Water uptake by chlorate salts under Mars-relevant conditions

Chlorine is ubiquitous on Mars, some of it in the form of oxy-chlorine salts. Chlorine-containing salts have been found at several landing sites, including that of Phoenix and Curiosity, in the form of perchlorates and chlorides. Several intermediate states also exist, of which chlorate is the most stable. While perchlorates have received much attention in the past few years, chlorate salts are much less studied. The ratio of perchlorate to chlorate on Mars is not well-defined but may be approximately 1:1. Chlorate salts have similar properties to perchlorates: high solubility, low eutectic temperatures, and likely low deliquescence relative humidities. Laboratory studies were performed to determine the ability of sodium and magnesium chlorate salts to take up water vapor at low temperatures (296 K to 237 K). These studies were performed using a Raman microscope equipped with an environmental chamber and a single particle optical levitator equipped with a Raman spectrometer. The deliquescence of sodium chlorate (NaClO3) was found to be temperature-dependent with the average relative humidity (RH) values ranging from 68% RH at 296 K to 80% RH at 237 K. Additionally, there was a slight deviation between experimental deliquescence values for this salt and those predicted by equilibrium thermodynamics. The observed efflorescence (recrystallization) of NaClO3 occurred at lower RH values ranging from 18% RH at 264 K to 24% RH at 249 K, demonstrating the hysteresis common to salt recrystallization. Several experiments were performed below the reported eutectic temperature of NaClO3 which resulted in supercooling of the brine and depositional ice nucleation. Based on the supercooling effects observed during our experiments, a revised metastable eutectic temperature of 237 K is suggested for NaClO3 compared to the previously reported value of 252 K. Two phases of magnesium chlorate (Mg(ClO3)(2)) were observed and exhibited different water uptake behavior. The most common form of Mg(ClO3)(2) appeared to be a hydrated, amorphous phase, Mg(ClO3)(2) . X H2O(a) that continuously took up water when the RH was increased. This water uptake behavior was even observed at very low humidity values, 5.0 (+/- 1.9)% RH, with little temperature dependence. This detectable water persisted down to RH values close to 0%, averaging 0.5 (+/- 0.6)% RH with no visible temperature dependence. The deliquescence relative humidity (DRH) of the hexahydrate, Mg(ClO3)(2) . 6 H2O, was found to range from 50.9 (+/- 7.5)% at 227 K to 55.8 (+/- 6.6)% at 224 K and was consistent with thermodynamic calculations. Under conditions measured by the Remote Environmental Monitoring Station (REMS) instrument at Gale Crater and conditions modeled in the shallow subsurface, magnesium chlorate, if present, likely interacts with water vapor during some diurnal cycles.

Automated summary

Chlorine in various forms is widespread on Mars, with perchlorates and chlorides being commonly found. While perchlorates have been extensively studied, chlorate salts have received less attention. Laboratory studies revealed differences in water uptake behavior between sodium chlorate and magnesium chlorate salts at different temperatures, suggesting temperature-dependent deliquescence properties. Observations also showed the potential interaction of magnesium chlorate with water vapor under specific environmental conditions.