New views of global variations in surface emissivity and mineralogy from MGS-TES data

Global-scale variations in surface emissivity and modal mineralogy of martian low-albedo regions (defined here as surfaces with TES albedo values < 0.14) were determined from the Mars Global Surveyor Thermal Emission Spectrometer (TES) dataset. Distributions of previously-determined global spectral unit types (Surface Types 1 and 2) were refined. Martian low-albedo regions exhibit spatially-coherent variations in spectral character, which were classified into eleven representative spectral shapes. Groups of these spectral shapes can be averaged to produce spectra that are similar to Surface Types 1 and 2, indicating that the originally-derived surface types are representative of the average of all low-albedo regions. However, regional-scale spectral variations are present within areas previously mapped as Surface Type 1 or a mixture of the two surface types, indicating variations in mineral abundance among basaltic units. For example, Syrtis Major, which was the Surface Type 1 type locality, is spectrally-distinct from terrains that were also previously mapped as Type 1. Martian low-albedo regions were classified into four groups based on relative abundances of plagioclase, ferromagnesian minerals, and high-silica phase(s). Surfaces dominated by high-silica phase(s) (> 25%) are mostly confined to high-latitude regions > 45° and Solis Planum (28° S), consistent with prior studies; however, the distributions are not as widespread as previously indicated. While a secondary origin for high-silica phase(s) is consistent with the distribution of these surfaces in high-latitude regions, the global occurrences of these phases (including the north polar dunes, Nili Patera, and Solis Planum) indicate that a single process cannot explain the origin of all surfaces dominated by high-silica phase(s) on Mars. Compositional units located between +/-45° latitude are moderately correlated with distinct provinces defined by surface age and crustal thickness, indicating that the observed spectral differences in equatorial regions (< 45° ) are strongly controlled by mineralogic variations associated with original crust-forming magmas. Some Hesperian terrains exhibit a difference in mineralogy from Noachian terrains and from each other, suggesting variations in degrees of fractional crystallization, assimilation, or source region compositions. The distributions of relative differences in mafic mineral abundances between regions are generally consistent with VNIR observations from Mars Express OMEGA data.