Investigation of the near-infrared spectral character of putative Martian chloride deposits

Putative chloride salt deposits observed throughout the southern highlands of Mars by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) display featureless red slopes in near-infrared ratio spectra. It is hypothesized that the admixture of anhydrous chlorides or unoxidized sulfides with silicate rocks or minerals could imitate this spectral behavior. Three different sets of spectra were collected: (1) simple mixtures of halite with labradorite and flood basalt at multiple concentrations, (2) halite crusts formed on both labradorite and flood basalt, and (3) simple mixtures of 25 and 50 wt % acid-washed pyrite with labradorite and flood basalt. In all three instances, multiple grain sizes were used to evaluate the effect of particle size on spectral results. Spectra of the mixtures and crusts were divided by pure labradorite and flood basalt spectra of the same grain size to produce ratio spectra comparable to the CRISM ratio spectra. Our study rules out pyrite as a possible component of these deposits, whereas flood basalt mixtures with halite reproduced the observed red slope under some conditions. This allows us to place some broad constraints on halite proportions and the effective grain sizes of these deposits.