Thermal and Evolved Gas Analysis of Magnesium Perchlorate: Implications for Perchlorates in Soils at the Mars Phoenix Landing Site

Introduction: Perchlorate salts were discovered in the soils around the Phoenix landing site on the northern plains of Mars [1]. Perchlorate was detected by an ion selective electrode that is part of the MECA Wet Chemistry Laboratory (WCL). The discovery of a mass 32 fragment (likely O2) by the Thermal and Evolved-Gas Analyzer (TEGA) provided additional confirmation of a strong oxidizer in the soils around the landing site. The purpose of this paper is to evaluate the thermal and evolved gas behavior of perchlorate salts using TEGA-like laboratory testbed instruments. TEGA ovens were fabricated from high purity Ni. Hence, an additional objective of this paper is to determine the effects that Ni might have on the evolved gas behavior of perchlorate salts. Materials and Methods: Mg-perchlorate was chosen as the test candidate because it is a leading candidate for the perchlorate salt in the Phoenix soils [1]. The first set of perchlorate experiments utilized a mixture of organic free SiO2 (i.e., quartz heated to 1000C in O2 for 12 hrs) and reagent grade Mg(ClO4)26H2O. The composition of this mixture was 90.14 wt. % SiO2 and 9.86 wt. % Mg(ClO4)26H2O. Another set of experiments were conducted using high purity Ni (99.9 % Ni) mixed with the Mg-perchlorate. The composition of the material was 50.17 wt. % organic-free SiO2, 47.00 wt. % Ni, and 2.83 wt. % Mg(ClO4)26H2O. The thermal and evolved gas laboratory testbed consisted of a Setaram SENSYS EVO differential scanning calorimeter (DSC) integrated with a Pfeiffer quadrapole mass spectrometer. The DSC has an operating temperature range -120C to 830°C. However the current set of experiments were run from ambient temperature to 820C, 275 mbar oven pressure, ultrapure N2 carrier gas, 10 sccm gas flow rate, and 20°C/min temperature ramp rate. The mass spectrometer has a 300 AMU range. The mass spectrometer data was collected for a predetermined set of individual masses (i.e., mass hopping) and recorded as a function of time rather than a full range mass scan. The carrier gas was allowed to flow through the system for approximately one hour before starting the oven ramp to reach a steady gas state and remove any trace gas contaminants. Samples were reheated after the initial temperature ramp to 820°C to establish the baseline data for the DSC data. Results: Reagent grade Mg-perchlorate has structural water and perchlorates are known to be notorious water adsorption media. Water evolved at an onset temperature of 85°C, which, corresponds to a well-defined endothermic peak (Fig. 1). Two additional water releases with corresponding endothermic peaks had onset temperatures of 155°C and 195°C. A strong exothermic reaction had an onset temperature at 435°C (Fig. 1). This exothermic reaction corresponds to the release of Cl species (Fig. 2) and O2 (Fig. 3). The release of the volatiles results in the formation of MgO and hence, the phase transition responsible for the exothermic reaction.