A story of changes in the climate of Mars is told by icy deposits. Remnants of a formerly extensive deposit composed of dry ice layered together with dust and water ice form what is known as the south polar residual cap.
This deposit is shrinking over time as the frozen carbon dioxide turns to vapor. Rounded valleys that give the deposit an appearance resembling Swiss cheese are enlarging over time, exposing an older surface below that is likely made up of water ice.
In the past, the South Pole of Mars was colder than it is today, with an average temperature below the freezing point of carbon dioxide gas. Carbon dioxide from the atmosphere condensed on the polar water ice cap, forming a layered deposit of dry ice together with trace amounts of dust and water ice. More recently, the South Pole of Mars has warmed to an average temperature greater than the freezing point of carbon dioxide gas, and the dry ice deposits are retreating… [More at link]