Sulfates on Mars, a Systematic Raman Spectroscopic Study of Hydration States of Magnesium Sulfates

Introduction: The existence of sulfates at various locations on Mars has been implied by sulfur enrichment and compositional correlations between S and Mg (or Ca in some places) from landed missions including Viking, Pathfinder, and the Mars Exploration Rovers [1-3]. Direct identifications of sulfate minerals have been made by Mössbauer experiments [4] and by OMEGA [5-7]. Abundant water-equivalent-hydrogen (WEH) has been detected over two large equatorial regions (including the landing sites of the Spirit and Opportunity rovers) using the neutron spectrometer component of the gamma-ray spectrometer suite on the Mars Odyssey orbiter. This WEH is assumed to reside mainly in hydrated minerals [8]. The WEH is abundant around Gusev crater (~7.5 wt% over a 600 km footprint) and is much higher than the water content (~ 3 wt%) estimated for the most sulfate-rich (~22 wt% sulfates) subsurface regolith within The Boroughs trench [9] assuming kieserite (MgSO4.H2O) to be the stable phase and hydration state of Mg-sulfate. Kieserite has been identified by OMEGA at many locations on Mars [3]. The relative humidity (RH) at the regolith surface in equatorial regions on Mars can vary from ~ 0% during the day to ~ 100% in early morning as a result of large temperature excursions (≥100K) during the diurnal cycle. Under such conditions, more than one hydration state of the Mg-sulfates may be stable. It is therefore crucial to develop instrumentation capable of distinguishing among the various hydrated phases of the Mgsulfates so that we may develop an understanding of their stability fields and determine the pathways of phase transformation between the phases. Experiments:, The starting materials were crystalline, reagent grade (98+% pure) β-MgSO4, kieserite and epsomite having grain sizes ≤150 μm. Five types of experimental procedures were used to produce the various hydrated Mg-sulfates. The humidity buffer method [10] was used to study the stability fields and the pathway in de-/re-hydration reactions as a function of RH at fixed temperature; Raman measurements were made with the samples in sealed, glass reaction vials; X-ray diffraction was used to verify phase structure (measurements were conducted with the XRD powder sample holder tightly sealed within an amorphous polymer wrap), and weight changes were monitored during de-/re-hydration. Raman spectra of magnesium sulfates: Raman spectra of ten Mg-sulfate (aqueous solution, 8 different MgSO4xH2O's, and β-MgSO4) were obtained in this study. They were found to have unique, characteristic spectral patterns that can be used for the identification of the individual phases. Figure 1 shows that the water bands in 2500-4000 ∆cm spectral region of the different phases have reproducible variations in details of the band structure, arising from the distinct crystallographic sites of water molecules in the structures. The vibrational modes in the fundamental region of the Raman spectrum (100 to 1300 ∆cm) are collectively affected by variations in the linkages between SO4 tetrahedra and MgOn(OH2)6-n octahedra, as well as by the hydrogen bonding between the water molecules and the oxygen in SO4 groups. The fundamental symmetric stretching vibration ν1 of the SO4 group at 980-1050 cm also