ESA released stunning images of an ancient river in Nirgal Vallis that once flowed on Mars By studying the surrounding craters, the branching remains are said to be between 3.5 and 4 billion years old Detailed images of Mars reveal an ancient river that once flowed on the red planet. Spanning nearly 435 miles across the surface, the valley stream is named Nirgal Vallis and experts said it was shaped by flowing water and impacts. By exploring the characteristics of the surrounding craters, the European Space Agency has estimated the system’s age to be between 3.5 and 4 billion years...