Water on Mars. Computer artwork of Mars seen from space about 3.5 billion years ago. Darker colours show the water filling the large Amazonis Planitia basin (upper centre). Water dominates the northern hemisphere, while the southern hemisphere consists of a vast continental landmass (lighter colours). Water created the erosion features seen on Mars today, but was ultimately lost because of the thin Martian atmosphere and weak gravity. Mars is now a desert world with only ice deposits left. The Olympus Mons volcano (upper right) is the largest mountain in the solar system. The topography was drawn with Terragen software, using data from the laser altimeter on the Mars Global Surveyor probe.