Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures

Abstract

Perchlorate salts, known to exist on Mars, can readily absorb water vapor and deliquesce into aqueous solutions even at low temperatures. The multiple soluble ionic species, such as chloride salts, present in the Martian subsurface may affect this deliquescence. Here we study the deliquescence (solid to aqueous transition) and efflorescence (aqueous to solid transition) of three perchlorate/chloride mixtures: KClO4/KCl at 253 K, NaClO4/NaCl at 243 and 253 K, and Mg(ClO4)2/MgCl2 at 243 and 253 K. A Raman microscope with an environmental cell was used to monitor the phase transitions of internally mixed ClO−4/Cl− particles as a function of the perchlorate mole fraction. The eutonic relative humidity (where deliquescence begins to occur regardless of ClO−4 mole fraction), deliquescence relative humidity (DRH, where complete deliquescence occurs), and efflorescence relative humidity (ERH) were measured for several perchlorate mole ratios for each cation system. At the temperatures studied, the eutonic relative humidity was measured to be 28% RH for Mg(ClO4)2/MgCl2 mixtures, 38% RH for NaClO4/NaCl mixtures, and 82% RH for KClO4/KCl mixtures. The DRH depends on the perchlorate mole ratio, but is below the DRH of the least deliquescent (highest DRH) pure salt. When humidity is lowered around an aqueous salt mixture, we find that efflorescence occurs at an RH below the DRH due to the kinetic inhibition of crystallization. The ERH values of the salt solutions were as low as 5% RH for Mg(ClO4)2/MgCl2 mixtures, as low as 13% RH for NaClO4/NaCl mixtures, and as low as 66% RH for KClO4/KCl mixtures. The low eutonic RH values for the Na+ and Mg2+ perchlorate/chloride mixtures are important: wherever Mg(ClO4)2 and MgCl2 or NaClO4 and NaCl coexist at the temperatures studied, mixtures will contain a stable aqueous phase above 28 or 38% RH, respectively, regardless of the perchlorate mole fraction. This liquid water may persist until 5 or 13% RH, respectively.