Ferromagnesian Clay Diversity Across Mars' Crustal Dichotomy: A Window Into Early Aqueous Environments

Abstract Extensive research over the past two decades has shown that early Mars likely had a warmer, wetter climate with widespread water activity. Ferromagnesian (Fe,Mg‐rich) clay deposits are compelling markers of these ancient environments, helping reconstruct Mars' hydrologic evolution, assess past habitability, and guide future exploration. This study analyzes hyperspectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard NASA's Mars Reconnaissance Orbiter , focusing on regions along the Martian crustal dichotomy—where clay deposits occur at the boundary between the ancient southern highlands and the younger northern lowlands. We systematically surveyed ∼1500 CRISM targeted observations (1–2.6 μm) to identify ferromagnesian clays, distinguish them from other hydrated minerals, and characterize compositional differences between Fe‐ and Mg‐rich species using diagnostic absorptions around 1.4, 2.3, and 2.4 μm. Results reveal spatial variations in clay mineralogy: Fe‐rich nontronites are prevalent around Mawrth Vallis, while Mg‐rich saponites are more locally distributed in Nili Fossae and Libya Montes. Oxia Planum—the Rosalind Franklin rover landing site—exhibits more compositionally intermediate clays such as vermiculites and ferrosaponites. These differences may reflect variations in the iron and magnesium abundance or in the iron oxidation state. Moreover, a recurring absorption near 2.5 μm suggests co‐occurring carbonates like magnesite and siderite, increasing the potential for biosignature preservation. These findings refine our understanding of Mars' aqueous history and offer an important mineralogical context for future rover and sample return missions. They also emphasize the need for a next‐generation orbital imaging spectrometer to succeed CRISM and extend its legacy.