Water Quantification of Amorphous and Crystalline Ferric Sulfates
Relevant to Mars
Abstract
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.