Reliable spectroscopic identification of minerals associated with serpentinization: Relevance to Mars exploration

Mars has become the preeminent target of astrobiology due to its many Earth-like features. Serpentinized environments on Mars are increasingly of astrobiological interest because they imply the presence of several of the “key elements” for life. The Mars 2020 rover carries a compelling set of spectral instruments with the intent to characterize past habitable serpentinized environments, search for potential biosignatures, and collect samples for potential return to Earth. Reliable spectroscopic identification of serpentinization minerals is, of course, a prerequisite for mission accomplishment. The current assignment of spectroscopic features is based on the databases derived from pure minerals. However, many studies have confirmed that mineral assemblage can complicate spectrum identification, often leading to misinterpretation of the data. Therefore, a rock-based library should be built, which will increase our capability to interpret the Martian spectroscopic data. As such, we performed a comprehensive mineralogical and spectroscopic survey of several rocks sampled from an ophiolite complex in Qaidam Basin, one of the largest Mars analogs on Earth, to build an ophiolite spectral database. X-ray fluorescence (XRF), visible and near-infrared (VNIR), Raman spectroscopy, and XRD were used to identify minerals in the rocks. The results show that serpentine in the rocks with talc could be misinterpreted as sepiolite only relying on the Raman vibrations, while the VNIR spectra can identify serpentine well in all rocks. In addition, the camera and Raman spectrometer on the Mars rover should work together to identify different polymorphs of serpentine, i.e., antigorite, lizardite, and chrysotile. Raman and/or VNIR spectroscopy is effective for other minerals associated with serpentinization, including brucite, dolomite, magnesite, magnetite, talc, and quartz. Our study provides a framework for detecting serpentinization minerals on Mars with spectrometers and can be used for data interpretation by the Mars 2020 mission. All the spectral data presented in the supplementary material facilitate further comparison with future in situ and orbital measurements on Mars.