Rivers on Mars. Computer artwork looking down on the mouth of a river thought to have created the Kasei Valles (Kasei Valley) on Mars 3.5 billion years ago. North is at top. Meltwater flowed from left as volcanoes erupted in the Tharsis region, entering the sea in the Chryse Planitia (Chryse Plain, right). The river divides around the Sacra Mensa (white, lower left) and the Sharanov Crater (centre right). The image is 1100 kilometres wide. The water was ultimately lost due to the thin Martian atmosphere and weak gravity. Mars is now a desert world. The topography was drawn with Terragen software, using data from the laser altimeter on the Mars Global Surveyor probe.