A 2‐Billion‐Year History of Water‐Alteration in Terra Sirenum, Mars: Volcanism's Influence on Aluminum Clay Formation and Chemically Distinct Waters Forming Sulfates and Chlorides Into the Amazonian

Abstract Terra Sirenum, a region of Noachian highlands southwest of the Tharsis volcanic complex, is unique in the number, proximity, and diversity of orbital detections of secondary minerals, as the sole region found to date hosting large‐scale deposits of all of Mars' major salts (chlorides, sulfates, carbonates) as well as diverse hydrated silicates. We combine mineralogical information, high‐resolution imagery, and elevation models to investigate the geologic context of these secondary minerals to understand the sources of water and ions for each type of deposit and their spatial/temporal relationships. Carbonates, where present, are part of Noachian basement rocks exposed through cratering and do not appear associated with evaporative sequences. Numerous small detections of the acid sulfate minerals alunite and jarosite mirror the dominant clay cation in the localities they are found—Al phyllosilicates and Fe phyllosilicates, respectively—suggesting in situ formation. We interpret a previously discovered kaolinite‐rich unit overlying Fe/Mg clays across northeast Terra Sirenum as remnants of a widespread ash unit rather than a pedogenic weathering sequence. Sulfate and chloride detections are decoupled, with sulfates in topographic lows likely precipitated from volcanism‐associated groundwaters, while chloride detections are consistent with surface water runoff, in some instances clearly post‐dating volcanic units capping sulfate detections. Volcanic resurfacing of craters in the region is progressively younger from west to east, and crater statistics‐based ages indicate localized sulfate‐ and chloride‐forming processes continue to occur from ∼3.5 to ∼1.4 Ga. We hypothesize that their decoupling points to disconnected, episodic surface and groundwater reservoirs, perhaps separated by a permafrost layer.