Water, Water Everywhere — On Mars?

The Mars Express orbiter has taken photos of river valleys on the Red Planet, showing evidence that liquid water once flowed across the Martian landscape. Once thought to be completely dry, Mars is now known to have water ice beneath its surface, up to 100 meters, or 330 feet, in depth. Billions of years ago, Mars lost most of its atmosphere, and water on its surface disappeared. Astronomers are still uncertain what caused this dramatic climatic change on the Red Planet.

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