Mars has ice caps on both its north and south poles. The ice caps are made of water ice and carbon dioxide ice (dry ice). There are two kinds of ice caps on Mars: seasonal ice caps and residual ice caps. Seasonal ice caps accumulate during the winter season, and evaporate during the summer. The residual caps remain during the entire year.

Mars seasonal ice caps are entirely dry ice that is about 1 meter thick. The southern seasonal cap measures about 4000 km across when its largest during southern winter, and the northern cap measures about 3000 km across at its largest, during northern winter. When summer temperatures rise above 150K (-120 C), the ice sublimes (passes directly from the solid state into the gaseous state, bypassing the liquid state) into the atmosphere. Large seasonal changes in the amount of carbon dioxide in the atmosphere cause large seasonal changes, up to 30% different, in the atmospheric pressure on Mars.