Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: Evidence for near-surface water

The Thermal Emission Spectrometer (TES) instrument on the Mars Global Surve yor (MGS) mission has discovered a remarkable accumulation of crystalline h ematite (alpha-Fe2O3) that covers an area with very sharp boundaries approx imately 350 by 350-750 km in size centered near 2 degrees S latitude betwee n 0 degrees and 5 degrees W longitude (Sinus Meridiani). Crystalline hemati te is uniquely identified by the presence of fundamental vibrational absorp tion features centered near 300, 450, and >525 cm(-1) and by the absence of silicate fundamentals in the 1000 cm(-1) region. Spectral features resulti ng from atmospheric CO2, dust, and water ice were removed using a radiative transfer model. The spectral properties unique to Sinus Meridiani were emp hasized by removing the average spectrum of the surrounding region. The dep th and shape of the hematite fundamental bands show that the hematite is cr ystalline and relatively coarse grained (>5-10 mu m). Diameters up to and g reater than hundreds of micrometers are permitted within the instrumental n oise and natural variability of hematite spectra. Hematite particles <5-10 mu m in diameter (as either unpacked or hard-packed powders) fail to match the TES spectra. The spectrally derived areal abundance of hematite: varies with particle size from similar to 10% (>30 mu m diameter) to 40-60% (10 m u m diameter). The hematite in Sinus Meridiani is thus distinct from the fi ne-grained (diameter <5-10 mu m), red, crystalline hematite considered, on the basis of visible, near-IR data, to be a minor spectral component in Mar tian bright regions like Olympus-Amazonis. Sinus Meridiani hematite is clos ely associated with a smooth, layered, friable surface that is interpreted to be sedimentary in origin. This material may be the uppermost surface in the region, indicating that it might be a late stage sedimentary unit or a layered portion of the heavily cratered plains units. We consider five poss ible mechanisms for the formation of coarse-grained, crystalline hematite. These processes fall into two classes depending on whether they require a s ignificant amount of near-surface water: the first is chemical precipitatio n that includes origin by (1) precipitation from standing, oxygenated, Fe-r ich water (oxide iron formations), (2) precipitation from Fe-rich hydrother mal fluids, (3) low-temperature dissolution and precipitation through mobil e ground water leaching, and (4) formation of surface coatings, and the sec ond is thermal oxidation of magnetite-rich lavas. Weathering and alteration processes, which produce nanophase and red hematite, are not consistent wi th the coarse, crystalline hematite observed in Sinus Meridiani. We prefer chemical precipitation models and favor precipitation from Fe-rich water on the basis of the probable association with sedimentary materials, large ge ographic size, distance from a regional heat source, and lack of evidence f or extensive groundwater processes elsewhere on Mars. The TES results thus provide mineralogic evidence for probable large-scale water interactions. T he Sinus Meridiani region may be an ideal candidate for future landed missi ons searching for biotic and prebiotic environments, and the physical chara cteristics of this site satisfy all of the engineering requirements for the missions currently planned.