Some 1500 sols (martian days) after they began, the Mars Exploration Rovers Spirit and Opportunity continue to explore the martian surface. Both rovers have accomplished their primary objective of discerning the ancient geologic environments of the places where they landed and both have found evidence for the roles and fate of water in those environments. Spirit, operating in Gusev Crater, found the crater floor to have been flooded by basaltic lavas, but of a different type than is known from the well-studied basaltic martian meteorites. Exploration of the Columbia Hills showed the hills to be an older geologic construct, possibly an ancient impact ejecta deposit. Some of these rocks bear abundant evidence of hydrothermal alteration. At the present time, the rover is exploring the volcanic terrain south of the Columbia Hills known as “Home Plate.” Rocks in this area are characterized by higher volatile contents than the plains basalts and a more energetic volcanic eruption style. Deposits rich in silica (SiO2) have been found in this area; these deposits are thought to derive from hydrothermal activity possibly associated with ancient fumaroles. On the other side of Mars, Opportunity continues its exploration of the Meridiani Plains through a combination of lateral traverse and depth profiling through the study of impact craters of different size and excavation depth. Opportunity discovered sulfate-rich, layered sedimentary rocks that formed from a system of ancient playa and interdune settings. These settings may represent the waning stages of surficial water on an early, wet Mars as it transitioned from thick to thin atmosphere. Recent orbital investigations have now pinpointed sulfate deposits elsewhere on Mars that are understood in part because of the ground-truth observations made by Opportunity at Meridiani Planum.

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