Optimizing Raman Spectroscopy for detecting organics in nontronite-rich Martian analogues.

Smectites, the most abundant phyllosilicates identified on Mars, are key indicators of past aqueous environments and represent primary mineralogical targets in the search for extinct life. Raman spectroscopy is a powerful, nondestructive technique that is integrated into current and future missions to enable the in-situ detection of clay mineral and organic compounds. However, an accurate interpretation of rover-derived datasets requires complementary laboratory analyses on Earth, integrating multiple techniques to quantitatively assess the significance of the Martian observations. This study uses Raman spectroscopy in combination with elemental microanalysis, X-ray diffraction and infrared spectroscopy to evaluate the optimal conditions for detecting organics in nontronite, an iron-rich smectite associated with warm aqueous conditions. Our results show that the green laser provides the most reliable Raman response for discerning both mineral and organic bands, and that sample texture significantly influences Raman performance. ATR and Raman spectra revealed strong interactions between organics and the nontronite interlayer, involving protonated amines and carboxylates (glycine) and amino groups (urea). Additionally, the presence of intercalated organic molecules appears to reduce the internal stress of the tetrahedral SiO₄ units upon compaction. Raman and ATR spectroscopy detected organics at concentrations of ~1–2 wt%, which exceeds the organic bulk levels (~0.02–0.1 wt%) measured in Martian mudstones by the SAM instrument onboard Curiosity. These findings underscore the importance of a multi-approach in situ detection strategy, emphasizing the necessity of sample-return missions and further research into the role of smectites in preserving organic matter under Martian conditions.