Effect of Hydration State of Martian Perchlorate Salts on Their Decomposition Temperatures During Thermal Extraction

Three Mars missions have analyzed the composition of surface samples using thermal extraction techniques. The temperatures of decomposition have been used as diagnostic information for the materials present. One compound of great current interest is perchlorate, a relatively recently discovered component of Mars' surface geochemistry that leads to deleterious effects on organic matter during thermal extraction. Knowledge of the thermal decomposition behavior of perchlorate salts is essential for mineral identification and possible avoidance of confounding interactions with organic matter. We have performed a series of experiments which reveal that the hydration state of magnesium perchlorate has a significant effect on decomposition temperature, with differing temperature releases of oxygen corresponding to different perchlorate hydration states (peak of O2 release shifts from 500 to 600°C as the proportion of the tetrahydrate form in the sample increases). Changes in crystallinity/crystal size may also have a secondary effect on the temperature of decomposition, and although these surface effects appear to be minor for our samples, further investigation may be warranted. A less than full appreciation of the hydration state of perchlorate salts during thermal extraction analyses could lead to misidentification of the number and the nature of perchlorate phases present. Key Points The hydration state of perchlorates has a significant effect on their decomposition during thermal extraction Variable hydration states may lead to incorrect interpretations of cation composition and falsely indicate the presence of multiple components MSL data may show evidence of various, possibly metastable, hydration states Plain Language Summary Missions to Mars look for evidence of organic molecules using thermal extraction techniques. Certain minerals, perchlorate salts, on the Martian surface break down during this heating, releasing oxygen and causing the combustion and destruction of both themselves and any organic matter which may have been present. Previously, the parent salts of these perchlorates have been identified by the temperature at which they release oxygen. However, our work shows that the hydration state of these minerals can affect the temperature of oxygen release just as much as the perchlorate cation. Consequently, incorrect identification of mineral species may occur if hydration state is not taken into account and a mixture of hydration states of one t