Why Does Mars’s South Polar Ice Look Like Swiss Cheese?

Parts of Mars’s south polar residual ice cap look like Swiss cheese because frozen carbon dioxide gradually turns directly into gas, carving rounded pits into the ice. This process, called sublimation, occurs when sunlight warms exposed surfaces. Over repeated Martian seasons, the pits deepen and change shape rather than melting into liquid-filled holes.

What exactly are the holes?

They are depressions in a layer of frozen carbon dioxide, also known as dry ice. The polar terrain contains pits, mesas and troughs: some patches lose material faster than others. Unlike holes in cheese, the depressions do not have to connect through the ice.

Why does the ice disappear instead of becoming a puddle?

Surface pressure on Mars is much lower than Earth’s. Under typical Martian surface conditions, carbon dioxide ice can go straight from solid to vapor when heated. Sunlight reaches pit walls at different angles, so their surfaces warm unevenly. This helps explain why some features develop steep sides and sculpted edges.

Is all Martian polar ice made of carbon dioxide?

No. Mars has substantial water ice, especially in its polar layered deposits. Its south pole also has a persistent residual carbon dioxide ice cap, and seasonal carbon dioxide frost expands across larger areas during winter. The north pole’s summer residual cap is predominantly water ice. The different materials are part of why the poles can look and behave differently.

Do the pits change every year?

Researchers compare images taken at different times to study changes in pit edges and seasonal frost. Some depressions grow as carbon dioxide sublimates; new frost may cover flatter surfaces again in the cold season. The precise changes depend on the location and local conditions, so they cannot be predicted from one picture alone.

Frequently asked questions

Are these pits evidence of underground life?

No. The observed shapes have a well-established physical explanation involving carbon dioxide frost and sublimation. They are not evidence of organisms.

Can astronauts walk on this ice?

Descriptions of the terrain do not establish that any particular location would be safe to traverse. Dry ice, steep depressions and extreme conditions would make a future polar mission technically challenging.

Where can I see more real images?

See NASA’s HiRISE view of Swiss-cheese terrain and the north polar cap image for a comparison.