Dense Eutectic Brines on Mars: They Could Be Both Common and Ca-rich

Introduction: The recent, somewhat controversial observation of young gullies apparently related to seepage or flow of liquid water on Mars [1] has raised the question of whether the fluids responsible (if any) might be concentrated, CaCl2-rich brines, which alone among aqueous fluids have the low vapor pressures and depressed freezing points [2,3] to be stable at the intermediate to high latitudes at which the distinctive gullies have been observed. The gullies invariably originate high on slopes, implying a perched aquifer. Even if the cause of these gullies is not brines (and many other possibilities have been suggested), we submit that dense, eutectic, CaCl2-rich brines would still be expected deep in the basaltic martian regolith, as a natural consequence of the presumably wet early history of Mars, followed by devolatilization, freezedown, and water-rock interaction. Evidence Against CaCl2-rich Brines: Mars apparently lacks evaporites. No evaporite deposits have yet been unambiguously observed on Mars [4]. However, evaporite minerals are difficult or impossible to detect if etched, coated with dust, or mixed with lithic or soil materials. Large evaporite deposits on Earth form in shallow basins on the edges of continental platforms, mainly as a result of plate tectonic processes [5], an environment presumably lacking on Mars. About 90% evaporation is needed before halite precipitates from terrestrial sea water; by the time this degree of evaporation had occurred on Mars, brines could have disappeared into the regolith. Mars has sulfate-rich soils and a CO2-rich atmosphere. The Viking and Pathfinder missions to Mars both performed chemical (but not mineralogic) analyses of soils in the vicinity of the landers. The soils, particularly an indurated soil called duricrust, are Srich, with less Cl [e.g., 6]. The S was assumed to be sulfate, which forms insoluble salts (gypsum or anhydrite) with Ca, apparently ruling out Ca-rich brines. Although Mg forms soluble sulfates, its carbonates (like those of Ca) are insoluble, and the Mars atmosphere is CO2-rich. The implication is that Mars brines could contain neither Ca nor Mg [7]. This result, widely accepted for the past 20 years, is based on two assumptions that are probably erroneous. These are 1) that a subsurface brine must be in equilibrium with the atmosphere, and 2) that the salt composition of the duricrust reflects that of a subsurface brine. The first assumption is patently false for virtually any terrestrial groundwater (e.g., those that deposit supergene sulfides in ore deposits). In reference to the second, duricrust forms not by simple quantitative evaporation of brine, but by water rising to the surface by capillary action. Terrestrial duricrust typically contains the least soluble salts (e.g., calcite and gypsum); its composition therefore need not reflect the content of the more soluble salts (i.e., chlorides) that remain in solution at depth. Mars can be modeled as a closed system. Simple evaporation or freezing models subtract the compositions of first H2O (as vapor or ice), then H2O plus crystallizing salts; the system is a sealed, inert bathtub, open to the atmosphere. Ca is removed early as sulfates or carbonates, and therefore is predictably absent in the last brine to freeze, at the eutectic point [8]. The problem is that once a brine has sunk into the regolith, or is covered with a layer of ice or of low-density fresh water, it is out of equilibrium with the atmosphere. Furthermore, its container is not inert, but is highly reactive, Ca-rich (basaltic) regolith, consisting of a brecciated mixture of impact melts, volcanic rocks, and other fines. Given the billions of years available, the brine will certainly exchange Na or Mg for some of the Ca in the rocks, making it Ca-rich. Similar reactions are common on Earth, both in Ca-rich brines that have reacted with sediments [9], and in seawater that has circulated through mid-ocean ridge basalts [10].