Understanding Sulfate Stability on Mars: A Thermo-Raman Spectroscopy Study

This work examines the impact of high temperatures from celestial shock events on the stability of sulfates found on Mars (gypsum) and those expected to be present (syngenite and görgeyite). Raman spectroscopy, a cutting-edge technique in space exploration, was used to track their stability. Specifically, a Renishaw inVia^™ micro-Raman confocal spectrometer was coupled with an external Linkam THMS600/HF600 temperature-controlled stage to monitor the sample temperature while measuring the main Raman band positions of the sulfates and those of water molecules in these salts across temperatures ranging from 313 to 673 K. Results showed a shift toward lower wavenumbers with increasing temperature for all compounds, up to each compound's inflection temperature, where phase transformations occurred. The linear trends identified in this study provide valuable insights for interpreting data from space missions equipped with Raman instruments and understanding Earth-based measurements. These trends enable the estimation of Raman band wavenumbers at specific temperatures, as well as the determination of the temperature at which a given spectrum was acquired. Additionally, the research demonstrated that the three heated salts fully rehydrated after at least 1 month under standard environmental conditions (23°C, 1 atm, and ∼80% relative humidity). This finding on reversibility is crucial for interpreting time-dependent results, such as characterizing meteorites that contain evaporite salts.