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Scientists have long talked about the possibility of water on Mars, pointing to ancient river channels, polar ice caps and trace amounts of moisture in the soil. But now there's evidence, based on data from the Mars Reconnaissance Orbiter, that briny water may actually flow down the hillsides of the red planet. Where it comes from – deep underground or collected from the thin atmosphere – and whether it shows signs of life remain unanswered questions. All the more reason to go there and find out.