Water on Mars. Computer artwork looking west over the rivers of the Kasei Valles (Kasei Valley) as the Sun sets on Mars 3.5 billion years ago. This region lies east of the Tharsis volcanoes and it is thought that meltwater from eruptions of these volcanoes supplied the water. Mars is now a desert world, but erosion features seen on Mars today suggest that water once flowed there. The water was lost due to the thin Martian atmosphere and weak gravity. The Rongxar Crater (lower centre) and the 100-kilometre-wide Sharonov Crater (centre right) are seen. The topography of the artwork was drawn with Terragen software, using data from the laser altimeter on the Mars Global Surveyor probe.