Hydrated sulfates on Mars’s surface: water cycle and S isotope tracking

Abstract:

We study a range of hydrated sulfate minerals from first-principles calculations based on density-functional and density-functional perturbation theory. We report the results extensively on the WURM website (http://wurm.info, Caracas and Bobcioiu, 2011). We find that hydration has a more pronounced effect on the spectroscopic properties than cation replacement. The Raman spectra of all phases present clear SO4 features that are easily identifiable. We use this to show one can use the vibrational spectroscopic information as an identification tool in a remote environment, like the Martian surface.

Based on the computed vibrational results we analyze the S isotope partitioning. We observe that in general hydration favors enrichment in the lighter S isotope 32S with respect to the heavier 34S, which is accumulated in the less hydrous structures. Thus we show for the first time that the signature of 34S/32S partitioning could be observed by in situ spectroscopy on the surface of Mars.

Finally we compute hydration energies. For example, in the hydrated magnesium sulfate series we find that epsomite and meridianiite with, respectively 7 and 11 water molecules per MgSO4 unit are particularly stable with respect to other individual or combinations of hydration states (Bobocioiu and Caracas, 2014). This can be related to the diurnal cycle of hydration and dehydration and hence it can improve the modeling of the water circulation on Mars.

References:

E. Bobocioiu, R. Caracas (2014) Stability and spectroscopy of Mg sulfate minerals. Role of hydration on sulfur isotope partitioning. Amer. Mineral., 99, 1216-1220.

R. Caracas, E. Bobocioiu (2011) The WURM project – a freely available web-based repository of computed physical data for minerals. Amer. Mineral. 96, 437-444.