Alteration history of aluminum-rich rocks at Jezero crater, Mars: Insights from deeply weathered terrestrial materials

Sedimentary sequences containing aluminum-rich clay minerals have been detected across the surface of Mars and likely indicate periods of intense aqueous alteration by liquid water billions of years ago. On Earth, these clay minerals can form as a result of rainfall-driven chemical weathering in tectonically stable landscapes over thousands to millions of years, but exactly how they once formed on Mars remains a mystery. During its traverse in Jezero crater, the Mars 2020 Perseverance rover discovered distinct light-toned, cobble-sized, aluminum-rich (30-40 wt. %) “float” rocks (rock fragments), with some bearing the spectral signature of kaolinite and/or halloysite. These rocks now fortuitously enable an investigation into the 3-to-4-billion-year-old kaolinite-bearing terrains of early Mars. To contextualize these materials with analogs from Earth, we compare their elemental compositions and reflectance spectra to deeply weathered terrestrial paleosols (ancient, lithified soils) and hydrothermal kaolin deposits from Earth’s geological record. Aluminum and titanium enrichments along with depletion of iron and magnesium observed in several Jezero crater float rocks are unlike hydrothermal kaolin deposits and instead comparable to the bleached "pallid zone" of deeply weathered Eocene (55 Ma) and Paleoproterozoic (2.2 Ga) kaolinitic paleosols that formed from intense rainfall (pedogenic weathering) during past greenhouse climates on Earth. However, either pedogenic or hydrothermal processes can form kaolinite, and until the source outcrop of this material is discovered, either remains a possibility. These rocks may have been associated with the wettest, and possibly most habitable periods of Mars’ early history.