Clay minerals show green in this false-color image made with two of the Mars Reconnaissance Orbiter's instruments. The region is Nili Fossae, and was shot on October 4, 2006. Features as small as 60 feet (18 meters) are resolved in the multicolor CRISM image, which reveals mineralogy; the HiRISE background topography, taken in 3 colors, shows features as small as 1 foot (30 centimeters) across. This observation resulted from a discovery by the Mars Express spacecraft that clay minerals existed in the area. Clay minerals form in wet environments, suggesting the presence of abundant water in this region earlier in martian history. Reddish areas indicate dunes of sand rich in the mineral olivine.