarse-grained fraction (>100μm), we establish calibration curves in the concentration range 2-20 wt% for non-resonant compounds (L-cysteine, phthalic acid, adenine) and even lower (<1 wt%) for pre-resonant anthracene. Despite large levels of heterogeneity, the Raman analyses of these solid dispersions verify that quantitative Raman analyses can be performed in the context of robotic exploration studies.

Detecting Kerogen as a Biosignature Using Colocated UV Time-Gated Raman and Fluorescence Spectroscopy

The Mars 2020 mission will analyze samples in situ and identify any that could have preserved biosignatures in ancient habitable environments for later return to Earth. Highest priority targeted samples include aqueously formed sedimentary lithologies. On Earth, such lithologies can contain fossil biosignatures as aromatic carbon (kerogen). In this study, we analyzed nonextracted kerogen in a diverse suite of natural, complex samples using colocated UV excitation (266 nm) time-gated (UV-TG) Raman and laser-induced fluorescence spectroscopies. We interrogated kerogen and its host matrix in samples to (1) explore the capabilities of UV-TG Raman and fluorescence spectroscopies for detecting kerogen in high-priority targets in the search for possible biosignatures on Mars; (2) assess the effectiveness of time gating and UV laser wavelength in reducing fluorescence in Raman spectra; and (3) identify sample-specific issues that could challenge rover-based identifications of kerogen using UV-TG Raman spectroscopy. We found that ungated UV Raman spectroscopy is suited to identify diagnostic kerogen Raman bands without interfering fluorescence and that UV fluorescence spectroscopy is suited to identify kerogen. These results highlight the value of combining colocated Raman and fluorescence spectroscopies, similar to those obtainable by SHERLOC on Mars 2020, to strengthen the confidence of kerogen detection as a potential biosignature in complex natural samples. Key Words: Raman spectroscopy-Laser-induced fluorescence spectroscopy-Mars Sample Return-Mars 2020 mission-Kerogen-Biosignatures. Astrobiology 18, 431-453.

Oriented attachment by enantioselective facet recognition in millimeter-sized gypsum crystals

Optical crystallography of precipitated hot supersaturated solutions of calcium sulphate shows oriented attachment of gypsum crystals at homochiral facets.