FLUID CHEMISTRIES IN GALE CRATER CALCULATED BASED ON IN-SITU GEOCHEMISTRY AND MINERALOGY FROM THE MARS CURIOSITY ROVER

For much of its initial year on Mars, the Curiosity rover deployed its geochemical instruments to investigate an area called Yellowknife Bay, less than half a km from its arrival point at Bradbury Landing, in Gale Crater. Grotzinger et al. (2014) summarize the observations of elemental analyses, gas geochemistry, depositional features, and mineralogy and conclude that the site is an ancient lake deposit, characterized by neutral to slightly alkaline pH, low salinity, supporting variable redox states of both iron and sulfur. Vaniman et al. (2014) used X-ray diffraction data to determine the mineralogy of 3 mudstones at the site, concluding that the clay minerals present formed at temperatures less than 60–80 °C, during burial and diagenesis.

We used TOUGHREACT V3.0-OMP, a non-isothermal, multi-component, reactive transport code (Sonnenthal et al., 2014) to constrain the range of compositions of fluids and secondary mineral phases that would develop from reactions with primary (non-alteration) mineral phases detected in the mudstones. In our approach, we assumed an initial atmosphere and a nominal dilute fluid chemistry, and we fixed the mineralogy based on in siturover analyses. Simulations were conducted using a single grid block (i.e., no flow), and oxygen fugacity was buffered by the mineral assemblage. We modeled primary and clay minerals as solid solutions of end-member phases and assumed a volume ratio of 40% water:60% mineral for these batch reactions. We applied nominal reactive surface areas, assigning proportionally large values for clay minerals and smaller values for other mineral phases. We used thermodynamic data and kinetic rate constants from published literature sources.

We focused on reaction series at 10° and 40°C and bracketed geochemical compositions of near-surface waters that may have been present on Mars. Reaction of primary mineral phases with water produced clay and Fe-oxide phases in agreement with observed mineralogy. Both reaction temperatures generated bicarbonate-rich fluids with near-neutral pH over the simulated reaction times of 1,000 to 10,000 years.

References: Grotzinger JP et al. (2014) Science 343:1242777; Sonnenthal EL et al. (2014) http://esd.lbl.gov/research/projects/tough/software/toughreact.html; Vaniman DT et al. (2014) Science 343:1243480.