Figure 18. Plots are shown for the lowest RMSE differences
between the endmember spectra for Mount Desert Island and Sands of
Forvie and Hapke nonlinear modeling. The wavelength interval centered on
1.00 to 1.08 µm is blanked out because of increased uncertainty of
retrieved spectra because of low sensitivity at the wavelength limits
for the S and L spectrometers.
Specifically, the MDI sand endmember best fit had an RMSE of 0.00165 and
an R2 of 0.997, and the SoF sand endmember best fit
had an RMSE of 0.00124 and an R2 of 0.994. Figures 19
and 20 also show these spectra, along with the simulated SSA spectra
that the procedure chose as the best fit combination to make the model
spectrum. The retrieved mineral abundances and grain sizes are also
shown. For both deposits, GMS glass and labradorite are the most
abundant phases, similar to the rankings of the amorphous phase(s) and
feldspar abundances as determined by CheMin for the MDI sand sample
(Rampe et al., 2020). GMS glass and labradorite, in addition to any
olivine present in both deposits, contribute to the broad 1 µm feature
observed in both MDI and SoF spectra (Figures 16 and 17). Between MDI
and SoF, the main spectral difference (other than albedo) is in the
relative proportions of the two pyroxenes (pigeonite and augite). The
long wavelength spectra rise for MDI is consistent with the model result
pigeonite/augite = 4.8, and the SoF ~2.2 µm spectral
minimum is consistent with the model result pigeonite/augite = 0.0
(Figures 19 and 20). Grain size results, which in the Hapke (2012) model
are much less constrained relative to phase abundances (Lapotre et al.,
2017a), show a slight trend of coarser grains for SoF as compared to MDI
deposits. This result is consistent with MAHLI-based observations from
Curiosity (Weitz et al., 2022).