This VIS image of the South Polar cap shows several different surface textures. It appears that the circular depressions (which look like swiss cheese) alter with time, appearing to lose the circular shape as the intervening walls of ice are removed.
Orbit Number: 65187 Latitude: -87.0386 Longitude: 276.053 Instrument: VIS Captured: 2016-08-24 07:47
http://photojournal.jpl.nasa.gov/catalog/PIA21003
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