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.