Nanoscale proteinaceous residues in a Mars-analogous secondary mineral assemblage: Implications for Mars sample return

Secondary minerals, such as clay minerals and Fe-Mn oxides, commonly form through aqueous processes mediated by microbial activity, making them key biosignature targets. Microbial proteins can be preserved within these minerals. Although Mars return samples are expected to be rich in secondary phases, the nanoscale distribution and preservation of proteins within them remain poorly known. Using atom probe tomography combined with complementary techniques, we investigate natural Mars-analog secondary minerals and their protein preservation capacity. We achieve in situ, near-atomic-scale characterization of protein-mineral interactions in natural samples. Our results show that clay minerals and Fe-Mn oxides form intimately mixed assemblages with homogeneous spatial associations at the nanometer to subnanometer scale. Proteinaceous residues are preserved within these minerals, commonly distributed along the inner and outer edges of relatively large mineral grains. Incorporation into the mineral lattice likely contributes to their preservation. In contrast, smaller secondary mineral grains (<5 nm) generally retain little protein-related signal and may adsorb only limited amino acid fragments. These findings provide direct nanoscale insights into protein-mineral associations in natural settings, with important implications for detecting potential biosignatures in extraterrestrial samples, such as those returned from Mars.