Terra Sirenum, one of the oldest regions on Mars, holds important clues about the planet’s early crustal evolution. In this study, we identify a unique and extensive deposit of aluminum-rich clay minerals (Al clays) in this region, distinct from similar clays found elsewhere on Mars. This layered unit spans ~10,000 km², reaches up to 400 meters thick, and displays well-preserved horizontal bedding.

Our analysis suggests that this unit likely formed from the intense chemical weathering of felsic, glassy volcanic ash from surrounding small volcanic edifices. This is notable because most Martian clays form from the weathering of basaltic rocks, which are richer in Fe and Mg. The large volume of Al clays (>1,000 km³) in Terra Sirenum possibly linked to the shift in mineral composition hints at a deeper geological change, possibly involving the formation of a more evolved, continent-like crust. Overall, the presence of these altered volcanic materials in Terra Sirenum provides important clues about how Mars’ crust formed and evolved, preserve a rare window into Mars’ ancient crust.