Environmental Changes Recorded in Sedimentary Rocks in the Clay-Sulfate Transition Region in Gale Crater, Mars: Results From the Sample Analysis at Mars-Evolved Gas Analysis Instrument Onboard the Mars Science Laboratory Curiosity Rover.

Abstract

The Curiosity rover explored the region between the orbitally defined phyllosilicate-bearing Glen Torridon trough and the overlying layered sulfate-bearing unit, called the "clay-sulfate transition region." Samples were drilled from the top of the fluviolacustrine Glasgow member of the Carolyn Shoemaker formation (CSf) to the eolian Contigo member of the Mirador formation (MIf) to assess in situ mineralogical changes with stratigraphic position. The Sample Analysis at Mars-Evolved Gas Analysis (SAM-EGA) instrument analyzed drilled samples within this region to constrain their volatile chemistry and mineralogy. Evolved H2O consistent with nontronite was present in samples drilled in the Glasgow and Mercou members of the CSf but was generally absent in stratigraphically higher samples. SO2 peaks consistent with Fe sulfate were detected in all samples, and SO2 evolutions consistent with Mg sulfate were observed in most samples. CO2 and CO evolutions were variable between samples and suggest contributions from adsorbed CO2, carbonates, simple organic salts, and instrument background. The lack of NO and O2 in the data suggest that oxychlorines and nitrates were absent or sparse, and evolved HCl was consistent with the presence of chlorides in all samples. The combined rover data sets suggest that sediments in the upper CSf and MIf may represent similar source material and were deposited in lacustrine and eolian environments, respectively. Rocks were subsequently altered in briny solutions with variable chemical compositions that resulted in the precipitation of sulfates, carbonates, and chlorides. The results suggest that the clay-sulfate transition records progressivelydrier surface depositional environments and saline diagenetic fluid, potentially impacting habitability.