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Water on a prehistoric Mars, illustration. Today Mars is a cold desert world with a thin atmosphere of carbon dioxide. It is thought that Mars once had oceans of water, but that this water was lost due to the thin atmosphere and low Martian gravity. Any remaining water is trapped as ice below ground and at the poles. This illustration could also show terraforming of a future Mars. (KEYSTONE/SCIENCE PHOTO LIBRARY/DETLEV VAN RAVENSWAAY/SCIENCE PHOTO LIBRARY)