Perhaps the most surprising features that were revealed by the Viking spacecraft are the huge channels and dry riverbeds, such as those seen in Figure 7.7 More from the rover Opportunity showed grooves in rocks that were formed by the presence of water. The rover also detected large amounts of hematite, a mineral that forms in a wet environment.

Scientists infer from this evidence that liquid water once flowed on mars, even though there is no water present now. In fact, many scientists believe that huge lakes and small oceans once existed on Mars. The creats a big question : Where did the water go? A look at the atmosphere of Mars reveals the answer to this mystery.

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