The Mars Curiosity rover has discovered that Martian soil is made up of about 2 percent water. The Mars atmosphere contains a small amount of water vapor, but the majority of Mars' water is believed to be trapped in the polar ice caps as ice.
During the Mars summer, the ice caps at the north and south poles shrink as the ice goes directly from a solid to atmospheric gas, in a process called sublimation. In the winter, cooler temperatures cause the caps to grow halfway to the equator, pulling water out of the atmosphere. The polar ice is, on average, 2 miles thick, and if all the ice melted into liquid water, it could cover the surface of Mars with 18 feet of water. Scientists believe more water could lie just below the surface of the planet, covered by red dust. Sheets of ice at the bottom of deep craters have been found by spacecraft taking images of the surface of Mars from orbit.
In 2008, the stationary Phoenix lander turned over material that disappeared after several days; scientists concluded the most likely explanation for the material was water ice. Photos taken by the Phoenix lander over several days suggest small spherical objects seen in the photos merged, further providing evidence for water on Mars.