Deliquescence and efflorescence of calcium perchlorate: An investigation of stable aqueous solutions relevant to Mars

by Nuding, D. L.; Rivera-Valentin, E. G.; Davis, R. D.; Gough, R. V.; Chevrier, V. F.; Tolbert, M. A.

Calcium perchlorate (Ca(ClO4)(2)) is a highly deliquescent salt that may exist on the surface of present-day Mars; however, its water uptake properties have not been well characterized at temperatures and relative humidity conditions relevant to Mars. Here, we quantify the deliquescent relative humidity (DRH) and efflorescent relative humidity (ERH) of Ca(ClO4)(2) as a function of temperature (223-273 K) to elucidate its behavior on the surface of Mars. A Raman microscope equipped with an environmental cell was used to simulate Mars relevant temperature and relative humidity conditions and monitor deliquescence (solid to aqueous) and efflorescence (aqueous to solid) phase transitions of Ca(ClO4)(2). Deliquescence and efflorescence were monitored visually using optical images and spectroscopically using Raman microscopy. We find that there is a wide range of deliquescence RH values between 5% and 55% RH. This range is due to the formation of hydrates in different temperatures regimes, with the higher DRH values occurring at the lowest temperatures. Experimental deliquescence results were compared to a thermodynamic model for three hydration states of Ca(ClO4)(2). The model predicts that the higher hydration states deliquesce at a higher RH than the lower hydration states. Calcium perchlorate was found to supersaturate, with lower ERH values than DRH values. The ERH results were less dependent on temperature with an average 15 +/- 4%, but values as low as 3 +/- 2% were measured at 273 K. Levitation experiments were performed on single particles of Ca(ClO4)(2) and Mg(ClO4)(2) at 298 K. While efflorescence was observed around 15% RH for Mg(ClO4)(2), the efflorescence of Ca(ClO4)(2) was not observed, even when exposed to 1% RH at 298 K. Additionally, a 17-h experiment was conducted to simulate a martian subsurface diurnal cycle. This demonstrated Ca(ClO4)(2) aqueous solutions can persist without efflorescing for the majority of a martian sol, up to 17 h under Mars temperature heating rates and RH conditions. We find that Ca(ClO4)(2) aqueous solutions could persist for most of the martian sol under present-day conditions. The aqueous phase stability and metastability quantified for Ca(ClO4)(2) under Mars relevant temperature and relative humidity conditions has important implications for the water cycle and the stability of liquid water on present day Mars.

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1090-2643; 0019-1035