Water on a prehistoric Mars. Computer artwork of water on Mars. Mars, the fourth planet from the Sun, is smaller than Earth with a surface area about the same as Earth's land surface area. Today Mars is a cold desert world with a thin atmosphere of carbon dioxide. It is thought that Mars once had oceans of water, but that this water was lost due to the thin atmosphere and low Martian gravity. Any remaining water is trapped as ice below ground and at the poles. This artwork could also show terraforming of a future Mars.