We present observations of seasonal variations in polygonal crack patterns located in a polar trough on the south polar cap of Mars; previously, there was no direct observation showing that these patterns change. Polygonal patterns on Mars are attributed to thermal contraction cracking, which is commonly observed in periglacial environments on Earth. In this paper we discuss observations based upon the high-resolution image data of the Mars Orbiter Camera and focus on the reconstruction of the seasonal development. The image-based observations are further supported by temperature data. We show that the south polar trough pattern is located in an active geologic unit, which undergoes seasonal variations and annual crack formation. Furthermore, there are strong indications showing these contraction-crack processes take place in a thin layer that might be composed of water-ice and is located beneath the seasonal carbon dioxide ice cover.