Brine Pools With Round Caps in the Western Qaidam Basin: Application to Iron Mobility and the Evaporation Formation of the Iron Oxide and Sulfate Assemblage on Mars

The coexistence of sulfates and iron oxides is widespread on Mars, including at Meridiani Planum, the Opportunity rover's landing site. However, the formation mechanism is still poorly understood. Here, we report evaporitic iron oxyhydroxides and gypsum deposited as round caps in the center of brine pools in a playa area of the western Qaidam Basin in the northern Tibetan Plateau. Geochemical and mineralogical analyses indicate that the gypsum is precipitated in situ by strong evaporation of subsurface percolating brine containing dissolved iron. We suggest that the transport of iron from the anoxic subsurface and the evaporation of upwelling groundwater in an oxidized and hyperarid environment may have led to the co-deposition of sulfates and iron oxides at Meridiani Planum and other similar sites on Mars. Our results also suggest that the subsurface water cycle could be an important source of iron mobility even under an oxidizing atmosphere and cold climatic conditions on Mars. Groundwater evaporation during the desiccation of Mars after the late Noachian may have contributed to the heterogeneous distribution of iron and the widespread coexistence of sulfates and iron oxides on the martian surface. Surface geological processes under various climate conditions can produce different types of secondary minerals, such as clay minerals, sulfates, and iron oxides. The early Mars likely experienced warm and wet periods leading to extensive chemical weathering and the formation of clay minerals. When conditions on the surface of Mars became cold and dry, the water cycle and chemical weathering almost ceased, and intense evaporation resulted in the formation of evaporite minerals, mainly sulfates. Under these conditions, the surface iron mobility should have been inhibited. However, Mars hosts extensive sulfate deposits that often coexist with iron oxides, and the underlying process remains unclear. Through an analogous study in the western Qaidam Basin, we demonstrate that groundwater activity can sustain an active subsurface iron cycle even under the cold and hyperarid surface conditions of Mars. The intense evaporation of upwelling groundwater could have led to the formation of terrains rich in sulfates and iron oxides. Our findings contribute to a better understanding of geochemical processes in hyperarid climates on Mars. The coexistence of iron oxyhydroxides and sulfates was detected in a playa area of the western Qaidam Basin, a Mars analog environment Evaporation of upwelling groundwater may have led to the widespread coexistence of sulfates and iron oxides on Mars Groundwater activity favors Fe mobility even under an oxidizing atmosphere and in cold surface environments on Mars