Did Mars' Soil Salt Come From Evaporation or Freezing?

Modeling salt precipitation from brines on Mars: Evaporation versus freezing origin for soil salts

Authors:

Toner et al

Abstract:

Perchlorates, in mixture with sulfates, chlorides, and carbonates, have been found in relatively high concentrations in martian soils. To determine probable soil salt assemblages from aqueous chemical data, equilibrium models have been developed to predict salt precipitation sequences during either freezing or evaporation of brines. However, these models have not been validated for multicomponent systems and some model predictions are clearly in error. In this study, we built a Pitzer model in the Na–K–Ca–Mg–Cl–SO4–ClO4–H2O system at 298.15 K using compilations of solubility data in ternary and quaternary perchlorate systems. The model is a significant improvement over FREZCHEM, particularly for Na–Mg–Cl–ClO4, Ca–Cl–ClO4, and Na–SO4–ClO4 mixtures. We applied our model to the evaporation of a nominal Phoenix Lander Wet Chemistry Laboratory (WCL) solution at 298.15 K and compare our results to FREZCHEM. Both models predict the early precipitation of KClO4, hydromagnesite (3MgCO3·Mg(OH)2·3H2O), gypsum (CaSO4·2H2O), and epsomite (MgSO4·7H2O), followed by dehydration of epsomite and gypsum to kieserite (MgSO4·H2O) and anhydrite (CaSO4) respectively. At low residual water contents, our model predicts the precipitation of halite (NaCl), NaClO4·H2O, and Mg(ClO4)2·6H2O, whereas halite and NaClO4·H2O never precipitate in FREZCHEM. Our model predicts that calcite does not precipitate from evaporating WCL solutions at 298.15 K, which conflicts with other evidence for calcite in Phoenix soils. Previous studies that modeled freezing of WCL solutions found that calcite does form. Furthermore, our model predicts that ∼0.3 wt.% H2O is held in hydrated salts after the WCL solution has completely evaporated at 298.15 K, whereas previous studies have found that ∼1.3 wt.% H2O is held in hydrated salts if WCL solutions freeze. Given minimum water contents in Mars soils of 1.5–2 wt.% H2O measured from orbital spectra and in situ measurements, our modeling results suggest that salts at the Phoenix site were not formed during evaporation near 298.15 K, whereas formation during freezing remains possible.