Thick stacks of clay minerals indicate chemical alteration of thick stacks of rock by interaction with liquid water on ancient Mars as seen in this image from NASA's Mars Reconnaissance Orbiter.

Thick stacks of clay minerals indicate chemical alteration of thick stacks of rock by interaction with liquid water on ancient Mars. Aluminum clays overlying iron/magnesium clays here in the ancient terrains of Mawrth Vallis indicate a change in environmental conditions. Aluminum clays may form by near-surface leaching while iron/magnesium clays may form in the subsurface. The image is from the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter. (References: Wray et al., 2008; Loizeau et al., 2010.)

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