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Widely distributed perchlorate on the Martian surface and over three orders of magnitude variation in bromine abundances in surface samples are difficult to explain solely by chloride and bromide aqueous geochemistry. New experiments show that photochemical oxidation (ultraviolet wavelength 254 nm) of chloride- and bromide-bearing evaporative brines in the presence of silica beads produces substantial perchlorate (ClO − 4 ), chlorate (ClO − 3 ), and bromate (BrO − 3 ) under conditions relevant to Mars. Neutral to alkaline aqueous environments result in the dominance of chlorate over perchlorate. Preferential atmospheric recycling of Br over Cl causes variable Br/Cl ratios, consistent with numerous in-situ measurements of Cl and Br abundances on Mars. Bromate reacts with organics more readily than chlorate or perchlorate, and its presence in subsurface brines could challenge habitability in the Martian subsurface.

Benjamin Fernando and colleagues report on the international cooperation involved InSight's attempt to gather seismic data from the arrival at Mars of China's Zhurong rover.

The behaviors of bromide, chloride, and phosphate were studied experimentally under previously proposed Martian diagenetic conditions, involving jarosite (KFe3(OH)6(SO4)2), goethite (α-FeOOH), and hematite (α-Fe2O3). Experiments evaluated (1) the behavior of Cl−/Br− with and without aqueous phosphate during oxidation of Fe2+ to Fe3+, (2) the stability of halogen-bearing jarosite, and (3) the uptake of Cl−, Br−, H2PO4−, and SO42− 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−, SO42−, and H2PO4− on halogen-free-hematite, -goethite, and -jarosite demonstrates that in a sulfate-dominant aqueous system, Cl−, Br−, and H2PO4− could not compete with SO42−. 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.