RAMAN SPECTRA OF HYDRATED MG- AND CA-SULFATES AND FIELD TESTING THE MARS MICROBEAM RAMAN SPECTROMETER (MMRS)

The Mars Exploration Rovers, Spirit and Opportunity, have identified Mg-, Ca-, and Fe-sulfates, at Gusev Crater and Meridiani Planum. In-situ characterization of hydration states of sulfates is very important in future surface exploration of Mars. Raman spectroscopy is powerful in its capability to characterize structural and compositional features of rock-forming minerals, in natural settings and without sample preparation, and has great potential for the analysis of rocks and soils on the surface of other planets, especially Mars, and on the Moon and asteroids. In addition, this method is extremely sensitive to variations in hydration states of sulfates, carbonates, nitrates, and phosphates. It provides not only the distinct vibrational peaks of structural water and hydroxyl that are characteristic of their crystallographic sites and the bonded neighboring cations in the 3000-4000 wavenumber (1/cm) spectral range, but the spectra also reflect the overall structural changes induced by the variation of hydration state, expressed by systematic changes in fundamental vibrational modes of major covalent anionic groups, such as SO4. The Raman peak from the symmetric stretching vibrational mode monotonically shifts up from 984/cm to 1048/cm when the hydrated Mg-sulfates lose structural water and change progressively from epsomite (MgSO4.7H2O) to kieserite (MgSO4.H2O). Distinct Raman spectral patterns and major peak positions are observed for each of seven different hydration states of Mg_sulfates. A similar trend is observed in the Raman spectra of hydrated calcium sulfate minerals gypsum, bassanite and anhydrite. We have been developing a Mars Microbeam Raman Spectrometer (MMRS) with support through NASA programs, the Jet Propulsion Lab, and Washington University. The MMRS has reached a general Technical Readiness Level (TRL) 6. A field test of the MMRS was conducted in 2004 in the Pasadena, California. The test results will also be reported.