NEAR INFRARED SPECTRAL ANALYSIS OF MIXTURES OF HALITE AND LABRADORITE FOR APPLICATION TO PUTATIVE CHLORIDE DEPOSITS OBSERVED BY CRISM

Introduction: Putative chloride salt deposits have been detected in Mars Odyssey THEMIS daytime infrared images. The mid-IR spectra of these deposits are characterized by an unusually high apparent emissivity in processed data at shorter wavelengths in comparison to surrounding units [1]. CRISM visible to near infrared (VNIR) FRT and HRL images covering these deposits also indicate the presence of this probable chloride salt based on a red spectral slope lacking distinctive absorptions throughout the 1-2.7μm range and a lack of the 3μm water absorption band in spectra ratioed to spectrally neutral material [2]. However, diffuse reflectance spectra of halite taken in the laboratory setting do not result in a red spectral slope in the near infrared [3]. We hypothesize that this red spectral slope is due to the mixing of chloride with a feldspar mineral such as labradorite, which is found on the Martian surface, and shows a slope in this region due to the Fe 2+ absorption band centered at about 1.3μm [4]. Here we report on diffuse reflectance spectra of labradorite and halite mixtures. By investigating the effect of adding various amounts of labradorite to halite, we hope to find a suitable combination which allows for the red spectral slope viewed in the ratioed CRISM spectral data. Methodology: CRISM data were investigated in areas where putative chloride deposits are observed in THEMIS daytime decorrelation stretched (DCS) images. We confirmed the unique spectral character of these deposits by extracting the atmospherically corrected THEMIS emissivity spectra from the images. CRISM DCS images were made using CRISM multispectral summary products [1, 5]. To display phyllosilicates in association with the chloride deposits, red, green and blue were assigned to the reverse of the IR spectral slope parameter, D2300 (2.3μm drop off), and HCPINDEX (high calcium pyroxene) respectively. An example of this can be seen in Figure 1, where the putative chlorides appear to be red, phyllosilicates appear to be green and pyroxene appears to be blue. Spectra were extracted from atmospherically corrected CRISM hyperspectral image cubes by averaging pixels over areas bearing chloride (red) and phyllosilicate (green). In order to remove any remnant atmospheric effects or instrumental errors, these spectra were then ratioed to regions in the image that were considered to be spectrally neutral. Generally dust is the desired spectra for conducting ratios, however the Figure 1. Projected CRISM image FRT0000A102_07 mapped where red, green and blue are chloride, phyllosilicate and pyroxene respectively. Boxes indicate pixels where spectra of glowing terrain and phyllosilicate have been averaged and the neutral region that is used in spectral ratios (Figure 2). Colors in this figure correspond with those in the ratioed CRISM spectra.