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Microbes are considered the most probable form of life on Mars, if life ever exists there. Interactions between clay minerals and microbes are common on Earth, and these interactions can leave distinctive imprints in clay minerals, including crystal composition and structure. Infrared spectroscopy, the most important method to remotely study planetary minerals, is sensitive to crystal-chemistry/structure of clay minerals, thus giving us a window to search for life relics on Mars. Nontronite is the most common hydrous minerals on the surface of Mars. In this study, a series of microbe-reformed nontronites were synthesized in the laboratory, resulting primarily in nontronite-illite mixed layers. The changes in crystal chemistry and structure due to microbial reformation were analyzed using techniques such as FTIR, XRD, and TEM. The results indicate that the changes in chemical composition during microbial illitization differ significantly from those observed of geologic origins. These different reaction pathways between biotic and abiotic illititzation can also be reflected by near-infrared spectra. Based on these findings, we propose a near-infrared spectroscopy index that can be used to identify ancient biotic illitization processes. This index has potential applications in Martian remote sensing by orbiters and rovers, the selection of landing sites for China's TW-3 mission, and even the analysis of samples returned from Mars, with the goal of determining whether microbial activity once existed on this planet.

Additional information: Mr. HOU Xingao, u3011666@connect.hku.hk