Figure 22. Calculated SiO2 vs. MgO + FeO for
all accepted model runs for (a) MDI and (b) SoF, displayed as heatmaps.
Dark red color indicates a higher concentration of points, while white
means there are no points in that space. Mineral endmembers are
displayed as colored circles, while the best fit model run for each MDI
and SOF is a star.
8 Conclusions and Implications
Multiple CRISM false-color image data show that the linear Bagnold dunes
in Gale Crater have a reddish-brown color, while sand sheets to the
south of the dunes are darker and lack the reddish-brown color. The
traverse of the Curiosity rover has led to observations of the
linear dunes at Mt. Desert Island (MDI), and a sand sheet to the south
at the Sands of Forvie (SoF). SSA spectra retrieved from CRISM scene
FRT00021C92 show that the difference in the false-color images can be
attributed to a long wavelength (1.7 to 2.5 µm) rise in the MDI sand
spectrum that is absent in the SoF spectrum. Instead, the SoF sand
spectrum is characterized by a broad ~2.2 µm absorption
feature.
To classify the potential mineralogic controls on these different
spectral features, we performed checkerboard unmixing of the SSA image
cube to extract the extreme spectral endmembers from both sand deposits.
These spectra were modeled using carefully selected laboratory spectra
and Hapke (2012) theory to invert for mineral abundances and grain size
information. Model results, including inversions to equivalent APXS
oxide compositions, show that spectral endmembers from both sites are
dominated by basaltic glass (i.e., an amorphous phase) and labradorite
or other spectrally equivalent feldspar. The broad 1 µm feature in both
MDI and SoF is primarily controlled by olivine, with some contribution
from basaltic glass. Inclusion of a pigeonite spectrum explains the
increase in SSAs with increasing wavelength for the MDI spectral
endmember, while the absence of pigeonite and presence of augite
explains the deep, broad ~2.2 µm absorption for the SoF
spectral endmember. Modeling results also suggest that the SoF deposit
has coarser grains than the MDI deposit, consistent with measurements
from MAHLI data (Weitz et al., 2022). Importantly, modeling reflectance
data displayed the non-uniqueness of deriving chemical composition from
CRISM data and highlighted the importance of statistical analysis of the
data.
The primary emphasis of the paper was to retrieve the mineralogy and
grain sizes for MDI and SoF and to compare to observations acquired by
Curiosity, but there are also implications for aeolian processes. First,
we note that the two sand bodies are only ~2.5
kilometers from one another. Even given their proximity, they have
subtle yet distinctly different mineralogy and grain sizes. We speculate
that the differences are a consequence of the topographic settings
intensifying wind sorting effects over time. The Bagnold linear dunes
are not encumbered by topography, but rather are migrating from NE to SW
associated with the dominate wind direction (Sullivan et al., 2022). On
the other hand, the SoF is bordered on the south and southwest by
topographic obstacles, specifically the hills associated with Mount
Sharp and the scarp that defines the edge of the Greenheugh pediment
(Figure 23). The sands in SoF are thus unable to migrate in an
unencumbered manner. We infer that the finest grained sands
preferentially migrate onto the pediment surface, leaving behind overall
coarser material and a slightly different mineralogy as compared to the
Bagnold dune sands. The remaining grains left behind can become
segregated by wind conditions saltating smaller grains but causing creep
of larger grains. This can lead to coarsening of the ripples, ultimately
causing the formation of coarse-grained megaripples as observed in SoF
(Lapotre et al., 2018). This cycle causes the variation seen in the
spectral data, that is, SoF contains larger ripples with concentrated
coarser grains while MDI lacks this coarsening and has smaller ripples.