Colours indicate infrared emission signatures in this mosaic from NASA's Mars Odyssey orbiter, showing a region of troughs named Nili Fossae. Analysis from Odyssey's Thermal Emission Imaging System suggests that a deposit rich in the mineral olivine is about four times larger than earlier thought. The olivine-rich exposures appear magenta to purple-blue in this colour-coding. Olivine can turn into other minerals rapidly in the presence of water. This deposit, in a relatively old region of Mars' surface next to one of the planet's largest volcanoes, Syrtis Major, suggests the region may never have seen much water.
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