CHARACTERIZING SECONDARY X-RAY AMORPHOUS MATERIALS ON MARS VIA MISSION DATA AND ANALOG STUDIES

Abstract

X-ray amorphous materials (i.e., lacking long-range crystallographic order) have been identified on the martian surface via orbital and in situ measurements, and their composition and formation mechanisms have been a subject of debate for two decades. Hypotheses for their origins include volcanism, impacts, and aqueous processes. We use orbital visible/near-infrared (VNIR) and thermal infrared (TIR) spectroscopy, in-situ measurements, and laboratory analyses of synthetic and natural amorphous materials to identify amorphous materials on the martian surface and constrain their formation environments to better understand Mars’ aqueous past. Models of orbital TIR data from the Thermal Emission Spectrometer indicate broad regions on Mars contain amorphous silicates, and laboratory VNIR and TIR spectral measurements of poorly crystalline aluminosilicates, like allophane and imogolite, and volcanic glass leached by acidic fluids have allowed for their identification in many of these regions. The CheMin X-ray diffractometer on the Mars Science Laboratory Curiosity rover has identified X-ray amorphous materials in abundances of ~10-70 wt.% in all samples measured to date, including ancient fluvio-lacustrine and eolian rocks and modern eolian sediments. Mass balance calculations using CheMin mineral abundances and bulk chemical composition from the Alpha Particle X-ray Spectrometer indicate the amorphous components are variably enriched in Si, Fe, and S, with bulk compositions inconsistent with glass alone, suggesting some fraction of the amorphous materials formed via aqueous alteration. Our studies of mafic sediments in Mars-analog proglacial environments on Earth demonstrate that secondary amorphous materials are common in these settings and are variably enriched in Si, Fe, and Al. Data from Mars and Earth indicate that amorphous materials on the martian surface are heterogeneous in composition, likely resulting from different alteration processes and provenance variability, and may have formed via limited water-rock interactions during the last gasps of liquid water on the planet.