Global mapping of Martian hematite mineral deposits: Remnants of water-driven processes on early Mars

Near-global (60 degreesS to 60 degreesN) thermal infrared mapping by the Th ermal Emission Spectrometer (TES) on Mars Global Surveyor has revealed uniq ue deposits of crystalline gray hematite (alpha -Fe2O3) exposed at the Mart ian surface in Sinus Meridiani, Aram Chaos, and in numerous scattered locat ions throughout Valles Marineris. The Sinus Meridiani material is an in-pla ce, rock stratigraphic sedimentary unit characterized by smooth, friable la yers composed primarily of basaltic sediments with similar to 10-15% crysta lline gray hematite. This unit has outliers to the north that appear to hav e formed by stripping and removal. The hematite within Aram Chaos occurs in a sedimentary layer within a closed basin that was likely formed during th e basin infilling and predates the formation of nearby chaos and outflow te rrains. This unit appears to be exposed by erosion and may be more extensiv e beneath the surface. The Valles Marineris occurrences are closely associa ted with the interior layered deposits and may be in place within the layer s or eroded sediments. Overall, crystalline gray hematite is extremely unco mmon at the surface, yet in all observed locations it is closely associated with layered, sedimentary units. Here we argue that these hematite deposit s have formed by a process involving chemical precipitation from aqueous fl uids, under either ambient or hydrothermal conditions. Thus the TES mineral ogic data provide evidence that liquid water has been stable at or near the surface, probably for millions of years by analogy with terrestrial iron f ormations, in specific locations on early Mars.