Clay minerals on Mars: An up-to-date review with future perspectives

Clay minerals, or analogously phyllosilicates, are some of the most astonishing minerals ever discovered on Mars due to their roles as indicators of water-rock interaction. Their types, abundances, and locations provide hints to ancient environmental conditions of Mars and to the possible places where present-day mineral-bound water and/or biosignatures are likely to be detected. In this contribution, the definition, structures, and hydrated states, the global distribution and formation mechanisms, the significance of occurrence, and the developing detection techniques of clay minerals on Mars are summarized and discussed. The definition and structure-based classification of martian clay minerals build upon their Earth analogues; some martian clay minerals contain less water in their structure and thus exhibit smaller interlayer spacings. Clay minerals on Mars have been widely detected in ancient terrains of Noachian and Early Hesperian age (>3.5 Ga) across the planet. They have been formed mainly by chemical weathering, sedimentation, and hydrothermal alteration, at the surface or in the subsurface. Many techniques, including telescopic observations from Earth, remote sensing from Mars orbiters, in-situ characterizations by Mars landers/rovers, and lab studies of martian meteorites and terrestrial analogues and geochemical modeling, have been developed to detect, identify and further understand clay minerals on Mars. Among these techniques, visible and near-infrared reflectance spectroscopy onboard orbiters is the most powerful at global or regional scales while in-situ X-ray diffraction is the most definitive at a much smaller scale. The occurrence of clay minerals on Mars provides evidence for the presence of liquid water, the evolving geological alterations under varied environments and climates, and the potential habitability. Clay minerals on their own can serve as water sources for rocket fuel, human exploration, and immigration. Although many revolutionary advances have been made on martian clay minerals, many intriguing questions remain, including but not limited to the precise identification and quantification of clay minerals, the effects of impact on the detection of clay minerals, the formation and preservation of short-range ordered clay minerals, the co-occurrence of clay minerals and other secondary minerals, and the detection of clay minerals beyond Earth and Mars.