The Principal Component Analysis (PCA) applied on MDIS  photometrically-corrected images acquired
during the first two MESSENGER flybys revealed several spectral units on Mercury [2-5], mainly distinct for
differences in reflectance and spectral slope. All units have a positive spectral slope, but they are described
as “red” or “blue” depending on a steeper or less steep slope than the average Mercury terrain in MDIS
spectral range (395-1040 nm). The three major terrain classes are the Low-Reflectance Material (LRM), the
Intermediate Terrain (IT), and Smooth Plains, which are subdivided into High-reflectance Red Plains (HRP),
Intermediate Plains (IP), and Low-reflectance Blue Plains (LBP). HRP are the brightest and reddest smooth
plains; IP and IT are spectrally similar to the global mean; the LBP is intermediate between IP and LRM, the
latter is the darkest unit on Mercury . Minor classes include fresh crater ejecta, hollows, and “red units”
[2,5]. Fresh crater ejecta are interpreted to be among the least space-weathered materials on Mercury,
having higher reflectance and bluer spectral slopes than their mature counterparts . Hollows are shallow,
irregularly shaped, and rimless flat-floored depressions with bright interiors and halos and flat spectral slopes
. The red units are distinguished in red material (RM) emplaced during the impact process  and
pyroclastic deposits, formed after explosive eruptions [2,3,7]. The spectral units on Mercury were also
classified with MASCS data : a PCA and a cluster analysis was applied on MASCS spectra acquired during
the first two MESSENGER flybys and not corrected for viewing geometries.
In this work, we applied both the PCA and the Spectral Angle Mapper (SAM ) on photometrically-
corrected and quality-filtered MASCS spectra acquired during the MESSENGER orbital phases. PC1, PC2, and
PC3 are responsible for 98.72%, 0.95%, and 0.17% of the spectral variability in the dataset, and appear
correlated with the reflectance at 550 nm, the VIS Slope, and the UV Slope, respectively. By combining the
PC1, PC2, and PC3 maps in an RGB image, hollows, fresh ejecta, and red units are readily distinguishable, as
well as the IT, LRM, HRP, and LBP. Mean spectra of representative regions for each major and minor terrain
are used as end-members driving the SAM classification. As a result, 87.5% of the data is classified as smooth
plains, 2.8% as IT, 6% as LRM, and the minor classes involve 3.8% of the dataset. Results from PCA and SAM
classification are compared to derive physical/chemical properties of Hermean terrain classes.
Hawkins et al. (2007) SSR 131. Robinson et al. (2008) Science 321. Murchie et al. (2008) Science 321.
Denevi et al. (2009) Science 324. Blewett et al. (2009) EPSL 285. Ernst et al. (2010) Icarus 209. Head
et al. (2008) Science 321. McClintok and Lankton (2007) SSR 131.  D’Amore et al. (2011) LPSC XLII.
Yuhas et al. (1992) JPL Publication 92–41.