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.