Like on Earth, both Martian poles have permanent deposits of ice. Scientists once thought the southern polar cap is composed of mostly frozen carbon dioxide (dry ice), but recent measurements of the polar temperatures indicate both permanent caps are mostly H2O ice, with some CO2 ice mixed in. In the winter, the polar weather is so cold that CO2 freezes out of the atmosphere and makes much larger seasonal caps of dry ice deposits a few meters thick. In a similar way, our winter snow and ice deposits spread across Canada and the northern United States, only to shrink back in spring. The amount of ice is substantial. In 2004, Mars Express radar showed that ice in the southern polar cap extends to a depth of 3.7 kilometers. In 2010, radar on the Mars Reconnaissance Orbiter measured 820,000 culbic kilometers (200,000 cubic miles) of in in the northern polar cap. If melted, it would cover Mars to a depth of 5.6 meters! It is possible that there are "seas" of liquid water under these ice deposits.
Northern ice cap of Mars in 1999. Click here for original source URL.
Seasons on Mars resemble those on Earth, because the tilt of their axes is very similar. But the red planet’s eccentric orbit brings it a lot closer to the Sun during the southern hemisphere’s summer. This makes the southern summers significantly warmer, so the southern polar cap shrinks more than the northern one does. However, because of Kepler’s second law of orbital motion, Mars also moves a lot faster in its orbit during the southern summer. So the warmest part of the Martian year races by, and the southern ice cap never completely melts.
Carbon dioxide, unlike water, has no liquid state. It goes directly from solid to gas as it heats, and from gas to solid as it cools. So when summer temperatures reach about 150 K (-120 ºC), large amounts of CO2 are released into the atmosphere. This can increase the atmospheric pressure on Mars by as much as 25%. Conversely, when CO2 freezes out of the atmosphere during the winter, it not only increases the size of the polar ice cap, it also decreases the overall atmospheric pressure.
Measurements also suggest water is present in the form of permafrost ice deposits mixed in soil beneath the Martian surface at lower latitudes, analogous to Canadian tundra. Mars today has no water in the liquid state; water is frozen in the ice caps and permafrost, and incorporated into molecules that are trapped in mineral deposits. Channels that were once carved by liquid water require a dramatically different Martian climate in the past. While Mars has no liquid water on its surface, it does have at least one "ice lake." In 2005, the Mars Express orbiter spotted a sheet of ice at the bottom of an unnamed crater in the northern latitudes. The depth of the frozen lake is up to 200 meters in places, and it persists throughout the Martian year.
Intricate dark and light bands are visible in both polar ice caps. These are probably alternating layers of ice and dust, which are then carved away by erosion to reveal the banded patterns. They were laid down during hundreds of thousands of seasonal cycles. They could provide clues to a changing Martian climate, just as ice cores from Antarctica can tell scientists about Earth’s past climate.
South polar cap of Mars in 2000. Click here for original source URL.