The Ice Caps of Mars: Two Frozen Poles That Breathe With the Seasons

What you would actually see at the poles

If you hovered above the Martian north pole in late northern winter, you would see a brilliant white cap of frost stretching far south — sometimes reaching down to about 50° latitude, the Martian equivalent of covering everything on Earth from the pole to the middle of Canada or the northern United States. This is the seasonal cap, and it is almost entirely frozen carbon dioxide — dry ice, the same stuff that fogs a stage. As spring arrives it retreats rapidly, and the frost peels back to expose a much smaller, permanent white core.

That core is the residual (permanent) cap. In the north it is called Planum Boreum, roughly 1,000 km across — comparable to the state of Texas or the country of France — and made of remarkably clean water ice. In the south, the residual cap is Planum Australe, smaller and offset from the geographic pole, and here the summer surface stays cloaked in a thin skin of CO₂ ice that never fully disappears.

Look closer and the caps are not smooth domes but sculpted spirals. Both permanent caps are carved by spiral troughs — canyons that wind outward from the center like the arms of a pinwheel, typically hundreds of meters deep and reaching up to roughly 1 km at their deepest. The north cap is also split by Chasma Boreale, a colossal canyon roughly 560 km long and up to 2 km deep, comparable in scale to parts of Earth's Grand Canyon system but cut into ice and dust rather than rock.

The mechanism: an atmosphere that freezes and thaws

Mars has a thin atmosphere — surface pressure averages about 6 millibars, less than 1% of Earth's — and it is over 95% carbon dioxide. That combination sets up something no other planet does at this scale. In polar winter, with no sunlight for months, the surface plunges to about 148 K (−125 °C), which is the temperature at which CO₂ condenses directly into ice at Martian pressures.

So the air itself freezes. Roughly 25 to 30% of the entire atmosphere snows out onto the winter pole each year, building a seasonal frost layer that is typically around a meter thick. This is the planet's dominant seasonal cycle:

• Autumn/winter: CO₂ condenses at the dark pole, the cap grows, and atmospheric pressure across the whole planet measurably drops.
• Spring: returning sunlight sublimates the dry ice straight back to gas, the cap shrinks, and global pressure rises again.
• Summer: in the north the CO₂ is gone entirely, exposing bare water ice; in the south a stubborn CO₂ remnant survives all year.

Because the two hemispheres are out of phase, and because Mars's elliptical orbit gives the southern winter a longer, deeper freeze, the caps trade mass back and forth in a planet-wide seasonal breathing. The permanent water-ice caps, by contrast, do not sublimate away — water ice is far more stable, and it accumulates and erodes far more slowly, over geological time.

The numbers: how much ice, how deep, how much water

The permanent caps are genuinely massive. Radar sounding from orbit — chiefly the SHARAD instrument on NASA's Mars Reconnaissance Orbiter and MARSIS on ESA's Mars Express — has measured through the ice to the rock below, revealing stacks of layered material called the Polar Layered Deposits (PLD) up to about 3 km thick.

The northern cap alone holds an estimated 1.6 million km³ of water ice. Melted and spread evenly over the whole planet, the combined polar water would form a global ocean several meters deep — a serious inventory of accessible water for a world that looks bone-dry at the surface today.

The south hides a different treasure. SHARAD found a buried deposit of solid CO₂ ice within the southern layered deposits totaling roughly 9,500 to 12,500 km³ — about 30 times the amount in the visible southern residual cap. That is enough frozen carbon dioxide that, if it all sublimated, it could roughly double the total mass of the Martian atmosphere. The dry ice is sandwiched between and capped by thin bounding layers of water ice tens of meters thick.

For comparison: Earth's Antarctic and Greenland ice sheets together hold on the order of 30 million km³ of ice, so Mars's polar caps are far smaller in absolute terms. But relative to a much smaller, drier planet with a wisp of an atmosphere, they are a dominant reservoir — and the CO₂ they can release or trap directly regulates the whole climate.

Spiders, geysers, and the strangest springtime on any planet

The southern seasonal cap hosts a phenomenon with no Earth analog. As spring sun reaches the translucent slab of seasonal CO₂ ice, it passes through the ice and warms the dark ground underneath. The dry ice then sublimates from the bottom up, building a pocket of high-pressure gas trapped beneath a solid ice roof.

Eventually the pressure ruptures the ice. Gas rushes toward the crack, tearing loose dark sand and dust and blasting it upward in a jet — a genuine cold geyser venting carbon dioxide. This is the Kieffer model, named for planetary scientist Hugh Kieffer, who proposed it in the early 2000s. Over many seasons the escaping gas carves branching, radial channels into the ground that, seen from orbit, look uncannily like spiders — formally called araneiform terrain. The dust the geysers eject settles into dark fans and spots, all pointing downwind, mapping the surface winds of that spring.

These are not ancient features but active geology happening right now, every Martian spring, imaged repeatedly by MRO's HiRISE camera. It is one of the clearest reminders that Mars is not a dead planet: its poles are a working machine, sublimating, cracking, and venting on an annual clock. Laboratory experiments in 2024 successfully recreated the spider-carving process, strengthening the case for the Kieffer mechanism.

Reading the climate record — and the liquid-water controversy

The Polar Layered Deposits are Mars's greatest climate archive. Each layer records a period when ice and airborne dust accumulated together, and their alternating brightness is thought to track cycles in Mars's orbit and spin — the Martian version of Milankovitch cycles. Mars's axial tilt (obliquity) swings far more wildly than Earth's, and during high-tilt epochs ice migrates off the poles toward mid-latitudes. Radar layering suggests the poles emerged from the most recent Martian ice age roughly 370,000 years ago, accumulating tens of thousands of cubic kilometers of ice since.

The most debated claim came in 2018, when Roberto Orosei and the MARSIS team reported a bright radar reflection beneath the south polar deposits — centered near 81°S — that they interpreted as a stable body of subglacial liquid water about 1.5 km down, kept unfrozen by dissolved salts. It would be the first known liquid water lake on Mars.

Be careful here, because this is genuinely unsettled. The interpretation is contested:

• The temperatures at that depth appear too cold for liquid brine without an implausibly large local heat source.
• Later work showed that ordinary geology — thin layers of clay, salty ice, or dry conducting material — can mimic the bright radar reflection without any liquid water at all.

So the honest answer is that the south-polar bright reflectors are real, but whether they are lakes remains an open question. It is a live scientific debate, not a settled discovery.

How we came to know the caps: from telescopes to orbiters

The Martian poles were the very first Martian weather anyone recorded. In the 17th and 18th centuries observers including Christiaan Huygens and Giovanni Cassini saw the bright polar spots through telescopes, and by the late 1700s William Herschel watched them grow and shrink with the seasons — correctly guessing they were ice or snow, and even using their behavior to argue Mars had seasons like Earth's.

The composition took far longer. Only in 1966 did Robert Leighton and Bruce Murray predict the caps should be largely frozen CO₂, and NASA's Mariner 7 flyby in 1969 confirmed dry ice at the poles from its temperatures. The picture sharpened enormously with the orbiter era:

• Viking (1976–1980): showed the north residual cap survives summer as water ice, implying a permanent water reservoir.
• Mars Global Surveyor (from 1997): mapped the caps with laser altimetry, measuring their thickness and the spiral troughs in detail.
• Mars Odyssey (2001): detected vast buried water ice extending well beyond the visible caps.
• Mars Express (2003) and MRO (2006): their MARSIS and SHARAD radars finally saw inside the caps, mapping the layered deposits, the buried CO₂ store, and the disputed south-polar reflectors.

Today the poles are among the best-studied places on Mars, watched season after season — and a leading target for future missions that hope to drill the layered deposits and read Mars's climate history layer by layer.

North vs. south permanent (residual) polar cap of Mars

Property	North cap (Planum Boreum)	South cap (Planum Australe)
Summer surface composition	Almost pure water ice	Thin CO₂ ice (~8 m) over water ice
Approx. diameter	~1,000 km	~350–400 km (bright residual patch)
Summer surface temperature	Rises to ~205 K (−68 °C)	Held near ~148 K (−125 °C) by CO₂
Elevation	Low-lying northern plains	~6 km higher, on southern highlands
Buried surprise	Layered dust + ice climate record	Vast buried CO₂ ice (~9,500–12,500 km³)