The northern ice cap is permanently frozen water covered by frozen carbon dioxide (dry ice), which solidifies from the atmosphere at cold temperatures.
During summer in the north, the ice cap shrinks, as some of the carbon dioxide reverts to gas in the atmosphere.
In the winter, it grows much larger again.
The ice cap in the southern hemisphere is almost all frozen carbon dioxide.
It also grows and shrinks seasonally, but to a greater extent than the northern cap, as the carbon dioxide solidifies and then vaporizes.