Like Earth, Mars has ice caps at its poles. Water reaches the poles as vapor and is frozen into thin layers that build up thick deposits. Mixed with this water is dust picked up by the wind, so the caps have bright and dark layers of "clean" and "dirty" ice.
During winter at each pole, temperatures are so low that carbon dioxide freezes from the atmosphere and forms additional layers of "dry ice." Much water is also trapped as permafrost surrounding the polar regions.
Like Earth, Mars spins on an axis tilted about 25 degrees from its orbital plane. Mars has no large satellite like the Moon, just its two small moons Phobos and Deimos. As a result, the tug of gravity from the Sun and the large planets causes a slow wobble in the tilt, or obliquity, of its axis. During periods of higher obliquity, the atmosphere is thicker, dust storms are more intense, and water now trapped at the poles moves to the equatorial region to form mountain glaciers. Many glacial landforms from the last time this occurred can still be seen on Mars.
Courtesy of F. Forget, F. Costard, P. Lognonne, and Springer Press
This image shows a "rock glacier" east of the giant Hellas Basin. Ice-rich material appears to have flowed from one crater (9 kilometers wide) into another. These glaciers may have formed when the planet's spin axis was steeply tilted.
Mars Express, HRSC
ESA/DLR/FU Berlin (G. Neukum)
Radar signals can probe the interior structure of icy deposits and create a cross-sectional view of the polar caps.
Radar echoes reveal a complex pattern of layering in the north (top) and south (bottom) polar deposits. In these cross-sectional views, brighter radar reflections indicate greater amounts of dust trapped in ice layers deposited over several million years. Darker regions are probably almost pure water ice.
Like tree rings, this layering records climate changes related to the changing tilt of Mars over time. The surface topography is due to wind erosion, loss of ice to the atmosphere, and slumping of material near the cap edges. Great deserts lie beneath the polar ice, formed of sand left behind each time the caps have disappeared.
Mars Reconnaissance Orbiter, SHARAD
NASA/JPL-Caltech/ASI/University of Rome/SWRI image
A large amount of water from the ancient era of rivers and shallow seas remains trapped as permafrost.
Mars has impact craters with features not found on the Moon. Here, "ramparts" of material flowed like mud after being excavated by a cratering event, an indication of ice near the surface.
NASA/JPL/Arizona State University image
A more complete picture of ground ice comes from measurements of hydrogen in the Martian soil. This map shows an increasing amount of hydrogen in the upper meter of the soil heading north and south from the equator. The most likely source is water—H2O—trapped as permafrost in a dusty layer. Much more water may lie farther below the surface.
Los Alamos National Laboratory image