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Mars’s poles both grow seasonal carbon-dioxide frost in winter, but their summer remnants differ: the north cap is mainly water ice, while the south retains a relatively thin carbon-dioxide cover over water ice. Their terrain also looks different, and the full reason for the asymmetry is not settled.
Seasonal frost is different from the summer remnants
Each Martian winter, carbon dioxide from the atmosphere freezes onto the polar surface, forming a seasonal cap. NASA describes this frost as carbon dioxide mixed with smaller amounts of water-ice frost and dust. As spring warms the surface, the carbon dioxide sublimates—changing directly from solid to gas—and much of the seasonal cover retreats.
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What remains through summer is called a residual cap. Its boundary can vary from one summer to another, so “permanent” is a useful contrast with seasonal frost, not a claim that the cap never changes. NASA/JPL explains the distinction in “What is a ‘Residual’ Polar Cap?”
How the north and south caps differ in composition
| Feature | North pole | South pole |
|---|---|---|
| Winter seasonal cover | Carbon-dioxide frost forms and retreats seasonally. | Carbon-dioxide frost forms and retreats seasonally. |
| Summer residual | Mainly water ice; NASA/JPL’s 2015 radar report describes the remaining north cap as all water ice during northern summer. | Water ice remains beneath a relatively thin carbon-dioxide layer, according to NASA/JPL’s 2015 radar report. |
| Described surface | Relatively flat and pitted, with cracks and knobs also noted in spacecraft observations. | Larger pits, troughs, and flat mesas are described in spacecraft observations. |
The south’s surface can be described as “mostly carbon dioxide” in older NASA image accounts, while the later radar summary distinguishes that persistent surface cover from water ice beneath it. These descriptions need not mean that the entire southern deposit is a solid mass of dry ice. For the later composition account, see NASA/JPL’s 2015 radar report; the older north-pole image page is NASA Science’s “North Polar Cap”.
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What the different surfaces look like
NASA/JPL’s 2000 comparison described the north residual cap as relatively flat and pitted, with a “cottage-cheese” appearance. It compared the south’s larger pits, troughs, and flat mesas to “Swiss cheese.” These are visual analogies for observed terrain, not descriptions of what the caps are made of. An archived NASA technical report also records pits, cracks, and knobs in the north and layered southern terrain modified by collapse and erosion, including troughs and polygonal depressions. The observations apply to the spacecraft views described, not necessarily every part of either polar region.
See NASA/JPL’s 2000 comparison and the NASA technical report on residual-cap differences.
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What the layers can tell us about Martian climate
Polar deposits preserve layers that record changes in Mars’s climate over time. A NASA Hubble image description notes that retreating northern carbon-dioxide frost exposes the water-ice remnant and layered terrain interpreted as evidence of past climate change. NASA technical work also discusses how long-term shifts in Mars’s axial tilt can affect polar deposition and climate. That context does not mean a visible layer can be assigned to one particular year or event.
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Sources: NASA’s Hubble image description of the north pole and NASA’s paper on polar stability and climate.
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Why are the two poles different?
The north–south contrast is observed, but the cited sources do not establish one complete explanation for it. Modeling work examines how seasonal retreat and changes in atmospheric pressure could contribute to the asymmetry. Those are proposed factors to investigate, not a settled single cause. See the NASA modeling paper on polar-cap asymmetry.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why shrinking south-polar mesas do not prove the whole cap is shrinking
NASA describes pits growing in south-polar dry-ice mesas as carbon dioxide turns to vapor and exposes an older surface likely made of water ice. At the same time, nearby flat areas can accumulate new dry ice; NASA says the total frozen carbon dioxide at the South Pole may even be increasing. Local erosion therefore does not, by itself, demonstrate a net loss of the southern carbon-dioxide deposit. NASA distinguishes these local processes from Earth-like climate change in “The Changing Ice Cap of Mars.”
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In winter, both poles acquire seasonal carbon-dioxide frost. In summer, the north is mainly water ice, while the south’s water ice remains under a thin carbon-dioxide cover. Their visible terrain differs, but the complete cause of the hemispheric contrast remains unresolved in the cited sources.
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