Mineral and Lithologic Mapping of Martian Low Albedo Regions Using OMEGA

Introduction: Since January 2004, OMEGA/Mars Express data has revealed a diverse and complex Martian surface mineralogy [1]. The important findings, together with Thermal Emission Spectrometer (TES), Thermal Emission Imaging System (THEMIS), and insitu observations from Mars Exploration Rovers, include but not limited to (1) hydrated sulfates [1, 2, 3, 4, 5, 6, 7]; (2) iron oxides and oxhydroxides [2, 3, 5, 8, 9, 10, 11, 12, 13]; (3) hydrated alteration phyllosilicates [4, 2, 3]; and (4) mafic and ultramafic rocks containing pyroxene and olivine [1, 11, 13, 14, 15, 16]. These results are improving our knowledge regarding the Martian surface mineralogy and lithology, and the geological history of Mars. OMEGA imagery as noted above provides efficient and satisfactory hyperspectral information. It can be used not only to map individual minerals, but also their corresponding lithologic units. The purpose of this study was to identify the mineral and lithologic units using the OMEGA imagery. The minimum noise fraction (MNF) method was applied to derive the lithologic endmember units and then three spectral matching methods, spectral angle mapper (SAM), spectral feature fitting (SFF), and binary encoding (BE), were used to match minerals and lithologies. Three low albedo areas, Meridiani Planum, Ophir-Candor Chasma, and Syrtis Major, were chosen for this study. Meridiani Planum is Opportunity Rover’s landing site. Minerals such as pyroxene, hematite, jarosite, and phyllosilicates and lithologies such as basalt have been identified [2, 3, 11, 12]. The hydrated sulfates (kieserite and polyhydrated sulfates) on lighttoned layered terrains at the Ophir and Candor Chasmas have been detected [1, 5]. Syrtis Major is dominated by basalts, but with little olivine [13, 17]. These are available for testing results of this study. Dataset: OMEGA acquires spectrum in 352 contiguous bands covering 0.35 to 5.1 μm with a spatial resolution of 0.3 to 4 km/pixel and spectral resolution of 7 to 20 nm [1]. The spectral range and resolution have been chosen to allow for identification of major surface and atmospheric species by their diagnostic spectral absorption feature [1]. OMEGA data for three selected areas were downloaded from the ESA’s Planetary Science Archive. In this study, we mainly examined the spectral range from 0.4 to 2.5 μm. The data was pre-processed using a modified IDL program initially provided by ESA to a relative reflectance image (I/F), which was then utilized for atmospheric corrections using a LLEE model developed by Guan et al. [18]. Method: The atmospherically corrected image was then processed using ENVI for image classification and mineral and lithologic identification. The MNF method, conducted twice principal component analysis, was first run to produce noise-free principal components. The MNF band1 mostly contains the albedo information, but the MNF bands 2, 3, and 4 mainly contain the mineral and lithologic information, which can be used to produce a false-color endmember map (Fig.1). Spectra of these endmembers were then processed to match with the various standard spectral libraries from the USGS, John Hopkins University, and Brown University. Three spectral matching methods (SAM, SFF, and BE) were applied based on both spectra and/or continuum removal spectra for scoring individual minerals and lithologies from libraries with each endmember. The highest scores for the matched minerals and lithologies were then recorded for each endmember.