Modeling aqueous perchlorate chemistries with applications to Mars

NASA's Phoenix lander identified perchlorate and carbonate salts on Mars. Perchlorates are rare on Earth, and carbonates have largely been ignored on Mars following the discovery by NASA's Mars Exploration Rovers of acidic precipitated minerals such as jarosite. In light of the Phoenix results, we updated the aqueous thermodynamic model FREZCHEM to include perchlorate chemistry. FREZCHEM models the Na-K-Mg-Ca-Fe(II)-Fe(III)-Al-H-Cl-Br-SO4-NO3-OH-HCO3-CO3-CO2-O-2-CH4-Si-H2O system, with 95 solid phases. We added six perchlorate salts: NaClO4 center dot H2O, NaClO4 center dot 2H(2)O, KClO4, Mg(ClO4)(2)center dot 6H(2)O, Mg(ClO4)(2)center dot 8H(2)O, and Ca(ClO4)(2)center dot 6H(2)O. Modeled eutectic temperatures for Na, Mg, and Ca perchlorates ranged from 199 K (-74 degrees C) to 239 K (-34 degrees C) in agreement with experimental data. We applied FREZCHEM to the average solution chemistry measured by the Wet Chemistry Laboratory (WCL) experiment at the Phoenix site when soil was added to water. FREZCHEM was used to estimate SO42- and alkalinity concentrations that were missing from the WCL data. The amount of SO42- is low compared to estimates from elemental abundance made by other studies on Mars. In the charge-balanced solution, the dominant cations were Mg2+ and Na+ and the dominant anions were ClO4-, SO42-, and alkalinity. The abundance of calcite measured at the Phoenix site has been used to infer that the soil may have been subject to liquid water in the past, albeit not necessarily locally; so we used FREZCHEM to evaporate (at 280.65 K) and freeze (from 280.65 to 213.15 K) the WCL-measured solution to provide insight into salts that may have been in the soil. Salts that precipitated under both evaporation and freezing were calcite, hydromagnesite, gypsum, KClO4, and Mg(ClO4)(2)center dot 8H(2)O. Epsomite (MgSO4 center dot 7H(2)O) and NaClO4 center dot H2O were favored by evaporation at temperatures >0 degrees C, while meridianite (MgSO4 center dot 11H(2)O), MgCl2 center dot 12H(2)O, and NaClO4 center dot 2H(2)O were favored at subzero temperatures. Incongruent melting of such highly hydrated salts could be responsible for vug formation elsewhere on Mars. All K+ precipitated as insoluble KClO4 during both evaporation and freezing simulations, accounting for 15.8% of the total perchlorates. During evaporation, 35.8% of perchlorates precipitated with Na+ and 48.4% with Mg2+. During freezing, 58.4% precipitated with Na+ and 24.8% with Mg2+. Given its low eutectic temperature, the existence of Mg(ClO4)(2) in either case allows for the possibility of liquid brines on Mars today. FREZCHEM also showed that Ca(ClO4)(2) would likely not have precipitated at the Phoenix landing site due to the strong competing sinks for Ca as calcite and gypsum. Overall, these results help constrain the salt mineralogy of the soil. Differences between evaporites and cryogenites suggest ways to discriminate between evaporation and freezing during salt formation. Future efforts, such as sample return or in situ X-ray diffraction, may make such a determination possible. (C) 2009 Elsevier Inc. All rights reserved.