Water‐Limited Hydrothermalism and Volcanic Resurfacing of Eridania Basin, Mars

The Eridania Basin system, located in the martian southern highlands, may have hosted long‐lived lacustrine hydrothermalism. Despite its multi‐basin scale and suggested longevity that should pervasively alter the landscape, evidence for consistent compositional signatures of lacustrine hydrothermalism across varying spatial scales is poorly known. Here we synthesize orbital data sets (derived from the Gamma Ray Spectrometer, GRS; Thermal Emission Spectrometer, TES; and Compact Reconnaissance Imaging Spectrometer for Mars, CRISM) to characterize the composition of Eridania's landscape from regional to outcrop scales. The bulk geochemistry representing decimeter depths is broadly on par with mafic igneous compositions seen elsewhere on Mars. The landscape‐scale mineralogy of Eridania suggests a compositional distinction between Eridania's northernmost basin and the remaining basins. Meanwhile, chloride mapping and analyses of amorphous silica and acid‐altered clay phases at the outcrop scale both show that mineralogical evidence for hydrothermal alteration and putative lacustrine activity is broadly limited to the northernmost basin, and that most other amorphous silica in Eridania is consistent with low‐to‐minimal water/rock weathering. This suggests either limited regional hydrothermalism or extensive mafic resurfacing occurred in Eridania, the latter of which is consistent with previous studies. Our findings collectively suggest that Eridania underwent local crystal hydrothermal alteration, perhaps confined to the northernmost basin, while much of the remainder of the Eridania's basins either experienced low‐temperature weathering or extensive resurfacing to at least meter scale depths by later volcanism. This supports prior inferences of spatially and temporally limited surface hydrology on Mars and extensive volcanic resurfacing of basins, as in the Gusev Crater. Eridania Basin is a system of semi‐circular depressions in the Southern Highlands of Mars that is suggested to have once held a vast lake. This lake is inferred to have hosted hydrothermal activity for hundreds of millions of years. Such a long‐lived system should have led to mineralogical and geochemical signatures on large regional scales. Analysis of regional, local, and outcrop‐scale data—using orbital gamma ray, thermal, and near infrared spectra, respectively—indicates that the geochemistry of Eridania is consistent with relatively unaltered volcanic material, while mineralogical data suggest that aqueous activity and weathering may have been confined to a smaller region than previously thought. Materials indicating hydrothermal activities, such as chlorides and altered clays, are confined to the northernmost part of the basin system, while silica formed under low temperatures is found in the southern regions of Eridania. We propose that any hydrothermal activity in a lake setting was limited to the northernmost basin of Eridania, and that it may have been less long‐lived than previously anticipated. Evidence for ancient lakes may have also been obscured by volcanism. Across regional to outcrop scales, Eridania Basin landscapes present compositions consistent with mafic resurfacingEridania's northern basin experienced a distinct, possibly longer‐lived aqueous alteration than the southern basinsA smaller watershed for Eridania, or a volcaniclastic cap, may help explain the limited evidence for lacustrine alteration in the Gusev Crater Across regional to outcrop scales, Eridania Basin landscapes present compositions consistent with mafic resurfacing Eridania's northern basin experienced a distinct, possibly longer‐lived aqueous alteration than the southern basins A smaller watershed for Eridania, or a volcaniclastic cap, may help explain the limited evidence for lacustrine alteration in the Gusev Crater