The South Polar residual cap (the part that lasts through the summer) is composed of carbon dioxide ice.
South Polar Residual Cap
Posted inAtmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Habitable Zones, Mars, Press Release
InSight Pressure Data Recalibration, and Its Application to the Study of Long-Term Pressure Changes on Mars
Observations of the South Polar Residual Cap suggest a possible erosion of the cap, leading to an increase of the global mass of the atmosphere.