THERMAL INFRARED SPECTRAL EVIDENCE FOR IMMATURE SEDIMENTARY MATERIALS ON MARS

On Earth, sedimentary materials derived from granodioritic continental rocks become enriched in quartz, alkali feldspars, mica, and clay minerals with increasing maturity. Because the Martian crust is largely gabbroic/basaltic, Martian sediments should evolve differently – potentially becoming enriched in plagioclase, Fe-Ti-oxides, and clay minerals with increasing maturity. The mineralogy of Martian sediments can be constrained using thermal infrared spectra collected by the Thermal Emission Spectrometer (TES) and THEMIS experiments. TES spectra indicate high plagioclase/(pyroxene + olivine) ratios in some sedimentary deposits, including the Vastitas Borealis Formation. While this may be consistent with aqueous processing of mafic sediments, evidence for widespread/abundant clay minerals on Mars is lacking. However, TES spectra show evidence for poorly crystalline aluminosilicates and opaline silica that could occur as grain coatings and cements in mineralogically immature sedimentary materials on Mars. The predominance of poorly crystalline alteration products in the observable geological record of Mars, rather than crystalline clay minerals, favors geochemical degradation during episodic aqueous activity, rather than during sustained, warm, wet environments – in the represented time periods. Alternatively, poorly crystalline weathering products observed with TES may correspond to coatings on volcanic rocks and rock particles at the surface. Rock coatings - by common definition - are thin, exogenic sedimentary deposits and are of interest for understanding the history of water at the Martian surface.