Mars’s polar caps change size with the seasons mainly because atmospheric carbon dioxide freezes onto the poles during winter and sublimates back into gas during spring and summer. Beneath the temporary dry-ice frost, long-lasting water ice remains.
Winter builds a seasonal cap of dry ice
When sunlight is weak and polar temperatures fall, carbon dioxide gas from the thin Martian atmosphere condenses as solid CO₂ frost and ice. This enlarges the bright seasonal polar cap. The frozen material is different from ordinary water ice and is commonly called dry ice.
Spring sunshine causes sublimation
As sunlight returns, much of the seasonal CO₂ cover changes directly from solid back to gas—a process called sublimation. The visible cap shrinks. This seasonal exchange even changes the amount of carbon dioxide in the atmosphere over the Martian year, contributing to changes in air pressure.
Permanent ice is different from temporary frost
Mars has substantial deposits of water ice at both poles and layered polar terrains. The south polar region also has residual carbon dioxide ice that can persist through summer in places. A satellite image of a shrinking seasonal cap therefore does not mean all Martian polar water ice disappears each year.
Why is this interesting to scientists?
The seasonal frost cycle provides a way to study Martian weather and climate. Spacecraft track the advance and retreat of bright deposits, atmospheric pressure changes and unusual features produced as gas escapes from under seasonal ice. NASA imagery even documents pits forming in residual carbon dioxide ice.
Frequently asked questions
Are Mars’s caps made only of frozen water?
No. They contain water ice and seasonal carbon dioxide ice; their distribution and persistence depend on hemisphere and season.
Does melting make rivers of liquid water?
No. Most of the seasonal dry ice sublimates into gas. The visible seasonal changes do not imply flowing surface rivers.
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