Permanent and seasonal polar caps

In spring, the Sun vaporises carbon dioxide ice below the surface. High-pressure carbon dioxide gas creates cracks, escapes through them, and carries material from dust layers. Wind spreads it into fan-shaped deposits. No such phenomenon is known on Earth.

Sublimation of the upper carbon dioxide layer of the south polar cap creates round, Swiss-cheese-like holes. An older water-ice layer is exposed beneath it. The round pattern is produced by the warmth of the Sun circling the summer south polar sky.