Video narration
How can cold, dusty Mars still carry scars of ancient rivers? Think of rainwater running downhill on Earth: it cuts channels as it goes. On Mars, branching valley networks, deltas, rounded rocks, and water-altered minerals show that liquid water once flowed. The key is its atmosphere and climate. Long ago, Mars had a thicker atmosphere, and conditions sometimes let water stay liquid long enough to carve the ground. But it was not necessarily permanently warm and wet like Earth. Those periods were probably intermittent, and their exact pattern remains uncertain. Then Mars lost much of its atmosphere and cooled. Today, low pressure and freezing temperatures make surface water quickly freeze, evaporate, or sink underground. The river evidence is real; it records a different climate. Mars had rivers because its ancient climate let liquid water flow. Today, it is too cold and thin-aired to keep water at the surface.