How do carbon dioxide polar caps form on Mars?

The polar caps on Mars form through a seasonal process where atmospheric carbon dioxide freezes directly onto the surface at the winter pole, a phenomenon often described as "snowing out" of the atmosphere.1

As fall begins in a hemisphere, clouds develop over the polar region, and the cap begins to grow as carbon dioxide transitions from a gas to a solid state. This process is driven by the planet's extreme cold; because carbon dioxide is the primary constituent of the Martian atmosphere, these seasonal shifts cause significant annual variations in atmospheric pressure as massive amounts of gas—roughly 7.9 trillion metric tons—leave and reenter the atmosphere.21

In the spring, these seasonal caps recede as the frozen carbon dioxide returns directly to a gas through sublimation. While the northern carbon dioxide cap disappears completely during the summer, leaving behind a residual water-ice cap, the southern pole retains a small residual cap composed of both carbon dioxide ice and water ice.123