Freezing of perchlorate and chloride brines under Mars-relevant conditions

by Primm, K. M.; Gough, R. V.; Chevrier, V. F.; Tolbert, M. A.

Perchlorate and chloride salts on Mars could readily absorb water vapor and deliquesce into aqueous solutions. The deliquescence relative humidity (RH) as well as the efflorescence (recrystallization) RH of several Mars-relevant salts are now well known; however, the conditions that could cause a brine to freeze are not well established. It is often assumed that ice formation will occur whenever the saturation with respect to ice, S-ice, of the system is greater than or equal to unity; however, ice nucleation is often hindered due to a kinetic barrier. For ice to form, a critical cluster of the ice crystal must first be achieved, often requiring S-ice > 1. Here we use a Raman microscope and an environmental cell to examine the RH and temperature conditions required for Mg(ClO4)(2) and MgCl2 brines to freeze into ice. By examining the salt phase present both optically and spectrally under different low temperature conditions, it is found that both salts exhibit S-ice values much greater than unity, meaning that supersaturation readily occurs and brines can persist under conditions previously thought to lead to freezing. The RH range of ice formation for Mg(ClO4)(2) from 218 to 245 K is 83-95%, respectively, corresponding to S-ice = 1.30-1.54. The RH of ice formation for MgCl2 ranges from 80 to 100% for temperatures between 221 and 252 K, corresponding to S-ice = 1.30-1.35. In addition to ice nucleation, the deliquescence and efflorescence relative humidity values for MgCl2 were determined. Two hydrates for MgCl2 were observed, and exhibited different deliquescence relative humidity (DRH) values. The DRH for MgCl2.4H(2)O was found to be 12.8 +/- 0.3% at 243 K with slightly increasing DRH as temperature decreased. The DRH for MgCl2.6H(2)O was found to be 31.3 +/- 0.6% at 242 K with little temperature dependence. The DRH of MgCl2.6H(2)O was measured below the previously reported eutectic, 240 K, suggesting that the eutectic might be incorrect or that there is a different relevant hydration state. The aqueous solutions of MgCl2 recrystallized to the tetrahydrate at low RH in the range of 3-9% RH at temperatures 265-235 K. Together, the ice nucleation, DRH, and efflorescence relative humidity (ERH) results show that Mg(ClO4)(2) and MgCl2 brines on present day Mars may have the ability to exist for up to 2 h longer than previously predicted.

Geochimica Et Cosmochimica Acta
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