Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters

Cross crater is a 65 km impact crater, located in the Noachianhighlands of the Terra Sirenum region of Mars (30°S, 158°W), which hosts aluminum phyllosilicate deposits first detected by the Observatoire pour la Mineralogie, L’Eau, les Glaces et l’Activitie (OMEGA) imaging spectrometeron Mars Express. Using high-resolution data from the Mars Reconnaissance Orbiter, we examine Cross crater’s basin-filling sedimentary deposits. Visible/ shortwaveinfrared (VSWIR) spectra from the Compact Reconnaissance Imaging Spectrometerfor Mars ( CRISM) show absorptions diagnostic of alunite. Combining spectral data with high-resolution images, we map a large (10 km × 5 km) alunite-bearing deposit in southwest Cross crater, widespread kaolin-bearing sediments with variable amounts of alunitethat are layered in <10 m scale beds, and silica- and/or montmorillonite-bearing deposits that occupy topographically lower, heavily fractured units. The secondary minerals are found at elevations ranging from 700 to 1550 m, forming a discontinuous ring along the crater wall beneath darker capping materials. The mineralogy inside Cross crater is different from that of the surrounding terrains and other martianbasins, where Fe/Mg-phyllosilicates and Ca/Mg-sulfates are commonly found. Alunitein Cross crater indicates acidic, sulfurous watersat the time of its formation. Waters in Cross crater were likely supplied by regionally upwelling groundwaters as well as through an inlet valley from a small adjacent depression to the east, perhaps occasionally forming a lake or series of shallow playa lakes in the closed basin. Like nearby Columbus crater, Cross crater exhibits evidence for acid sulfate alteration, but the alteration in Cross is more extensive/complete. The large but localized occurrence of alunitesuggests a localized, high-volume source of acidic waters or vapors, possibly supplied by sulfurous (H_2S- and/or SO_2-bearing) waters in contact with a magmatic source, upwelling steam or fluids through fracture zones. The unique, highly aluminous nature of the Cross crater deposits relative to other martianacid sulfate deposits indicates acid waters, high water throughput during alteration, atypically glassy and/or felsicmaterials, or a combination of these conditions.