Shock metamorphism of clay minerals on Mars by meteor impact

A large fraction of clay minerals detected on Mars by infrared remote sensing represent materials exhumed from the subsurface by meteor impact, begging the question of whether the infrared features used to detect the clays are affected by shock associated with the impacts. We used X-ray diffraction and infrared and Mossbauer spectroscopy to evaluate the mineralogy of five clay minerals after experimentally shocking them to six shock pressures from similar to 10 to 40 GPa. The shocked clays exhibit three main relevant shock effects: (1) an overall decrease in infrared spectral contrast in the impact-fragmented materials, (2) oxidation of Fe in ferrous clays, and (3) loss of some spectral structure in relatively well-ordered clays such as kaolinite. Other than the widespread oxidation of ferrous clays, shock metamorphism likely has little effect on the accurate interpretation of clay mineralogy on Mars from remote sensing data. However, we are able to identify rare cases of extreme shock in some Martian clay deposits. Plain Language Summary One of the most significant achievements in planetary sciences in the last 20 years has been the identification and mapping of Martian surface mineralogy by infrared remote sensing. Of major interest is the characterization of clay minerals that formed in ancient habitable environments on Mars >3.5 Ga ago. Because most of these deposits are extremely ancient, they exist within crustal materials that have experienced numerous meteor impacts and therefore could have been affected by heat and pressure associated with those events. The paper contains three main points: (1) the main features that are used to interpret clay mineralogy from infrared data remain intact despite shock metamorphic effects up to pressures of similar to 40 GPa, (2) shocked clays display some key features that can be used to identify effects of shock remotely, and (3) we describe some of the global-scale spectral biases in interpretation of clays on Mars that are likely to arise from shock effects and we show some concrete examples of shocked Martian clays in certain cases. We believe that this would be the first publication of clear evidence for shocked minerals detected by remote sensing on any planet.