Early Mars had a climate that was warmer and wetter than today; its atmosphere was thicker and water flowed across the surface. Mars may even have had oceans. As the interior of Mars cooled, volcanism declined and the atmosphere of Mars thinned. Today’s atmosphere is made of 95% carbon dioxide, 3% nitrogen, and small amounts of other gases, including water, oxygen and methane. The atmospheric pressure on the surface of Mars is about 1/100 that of Earth’s atmospheric pressure at sea level. Because of the thin atmosphere and Mars’ distance from the sun, Mars is cold. Its temperatures range from -193 degrees F (-125 degrees C) to 23 degrees F (-5 degrees C), well under the freezing point of water and also cold enough to freeze carbon dioxide.
Because of the low atmospheric pressures, liquid water at the surface of Mars would evaporate into water vapor. So what happened to all that water that used to be on the surface of Mars? Some did evaporate into space. But much is frozen under the surface and in the polar ice caps. Mars has water ice!
Mars also has another type of ice — carbon dioxide ice — which is familiar to us as “dry ice.” Because Mars is so cold, in the winter the carbon dioxide in its atmosphere condenses and falls to the ground as carbon dioxide ice. In the summer, much of this changes from the solid form back into gas (sublimates).
Mars has ice caps at both its poles. The north pole ice cap is about 600 miles (1000 km) across — about the width of Montana! The southern ice cap is about 1/3 this size. Both ice caps are made mostly of water ice, but the southern ice cap has a permanent cover of carbon dioxide ice. The ice caps grow each winter as carbon dioxide ice is added to them, and decrease each summer as the carbon dioxide sublimates back to the atmosphere.
Like Earth, Mars’ climate has fluctuated through geologic time, sometimes getting warmer and sometimes getting colder. During colder times, its ice caps expanded and glaciers extended farther across the Martian landscape.