Characterizing phyllosilicate distribution, abundance, and origin on Mars

Secondary phyllosilicates are hydrated minerals formed in the presence of liquid water. On Earth, their formation is often indicative of a neutral, water-rich environment, capable of supporting and preserving organic matter. Different phyllosilicate species may be produced in different pH levels and water-to-rock ratios. The identification of mineralogically diverse phyllosilicates in small, localized exposures on Mars provides a complex record of their formation processes. While discrete outcrops of phyllosilicates have been previously identified in high-resolution visible/near-infrared images of Mars, regional coverage of these phyllosilicate-rich areas at better resolution is limited. Furthermore, spectra of minerals in this wavelength range do not mix linearly, and it is difficult to determine their modal abundances. To better understand the geologic context of these deposits, I used the global coverage of thermal infrared multispectral imaging to develop a technique for mapping new phyllosilicate deposits on Mars and present results from the Mawrth Vallis and Nili Fossae regions.

Unlike in the visible/near-infrared, spectra of minerals in the thermal infrared mix linearly and can therefore be unmixed to derive endmember abundance. At 100-m/pixel resolution, the THEMIS instrument is more likely to detect surfaces with predominately phyllosilicate-rich materials, rather than the 3x5 km/pixel resolution of the thermal infrared TES instrument, which easily dilutes the spectral signature of phyllosilicate components with surrounding materials. Presented here are THEMIS unmixing results from the Mawrth Vallis region. Fe/Mg-rich phyllosilicates are modeled in abundances of up to 40-60%, while Al-rich phyllosilicates are modeled at ~20-40%. Geochemical analysis suggests alteration that occurred was not solely in-place in a closed system.

Finally, phyllosilicate-bearing material in the Nili Fossae region associated with metamorphic assemblages reveals evidence for the presence of a talc component. This spectral component is found to be associated with magnesium carbonates and olivine. We hypothesize this relationship and the limited occurrence of serpentine associated with these units indicates carbonation of serpentine was a localized process in this region. If this carbonation reaction was a widespread phenomenon in other olivine-rich protoliths, it may have been an important process in the ancient Martian carbon cycle and provided a sink for CO₂ in the past.