Spectral classification and MGM-based mineralogical characterization of hydrated phases: The Nili Fossae case, Mars

Mars has a relatively tenuous atmosphere, the remote-sensed spectral data of the surface are affected by the presence of the atmosphere. In particular the water and CO2 absorption of the atmosphere are in the same spectral region as water, OH- and (CO3)(2-) in minerals. An accurate correction of the gaseous contamination is thus mandatory for applying powerful mineralogical assessment methods, such as MGM (Modified Gaussian Model), also in the spectral ranges affected by possible residual from atmospheric features. In this paper, for the first time we apply MGM on hydrated phases using OMEGA/MEX corrected data through the Surface Atmosphere Separation (SAS) method, which allows a more reliable gaseous absorption removal when working on reflectance spectra. Nili Fossae area is selected as a test case to prove the validity of this approach. SAS-corrected OMEGA hyper-spectral images are analyzed using a set of sequential steps, comprising image derived spectral library collection, supervised classification and spectral band assignment. Applying the PPI (Pixel Purity Index) on the MNF (Minimum Noise Fraction) transform, 14 endmembers are selected and used to spectrally classify the region through SAM (Spectral Angle Mapper) classification. The MGM algorithm is applied for the mineralogical interpretation of Nili Fossae in the 0.5-2.5 mu m spectral range, with particular focus on hydrated mineralogies. We recognize 5 main spectral units: A) dominated by orthopyroxene and iron hydroxides, B) and C) characterized by a mixture of orthopyroxene and clinopyroxene (with different mineral proportions for the two units), D) dominated by olivine and altered olivine and E) showing hydrated mineralogies: good matches are found with nontronite, chamosite and serpentine.