The carbon dioxide moves from the atmosphere to solid frost on the ground, depending on the seasons. During the long winter, the carbon dioxide will condense out of the atmosphere, particularly over the polar caps. This ice will form down to latitudes of 40^o (Cincinnati Latitudes). During the Spring, the carbon dioxide ices sublimate directly back into the atmosphere.

During the Northern Summer, all the carbon dioxide ices sublimate, leaving behind water ices and dust. The carbon dioxide covering the Southern Polar Cap never entirely disappears, so it is not known if water ice lies below it.

Does the water ice at the poles ever melt? Water exists in the ices of the N.Polar Cap (and probably the S.Polar Cap). Also Water is found as a vapor and ice crystals in the the Martian atmosphere. Even Clouds of moisture are known to form. However, when the water ice at the poles warms up, it immediately evaporates into the atmosphere. On Earth, at a pressure of 1 atm, liquid water can form. Mars' surface pressure is below the triple point of water. Presently, liquid water can not exist on Mars' surface because of its low atmospheric pressure.