Mars, the fourth planet from the Sun, has polar ice caps—two prominent white regions covering its northern and southern poles. These features are easily visible even from Earth-based telescopes. The Martian ice caps are fundamentally different from Earth’s water-ice-dominated caps. Their composition and behavior are linked to the thin Martian atmosphere and the planet’s distinct orbital mechanics, holding information about Mars’s past climate and volatile resources.
The Dual Composition: Carbon Dioxide and Water Ice
The physical makeup of the Martian polar caps is a layered structure, consisting of both water ice and frozen carbon dioxide (dry ice). The permanent portion of both caps is primarily composed of water ice mixed with dust, forming vast, layered deposits.
The two poles differ in the stability of the dry ice layer during summer. The North Polar Cap (Planum Boreum) sheds its seasonal carbon dioxide layer entirely, leaving a residual cap of almost entirely water ice. In contrast, the South Polar Cap (Planum Australe) retains a thin, permanent veneer of carbon dioxide ice, estimated to be about eight meters thick. This persistent dry ice layer makes the south pole significantly colder.
Beneath the surface, the bulk of both polar caps consists of the Polar Layered Deposits (PLD). These are thousands of alternating strata of water ice and dust, compressed over eons. The PLD record the history of dust storms and climate shifts, reflecting climatic distinctions between the northern and southern hemispheres.
Seasonal Dynamics: Growth and Sublimation
The annual cycle of the Martian polar caps is driven by the freezing and sublimation of carbon dioxide gas. During winter, atmospheric carbon dioxide freezes directly onto the surface, forming a vast, temporary seasonal ice cap that extends into the mid-latitudes. This formation removes a significant portion of the atmosphere from circulation.
An estimated 25 to 30 percent of the planet’s entire atmosphere cycles onto and off the poles each Martian year. This massive movement of gas causes global atmospheric pressure to fluctuate noticeably. When spring arrives, sunlight warms the cap, causing the solid carbon dioxide to bypass the liquid phase and turn directly into gas (sublimation). This rapid sublimation returns the gas to the atmosphere, driving wind and weather patterns.
The sublimation process on the south cap creates unique “Swiss cheese” terrain, characterized by flat-topped mesas riddled with circular depressions. These features form when translucent dry ice allows sunlight to warm the ground underneath, trapping pressurized carbon dioxide gas. The gas ultimately bursts through the ice layer in geyser-like eruptions, carrying dark dust to the surface.
The Permanent Water Ice Reservoirs
The permanent water ice stored beneath the seasonal carbon dioxide frost is the most substantial part of the polar regions. These deep reservoirs represent the largest known inventory of water on Mars today. The northern cap’s reservoir alone is estimated to hold approximately 821,000 cubic kilometers of ice, roughly one-third the volume of Earth’s Greenland ice sheet. This enormous volume is locked up in the Polar Layered Deposits (PLD).
The vast quantity of water ice offers a window into Mars’s climatic past. Scientists study the alternating layers of ice and dust in the PLD to understand how the planet’s axial tilt (obliquity) has changed over millions of years, driving global climate shifts. Variations in the layering suggest recurring cycles of ice deposition and erosion influenced by these astronomical changes.
The permanent water ice is also important for the future of human exploration. This accessible store of frozen water is a valuable resource for life support, drinking water, and producing rocket fuel through electrolysis. Utilizing these resources directly from the planet (in-situ resource utilization) is necessary for long-duration human missions.