High-temperature Hydrogen Chloride Releases from Mixtures of Sodium Chloride with Kieserite: Implications for the Chlorine-mineralogy as Determined by the Sample Analysis at Mars Instrument on the Curiosity Rover in Gale Crater, Mars

Abstract:

The Sample Analysis at Mars (SAM) instrument on the Curiosity rover has detected evolved hydrogen chloride (HCl) from several Gale Crater samples. Evolved HCl can be derived at low temperatures by the thermal decomposition of (per)chlorate, and at high temperatures (>500°C) through the reaction of melting chlorides (original or from (per)chlorate decomposition) with evolved water from the thermal decomposition of phyllosilicates. Evolved HCl was detected from some samples despite the absence of (per)chlorates and/or limited to no high temperature (>500°C) evolved water. The goal of this work was to explain the evolved HCl detections in the absence of (per)chlorate and high temperature water evolving phases. This work specifically evaluated the role of evolved SO₂ in the production of HCl. Mixtures of NaCl and kieserite, a Mars analog Mg sulfate, were prepared using two ratios: 1) a Mars-like ratio based on data from Curiosity, and 2) an equivalent ratio. Both mixtures were analyzed using a thermal gravimeter (TG)/differential scanning calorimeter (DSC) connected to a quadrupole mass spectrometer, configured to operate similarly to SAM evolved gas analysis (EGA). Samples were also analyzed using heated X-ray diffraction (XRD). NaCl and kieserite analyzed by themselves did not evolve HCl, however both NaCl/kieserite mixtures evolved HCl similar to several SAM samples. EGA, DSC, and heated XRD results indicated that kieserite caused an increase in HCl production from NaCl via two mechanisms: 1) kieserite depressed the melting point of NaCl and 2) SO₂ from kieserite decomposition reacted with NaCl and water (Hargreaves reaction). Results from this work demonstrate that (per)chlorates are not required for SAM-EGA detection of HCl, and provides further evidence for chlorides in Gale Crater. The presence of chlorides in Gale Crater materials can provide information about the chlorine cycle on Mars and about the aqueous alteration conditions in past depositional and diagenetic environments.