Visible/NIR photometric signatures of liquid water in Martian regolith simulant

The bidirectional reflectance of dry and wet Martian regolith simulant has been measured. Measurements were performed using a scattered light measurement apparatus and a variety of optical bandpass filters ranging from the blue to the near infrared. The characteristic darkening of the soil simulant that arises from the introduction of moisture into the simulant is shown to be strongest at low phase angles and weaker, or even absent, at phase angles around 130∘. Although the wet sample appears darker than the dry at low phase angles in all bandpasses, the reflectance contrast between dry and wet samples tends to increase with decreasing wavelength. Best fit Hapke model parameters [Hapke, B., 1981. Bidirectional reflectance spectroscopy. I-theory. J. Geophys. Res. 86, 3039-3054, Hapke, B., 1984. Bidirectional reflectance spectroscopy. III-correction for macroscopic roughness. Icarus 59, 41-59, Hapke, B., 1986. Bidirectional reflectance spectroscopy. IV-the extinction coefficient and the opposition effect. Icarus 67, 264-280] suggest that the presence of moisture influences the bidirectional reflectance by reducing the single scattering albedo, increasing the simulant's porosity, and reducing the angular width of the single particle scatter lobes. Additionally, unlike the dry sample, the single particle scattering function of the wet simulant favors forward scattering rather than back scattering. These radiometric signatures could provide useful clues towards the identification of localized regions of moisture on the Martian surface, if they exist, during the analysis of images acquired with orbiting cameras.