The effects of instrument parameters and sample properties on thermal decomposition: interpreting thermal analysis data from Mars

Abstract

Thermal analysis instruments have been used on Mars by the Viking, Phoenix, and MSL missions. These instruments can be very useful in identifying volatile-bearing minerals such as carbonates, sulfates, or phyllosilicates down to very low abundances. Mineral identification is done by comparing thermal decomposition behavior of samples with known mineralogy to samples with unknown mineralogy. However, thermal decomposition behavior can change with instrument conditions such as pressure and sample properties such as particle size. The Mars instruments flown to date have used much lower pressures and flow rates than traditional laboratory experiments. The objective of this work was to investigate whether an analytical model based on equilibrium thermodynamics can accurately predict changes in decomposition temperature in instruments operating under lower pressure/flow conditions. We find that while the model predicts the general trend that decomposition temperature drops with decreasing pressure, the difference between modeled and measured temperatures can be on the order of 100{\textdegree}C for carbonates and sulfates. These differences can be explained by factors such as sample particle size, carrier gas species, gas flow rate, and oven volume. A calcium carbonate sample shows how particle size can change decomposition temperature by almost 200{\textdegree}C (decomposition temperatures decrease with decreasing particle size) and that carrier gas species, flow rate, and instrument geometry can affect decomposition temperatures by 20-50{\textdegree}C. These results demonstrate that predicting changes in decomposition temperature based on a thermodynamic or empirical model is not sufficient and that samples must be run under instrument conditions relevant to the instrument that produced the data on Mars. Furthermore, the effects of particle size, carrier gas species and flow rate, as well as instrument geometry must be taken into account in order to compare Mars data to samples run in terrestrial labs. This work shows the magnitude of these factors, demonstrating why they must be taken into account, providing a framework for how to correctly interpret thermal analysis data from Mars.