Figure 13. Correspondence analysis plot showing the relationship between APXS elemental abundances and targets for bedrock regions around Mount Desert Island and Sands of Forvie, together with measurements of the sand deposits at both sites. Note that the two sand deposit targets overlap, with Fleurac and Fleurac_offset located between the rock and sand targets. Also note the tight clustering of sand measurements for the two sites relative to the rock target data.
Bedrock data are clustered on one side of the plot, including the Limeyrat_offset target located near the Fleurac sand experiment site. The MDI and SoF sand measurements are clustered together on the left side of the plot. Fleurac and Fleurac_offset trend from the bedrock targets toward the sand targets, respectively. Note that Ratharsair, the SoF sand trough target, is also shown and falls within the sand cluster. Overall, the MDI and SoF sand targets are indistinguishable on the CA plot and in a direction consistent with the compositional dominance of ferrous silicate minerals. In summary, the Fleurac measurements and the overall compositional trends suggest that the APXS measurements at the margins of MDI and SoF are representative of their respective basaltic sand with minimal contributions from grains sourced from breakdown of local bedrock.
5 Mean CRISM Spectra for MDI and SoF Sand Deposits
Characteristics of Mean Spectra: The first step in examining spectral patterns for the MDI dunes and SoF sand sheet was to generate regions of interest (ROIs) for each deposit, with care not to include surrounding bedrock outcrops. The ROI boundary for the MDI dunes was limited to the northern extent of the CRISM ATO FRT00021C92. The ROIs, superimposed on the HiRISE mosaic, are shown in Figure 14 together with the mean spectra for the two areas. The spectral data extend from 0.50 to 2.6 µm, with the short wavelength cutoff associated with increased noise at shorter wavelengths and the long wavelength cutoff chosen to avoid the rapid downturn associated with the presence of a deep ~3 µm absorption due to the presence of H2O and M-OH, perhaps as adsorbed and/or absorbed molecules (e.g., He et al., 2022). Also shown on Figure 14 are the wavelengths used to generate the CRISM RGB false color images shown in Figures 1-3. The reddish-brown color of the Bagnold dunes, including the ROI, is associated with the upward slope of the MDI mean spectrum as opposed to the relatively flat longer wavelength trend for the SoF mean spectrum. Note that the SoF mean spectrum also has an overall lower SSA amplitude as compared to the MDI mean spectrum. This difference is intrinsic of the sands because the retrievals remove albedo variations associated with differences in lighting and viewing conditions.