New Mineralogic Diversity in the Noachian Crust of Tyrrhena Terra, Mars

Abstract:

The cratered Noachian highlands region of Mars is host to hundreds to thousands of exposures of hydrated minerals. Areas of the highlands that are relatively free of dust, such as Tyrrhena Terra, have been previously examined using Mars Express/OMEGA and MRO/CRISM visible/near-IR reflectance data. The inferred alteration mineralogy is consistent with formation under low water to rock ratio, relatively low T/P and shallow crustal conditions. The vast majority of occurrences are associated with impact craters with a wide range of diameters, and within not only central peaks, but also rims and ejecta. This implies that most outcrops are a result of the excavation of pre-existing mineralology, not formation in-situ by post-impact hydrothermalism. Types of minerals reported include, most commonly, Fe/Mg phyllosilicates (smectite, chlorite, mixed-layer clays, prehnite, serpentine, illite), and less commonly, Al-phyllosilicates (e.g., kaolinite), carbonates, zeolites (e.g., analcime), and hydrated silica.

Here we document and characterize the occurrence of an additional class of mineralogy in Noachian terrain: hydrated sulfates. Hydrated sulfates are observed in the central peaks and (in one case) the proximal ejecta of 3 craters dispersed across the region. In each crater, a diverse assemblage of other hydrated alteration minerals is also apparent and includes prehnite, carbonate, zeolite, Fe/Mg smectite, and chlorite. Two of the three craters are approximately 10 km in diameter, with resulting sulfate exhumation depths in the central peaks approaching 1 km. The third crater has a diameter of 25 km; however, uplift and ejecta from several pre-existing impacts reduce the possible sulfate excavation depth to 1.3 km. CRISM spectra are consistent with a poly-hydrated sulfate, with major absorptions centered at 1.45, 1.95, and 2.4 micrometers – similar to, but distinct from, many zeolites. While mono-hydrated and poly-hydrated sulfates are not uncommon to other areas of Mars, they are generally relegated to younger, layered sedimentary settings (e.g., Valles Marineris, Meridiani Planum). If confirmed, the formation of hydrated sulfates in Noachian-aged terrain substantially adds to our understanding of the complexity of the early Mars environment.