Examining wet Martian regolith simulants from the mid-infrared through the visible with reflectance spectroscopy: A laboratory perspective on recurring slope lineae

The Mars Reconnaissance Orbiter’s (MRO) CRISM instrument has observed the ‘ebb and flow’ of recurring slope lineae (RSL) on present-day Mars. The mechanism responsible for RSL has been heavily debated over the past decade. Proposed causes include: 1) brine present in the regolith or 2) granular, sand-like regolith. Both mechanisms subject the surface material to flow downward over time. This process is accompanied by seasonal patterns of lightening and darkening of the surface, the cause of which is also undetermined. To shed light on the cause of RSL, this study examines how sample moisture, sample wetting method, and the presence of brines affect the spectra of Martian regolith simulants. Using high-fidelity Martian regolith simulants (MGS-1 Mars Global Simulant, MGS-1S Sulfate ISRU, and MGS-1C Clay ISRU) produced at the Exolith Laboratory at the University of Central Florida, diffuse reflectance spectra of dry, wet, and briny simulants are collected from the mid-infrared (25 microns) through the visible (0.4 microns) regions. The salts selected in this study, sodium perchlorate and sodium chloride, are relevant to Mars’ surface composition. This study performs a comparison between a sample preparation method from previous studies in which water is misted onto the surface of the sample and a new method in which the water or brine are added to the sample cup prior to adding the simulants. The new method aims to more closely imitate the presence of subsurface solutions possibly responsible for the RSL. This analysis can provide insights about both proposed causes of RSL by examining changes in spectral features due to chemical and mineralogical alteration or changes in albedo as a result of the presence of brine or moisture content in the regolith.