Results from the Mars Exploration Rovers and from spectrometers on orbiting spacecraft show that the ancient highlands are compositionally distinct from the younger plains. The rover Spirit landed on a basalt plain that may be typical of plains. The rocks on the plains are mostly typical basalts with only thin alteration rinds high in sulfur, chlorine, and other volatile elements. The rinds probably formed by interaction of the basalts with acid fogs. The rover then traveled into the older Columbia Hills, where the rocks are very different. They are mostly basalts and impact breccias, but many are pervasively altered and rich in sulfates and hydrated minerals. Soils consisting almost entirely of sulfates or silica are also present. Many of the rocks appear to have been permeated by warm volcanic fluids or to have been weathered as a result of warm surface conditions. This mix of rocks and soils may be typical of the highlands in general. The results from orbit tell a similar story. Globally, the plains consist mostly of primary, unaltered basaltic minerals such as olivine and pyroxene. In contrast, alteration minerals such as clays are common throughout the ancient cratered terrain. The results indicate that surface conditions changed dramatically around 3.7 billion years ago. Prior to that time, warm and wet conditions were common and resulted in extensive rock alteration; after that time such conditions were rare, and rock alteration was minor.
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