Quantification of H2O in Amorphous and Crystalline Ferric Sulfates Relevant to Mars

Abstract Crystalline ferric sulfates (e.g., ferricopiapite and Fe(OH)SO 4 ) 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. Ferric sulfates with different hydration degrees may play a key role in the water cycle on Mars. In order to understand the role of the hydrated ferric sulfates in the water cycle and their exact hydration states on Mars, five crystalline ferric sulfates and seven amorphous ferric sulfates with different hydration states were synthesized in the laboratory. The water content (number of H 2 O molecules per ferric sulfate molecule) 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 SO 4 tetrahedron main Raman feature position around 1,000 cm −1 , shown as the intensity and area of water feature around 3,500 cm −1 over SO 4 tetrahedron main Raman feature around 1,000 cm −1 , respectively. Twelve ferric sulfates' Hα emission line area at 656.7 nm was 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 Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals data interpretation and their use to quantify the water content in detected targets.