Behavior of bromide, chloride, and phosphate during low‐temperature aqueous Fe(II) oxidation processes on Mars

The behaviors of bromide, chloride, and phosphate were studied experimentally under previously proposed Martian diagenetic conditions, involving jarosite (KFe 3 (OH) 6 (SO 4 ) 2 ), goethite ( α ‐FeOOH), and hematite ( α ‐Fe 2 O 3 ). Experiments evaluated (1) the behavior of Cl − /Br − with and without aqueous phosphate during oxidation of Fe 2+ to Fe 3+ , (2) the stability of halogen‐bearing jarosite, and (3) the uptake of Cl − , Br − , H 2 PO 4 − , and SO 4 2− by halogen‐free‐hematite, ‐goethite, and ‐jarosite through adsorption. Our results demonstrate that when precipitated from a solution, in which Cl − is higher than Br − , jarosite preferentially incorporated at least an order of magnitude more Br − than Cl − . Such enrichment of Br − over Cl − in the solids compared to initial solutions suggests that jarosite could be a host for elevated Br on the Martian surface, and the fluids from which jarosite forms could be depleted in Br − with respect to Cl − . Moreover, the incorporation of halogens in jarosite would affect its stability during aqueous alteration, and the dissolution rates of four types of jarosite at both 25°C and 70°C were in the same order: Br,Cl bearing > Br only > halogen free > Cl only. In addition, competitive adsorption of Cl − , Br − , SO 4 2− , and H 2 PO 4 − on halogen‐free‐hematite, ‐goethite, and ‐jarosite demonstrates that in a sulfate‐dominant aqueous system, Cl − , Br − , and H 2 PO 4 − could not compete with SO 4 2− . This observation suggests that the adsorption may not result in an enrichment of phosphate or halogens in Fe oxides in a sulfate‐dominant aqueous system like Meridiani Planum, consistent with the absence of significant correlations of Cl and P with nanoparticle Fe oxides found in Martian soils.