Impact cratering and erosion combine to reveal the composition of the Martian underground by exposing materials from the subsurface. Investigation of exposed clay minerals at thousands of Martian sites by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on NASA's Mars Reconnaissance Orbiter suggests a long period of wet, warm conditions, mostly underground.
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Image Credit: NASA/JPL-Caltech/JHUAPL
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