The ice caps of Mars

"Swiss cheese" holes in the south pole of Mars. The white substance in the picture is dry ice - made of carbon dioxide. The depressions in the dry ice layer reach a depth of only 10 meters. They are formed in the red summer season, when the sun's rays strike at a low angle in the polar region, and reach mainly the steep walls of the depressions - which encourages development laterally rather than in depth. The photograph, in its full resolution, has a resolution of 50 centimeters per pixel. Taken on March 25, 2017 by NASA's Mars orbiter MRO. Credit: NASA/JPL-Caltech/Univ. of Arizona.

Why does part of the south pole of Mars look like Swiss cheese?