Why do Mars' polar caps grow and shrink?
In each hemisphere's winter, so much of the air freezes out as dry ice that the caps spread halfway to the equator.
Mars' air is mostly carbon dioxide. In polar winter it gets cold enough (−125 °C) for that gas to freeze onto the ground as frost, burying the permanent water-ice caps under a seasonal layer.
About a quarter of the whole atmosphere freezes out this way each year, then turns back into gas in spring — the air pressure over the entire planet rises and falls with the seasons.
The globe shows the typical extent of the frost for the current Martian season.