LONG DRY SPELL. Colors ranging from magenta to purple-blue map large exposures of olivine-rich rocks in the Nili Fossae region of Syrtis Major. The olivine, which weathers quickly on exposure to water, is about four times as extensive as scientists previously thought, suggesting the Martian climate has been mostly cold and dry. The image was taken by the Thermal Emission Imaging System on Mars Odyssey, and it maps olivine first discovered by TES. This image is about 350 kilometers (220 miles) wide. NASA/JPL/Arizona State University