Figure 3. FRT00021C92 regularized and map projected data overlain with 50% transparency on the HiRISE red image mosaic using the RGB assignments delineated in the caption for Figure 1. CRISM data are in SSA units. White boxes delineate locations that include Curiosity ’s stop for sand-based measurements. These areas are shown using color HiRISE data in Figures 4 and 7. Sols 1647 and 2991 correspond to times when Curiosityacquired Mastcam image data of the sand deposits as shown in Figures 5, 6, 8, and 9.
2 Summary of Previous Results
Three orbital instruments have been used to infer the mineralogy of the Bagnold dunes. The Mars Global Surveyor’s Thermal Emission Spectrometer (TES) acquired data interpreted to indicate the dominance of ferrous silicate minerals, thereby pointing to a basaltic source (Lane & Christensen, 2013). Rogers and Bandfield (2009) reported similar results using a combination of TES and Mars Odyssey’s Thermal Emission Infrared Imaging System (THEMIS). Seelos et al. (2014), using CRISM data projected to 18 m/pixel and covering the northern part of the Bagnold dunes, inferred that the Bagnold barchan dunes exhibit an enhanced olivine spectral signature as compared to an enhanced pyroxene signature for the linear dunes. Edwards et al. (2018) inferred sand-sized particles (0.11 to 0.35 mm) for the Namib dune area within the Bagnold complex from Curiosity and THEMIS data. In summary, orbital data indicate that the Bagnold dunes are dominated by sand sized particles associated with ferrous silicate minerals and glass, i.e., basaltic sands. Bennett et al. (2018) employed THEMIS and two CRISM scenes projected at 18 m/pixel to investigate color and spectral reflectance variations between the Bagnold dunes and a sand sea located in the western part of Gale crater, farther west than the deposits in this study. They concluded that the color and spectral properties of the western sand sea were likely influenced by dust cover as compared to the Bagnold dunes, yet they discussed and did not rule out variations due to grain size sorting.
CRISM-based SSA spectral data were employed by Kreisch et al. (2017) to infer mineralogy and grain sizes using Hapke (2012) non-linear modeling for along-track oversampled (ATO) scene ATO0002EC79 covering the Namib dune area. They concluded that the mineral abundance and grain size retrievals were compatible with the Gobabeb sand sample from within the Bagnold dune complex measured by Curiosity ’s Chemistry and Mineralogy (CheMin) X-ray diffraction instrument (Achilles et al., 2017). Nonlinear modeling of the CRISM dune spectra retrieved abundances in decreasing order of amorphous material, olivine, plagioclase, pyroxene, and magnetite. This work was followed by a more extensive analysis of CRISM data covering the Bagnold dunes that showed minerals are spatially distributed and concluded that nonlinear modeling results should be viewed in a strict statistical sense to understand how well the results are constrained by the available endmembers and the spectral characteristics of CRISM input spectra (Lapotre et al., 2017a, 2017b). Rampe et al. (2018) included both CheMin and CRISM data in their analyses of Gobabeb (Namib dune sample) and Ogunquit Beach (MDI dune sample). The CheMin results showed the presence of XRD-amorphous material in addition to plagioclase, olivine, augite, and pigeonite, in decreasing order of abundance for both Gobabeb and Ogunquit Beach. CRISM SSA spectra retrieved from scene HRL0000BABA at 12 m/pixel in the MDI area were modeled using Hapke (2012) theory in their paper. Model results indicated abundances ranked in decreasing order as plagioclase, an amorphous phase, augite, olivine, pigeonite, and magnetite. The authors emphasized caution when drawing direct comparisons between CRISM data and CheMin, resulting from differences in the nature of the instruments and the data they acquire. For example, CRISM spectral radiances sample only micrometers into surface materials and are averages over areas tens of meters square, whereas CheMin analyzes a small volume (~10 mm3) of scooped sand with a grain size <0.15 mm (Blake et al., 2012).
Ehlmann et al. (2017, and references therein) provided an extensive summary of measurements acquired by Curiosity for the northern Bagnold dunes, specifically Namib and High dunes. The sands range in grain size from 0.045 to 0.5 mm, are loose, rounded, variably colored, and have significant fractions of an amorphous component. Plagioclase, olivine, and pyroxenes comprise the vast majority of the crystalline phases. Weitz et al. (2018, 2022) continued the analysis of sand deposits using Curiosity data, including sands found along the traverse between the Vera Rubin ridge to SoF. They found that 0.050 to 0.150 mm grains dominate active sand deposits, with much coarser grains (1-2 mm) coming from weathering of local bedrock. Christian et al. (2022) used CRISM ATO FRT00021C92 to derive apparent thermal inertias for the Bagnold dunes in the MDI area and found that values were lower than what was found for the SoF deposit, implying that SoF was dominated by larger grains. Sullivan et al. (2022) examined ripple orientations for sand deposits in Glen Torridon and inferred sand-driving winds moved toward the southwest.
3 Data and Methods
Five groups of data were utilized in the analyses presented in this paper. CRISM scenes HRL0000BABA and FRT0000C518 were used to provide an overview of the sand deposits within the vicinity of Curiosity ’s traverses (Figure 1). CRISM ATO FRT00021C92 was employed to identify and map mineral phases within MDI dune field and the SoF sand sheet. The mapping included spectral ranges for CRISM short wavelength (S) and long wavelength (L) data cubes after retrieving surface SSA spectra, as discussed below. An MRO High Resolution Imaging Science Experiment (HiRISE) mosaic (McEwen et al., 2007) covering the same regions as the CRISM data was also utilized (Calef & Parker, 2016). These data were further complemented by Curiosity ’s Navcam (Maki et al., 2012), Mastcam (Bell et al., 2017), the Mars Hand Lens Imager (MAHLI; Edgett et al., 2012), and the Mars Descent Imager (MARDI; Malin et al., 2017) image data acquired at the MDI and SoF sand deposits (Table 1). Compositionally, the Alpha Particle X-ray Spectrometer (APXS; Gellert & Clark, 2015) provides a means to evaluate the hypothesis that disaggregated bedrock grains mix with basaltic sands at the margins of both the MDI and SoF basaltic sand deposits.
Table 1. Reference information for data used in manuscript.