Water on Mars. Artwork of plains and craters on Mars as seen millions of years ago when filled with water and mud. Mars featured considerable quantities of water many millions of years ago, but all large expanses of liquid water have since disappeared, freezing as the planet cooled and evaporating off into space as much of the Martian atmosphere dissipated. Some water has remained as ice, and incorporated into the structure of water-rich substances such as mineral clay and sulphates.