Water Quantification of Amorphous and Crystalline Ferric Sulfates Relevant to Mars

Crystalline ferric sulfates (e.g., ferricopiapite and Fe(OH)SO4) have been proposed at multiple locations on Mars by the orbiter. While at Meridiani Planum and Gale Crater by rover missions, amorphous ferric sulfates were also suggested to exist in soils and sedimentary rocks. The ferric sulfates with different hydration degrees may play a key role in the water cycle on Mars. In order to understand in detail the role of the hydrated ferric sulfates in the water cycle and their exact hydration states on Mars, twelve ferric sulfates with different hydration states containing five crystalline ferric sulfates and seven amorphous ferric sulfates were synthesized in the laboratory. The water content (number of H2O molecules) was quantified by Raman spectroscopy and Laser-induced breakdown spectroscopy (LIBS), respectively. It was found that the amorphous ferric sulfates water content has a good relationship with the SO4 tetrahedron main Raman feature position around 1000 cm-1, the intensity and area of water feature around 3500 cm-1 over SO4 tetrahedron main Raman feature around 1000 cm-1, respectively. Twelve ferric sulfates’ Hα emission line area at 656.7 nm is normalized by the O emission line area at 778 nm in LIBS spectra. The crystalline and amorphous ferric sulfates all showed a good relationship between the values of normalized results and the water content. These results will aid us in precisely constraining the exact phases of hydrated ferric sulfates, provide a better reference for ChemCam, SuperCam, and SHERLOC data interpretation and their use to quantify the water content in detected targets.