Endolithic Mediation of Anhydrite in Hydrothermal Alteration Zones: Implications for Biosignature Exploration on Mars

Abstract The formation of Ca‐sulfate minerals on Mars is believed to have been driven by hydrothermal alteration processes during the late Noachian. On Earth, microbial involvement in the crystallization of Ca‐sulfate minerals could contribute to the development of distinctive biosignatures. However, understanding potential biosignatures in Martian hydrothermal alteration zones remains limited. In this study, we investigate anhydrite crystal formation in hydrothermally altered rocks colonized by endoliths. It is worth noting that anhydrite crystals within endolith‐colonized zones exhibited diverse morphologies, including prismatic, tubular, pseudo‐hexagonal, lenticular, and twinned shapes. These morphologies were randomly distributed on or around microbial cells, contrasting sharply with the uniform tabular and prismatic morphologies typically observed in abiotic processes on non‐colonized rock surfaces. The coexistence of varied crystal morphologies and iterative formation‐dissolution patterns indicates microbial involvement, making these features potential biosignatures. Our findings highlight the significance of microbial interactions in shaping Ca‐sulfate mineral morphology, offering critical insights for biosignature exploration on Mars.