“We’ve looked at the surface of the planet from thousands of images from the CRISM instrument, but we really see that only in a particular region on Mars, near the Isidus basin, which is a large impact basin early in Mars history, and in particular on the western portion of that basin, in an area called Nili Fossae, we see in over two dozen CRISM images the carbonate in a rock layer that’s about 20 meters thick, and it’s sandwiched in between iron-magnesium smectite clays below, which also form from alteration of water, and an unaltered cap rock above. So sandwiched in between is this layer that has both carbonate and in some places olivine. And so what we think we’re seeing here is evidence that an olivine-rich rock layer in the Nili Fossae region of Mars altered, in some places, by water, to form carbonate.”