Thermal and Evolved Gas Behavior of Calcite under Mars Phoenix Tega

Introduction: The Mars Phoenix Scout Mission with its diverse instrument suite successfully examined several soils on the Northern plains of Mars. The Thermal and Evolved Gas Analyzer (TEGA) was employed to detect organic and inorganic materials by coupling a differential scanning calorimeter (DSC) with a magnetic-sector mass spectrometer (MS). Martian soil was heated up to 1000°C in the DSC ovens and evolved gases from mineral decomposition products were examined with the MS. TEGA’s DSC has the capability to detect endothermic and exothermic reactions during heating that are characteristic of minerals present in the Martian soil. Initial TEGA results indicated the presence of endothermic peaks with onset temperatures that ranged from 675°C to 750°C with corresponding CO2 release. This result suggests the presence of calcite (CaCO3 CaO + CO2) [1]. Organic combustion to CO2 is not likely since this mostly occurs at temperatures below 550°C. Fe-carbonate and Mg-carbonate are not likely because their decomposition temperatures are less than 600°C. TEGA enthalpy determinations suggest that calcite, may occur in the Martian soil in concentrations of ~1 to 5 wt. % [1]. The detection of calcite could be questioned based on previous results that suggest Mars soils are mostly acidic (e.g., [2,3]). However, the Phoenix landing site soil pH was measured at pH 8.3±0.5 [4], which is typical of terrestrial soils where pH is controlled by calcite solubility [5]. The range of onset temperatures and calcite concentration as calculated by TEGA is poorly constrained in part because of limited thermal data of calcite at reduced pressures. TEGA operates at <30 mbar while most calcite literature thermal data was obtained at 1000 mbar or higher pressures. Past work [6,7,8] has indicated that as total pressure decreases, the onset temperature and enthalpy for endothermic reactions can decrease and increase, respectively. Proper identification of calcite onset temperatures in the TEGA data set requires that laboratory analysis of calcite and other carbonates be evaluated at TEGA operating conditions (e.g., ≤30 mbar, ~1 sccm flow rate). Furthermore, the enthalpy (J/g) of calcite decomposition at ≤30 mbar must be evaluated to ensure accurate calculations of calcite concentrations based on TEGA enthalpies. The objectives of this work are to (1) assess the onset temperature and (2) enthalpy of calcite decomposition at TEGA pressures (30 mbar). Results of this work will enhance the ability to assess the true onset temperature and enthalpy of calcite decomposition in the TEGA data sets.