Salty Hydration for Martian Soils: Amorphous Sulfates a Main Source of Regolith Water

This view was stitched together from 116 images taken by NASA’s Curiosity Mars rover’s Mast Camera, or Mastcam, on January 10, 2020, the 2,641st martian day, or sol, of the mission. Credit: NASA/JPL-Caltech/MSSS.

In situ analysis of martian soil in the Gale Crater by NASA’s Curiosity Mars rover has revealed that about a third of the samples are composed of non-crystalline materials (lacking a long-range ordered crystal lattice with a repeating unit structure) enriched in hydrated phases. Analysis of the total elemental composition collected by the ChemCam instrument — a chemical analysis tool equipped with a remote micro-imager and the first Laser-Induced Breakdown Spectrometer (LIBS) flown to another planet — by Gaël David from Université de Toulouse, France, and team demonstrated a direct link between the occurrence of water and the presence of sulfate salts in martian samples. Although ChemCam does not directly quantify hydrogen in granular samples, relative concentrations of hydrogen can be estimated using independent component analysis, a mathematical method for calculating individual components that contribute to the observed bulk signal.

The observed enrichment in hydration is significant in some samples, up to over three times the average hydration in bulk martian soils with an estimated total of about 6% by weight H2O. The increase in concentration of H from the ChemCam analysis did not correlate with concentrations of most other elements in the bulk soil (e.g., Si, Ti, Al, and Na), but it showed a strong linear correlation with S, indicating that soil hydration is related to S-bearing phases such as sulfates. Complementary analysis by the gas chromatograph mass spectrometer onboard Curiosity recorded the release of sulfur dioxide at temperatures consistent with the presence of crystalline and amorphous magnesium and iron sulfates in the hydrated samples. Both iron and magnesium sulfates can absorb a lot of water and commonly occur as hydrated salts carrying over 60% by weight H2O within their mineral structures. Correlation between H and S suggests that the main sources of water in martian soil are the hydrated sulfate salts. The in situ analysis results are consistent with orbital observations in demonstrating that sulfates are the main source of soil hydration in the Gale Crater. READ MORE