The distribution of materials in this composite image of the Nili Fossae region of Mars tells scientists that water resided there no more recently than nearly 4 billion years ago. Green indicates clay minerals that formed in a wet environment. Red depicts the mineral olivine, which formed about 3.8 billion years ago, according to the region's cratering record.
Earlier data showed the clays, but a spectrometer and a high-resolution camera on NASA's Mars Reconnaissance Orbiter now reveal that the olivine is in sand dunes and that the clays are in exposed bedrock. That juxtaposition indicates that the clay was laid down first, says Jack Mustard of Brown University in Providence, R.I. He unveiled the image Dec. 13 at a meeting of the American Geophysical Union in San Francisco.