Spatial and stratigraphic variations in Martian surface mineralogy determined from orbital thermal infrared data: Implications for upper crustal evolution and alteration.

摘要

Global- and local-scale variations in surface emissivity and modal mineralogy of martian low-albedo regions were determined from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and Mars Odyssey Thermal Emission Imaging System (THEMIS) datasets. Distributions of previously determined global spectral unit types (Surface Types 1 and 2) were refined. 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 is supported for the high-silica phase(s) found in high-latitude regions, the global distributions suggest that a single process cannot explain the origin of all surfaces dominated by high-silica phase(s) on Mars. 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. Martian low-albedo regions were classified into four groups based on relative abundances of plagioclase, ferromagnesian minerals, and high-silica phase(s). Compositional units located between +/- 45° latitude are moderately correlated with distinct provinces defined by surface age and geographic location, 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.; THEMIS multispectral and thermophysical information combined with TES hyperspectral and albedo data were used as a high spectral/spatial resolution tool to investigate the mineralogic heterogeneity of ancient martian crust exposed in Ares Vallis bedrock. The spatial and stratigraphic distribution of spectral units indicate that the Ares Vallis region likely experienced repeated episodes of olivine-enriched magmatism during the first 1.5 Gyr of crust formation. This olivine enrichment was likely due to less olivine fractionation relative to parent magmas of surrounding rocks. In Ares Vallis, the exposure of materials >3 Gyr in age that contain ∼25% olivine demonstrates that chemical weathering has been limited in this region and suggests that warm/wet conditions, including the formation of Ares Vallis, were limited in duration.