This "Swiss cheese" pattern forms in the Martian polar regions (here in the south) when a relatively high, smooth surface layer of frozen carbon dioxide ("dry ice") breaks up into depressions. Most likely, this pattern of depressions forms as the ice sublimates
(goes directly from a solid to a gas) as the temperature warms. Click here
for more information and to see a larger view.
NASA / JPL / Univ. of Arizona