What winter means on Mars: temperatures can plunge to −130°C, carbon dioxide falls as snow or freezes directly onto the ground, as much as 16% of the atmosphere becomes part of the winter polar cap, and air pressure planet-wide swings by roughly 25% over the seasonal cycle.

Winter on Mars does not simply make an already cold planet colder. At the winter pole, the atmosphere itself begins to become part of the ground.

Carbon dioxide, which accounts for about 95 percent of Martian air, freezes out of the polar darkness. Some falls from clouds as dry-ice snow. Some changes directly from gas to solid on the surface, forming frost. Spacecraft gravity tracking indicates that a winter polar cap can temporarily hold as much as 16 percent of the atmosphere’s mass.

When sunlight returns in spring, that dry ice turns back into gas. Pressure then rises not only near the cap but across Mars. Curiosity has measured the cycle near the equator, where the seasonal pressure range is roughly 25 percent.

These headline numbers need boundaries. Minus 130 degrees Celsius is a rounded lower extreme for the coldest times and places, not the temperature of every Martian winter day. The 16 percent cap figure and the 25 percent pressure swing also come from different measurements with different reference points. Both belong to the same carbon-dioxide cycle, but they are not interchangeable.

Familiar seasons on a less familiar orbit

Mars has seasons for the same basic reason Earth does. Its rotation axis is tilted by about 25.2 degrees, close to Earth’s 23.4 degrees. As the planet travels around the Sun, one hemisphere leans into stronger sunlight while the other leans away.

The timing is different. A Martian year lasts about 687 Earth days, so each season persists for months longer than its Earth counterpart. Mars also follows a much more elliptical orbit, making its distance from the Sun vary more strongly over a year.

That eccentricity makes the seasons unequal. NASA’s 2026 comparison of planetary seasons notes that northern spring occupies about 29 percent of a Martian year, while northern autumn takes only about 21 percent. Southern summer occurs near perihelion, when Mars is closest to the Sun, and is comparatively short and intense. Southern winter is long, and its polar cap has more time to grow.

The two hemispheres are not mirror images. Differences in elevation, season length, dust and circulation mean that the northern and southern caps accumulate and retreat differently.

What a low near minus 130 degrees actually means

Mars’s atmosphere is less than one percent as dense as Earth’s at sea level. It provides little insulation and stores little heat. Sunlit ground can warm sharply, while the same place can lose heat rapidly after sunset.

NASA gives about minus 129 degrees Celsius as a possible nighttime low on Mars. Polar winter values reach a similar range when the Sun remains below the horizon and the surface continues radiating heat into space. The minus 130 degree figure is therefore a useful rounded extreme, not a planetary winter average.

Temperature varies with latitude, altitude, local time, dust in the air and the thermal properties of the ground. A rock that absorbed sunlight can remain warmer than nearby loose soil after sunset. A slope turned away from the Sun may remain cold enough for frost while a neighbouring surface clears.

This range explains why winter imagery can be misleading. A bright coating may resemble terrestrial snow, but the material, temperature and atmospheric process can be quite different.

Mars has water snow and dry-ice snow

Martian snow comes in two forms. Water-ice snow has been detected, including by the Phoenix lander’s laser instrument in the northern polar region. Because the lower atmosphere is so dry, much of that water ice turns directly back into vapour before reaching the ground.

Carbon-dioxide snow forms only in colder conditions. NASA’s guide to winter on Mars explains that dry-ice crystals would be cube-shaped rather than six-sided like familiar water snowflakes, because solid carbon dioxide has a different crystal symmetry. Measurements indicate that the particles are smaller than the width of a human hair.

The Mars Climate Sounder on Mars Reconnaissance Orbiter supplied the clearest evidence that this dry-ice snow reaches the surface. During southern winter it detected a carbon-dioxide cloud about 500 kilometres across over the pole, along with smaller clouds farther north. The particles were large enough to fall during the clouds’ lifetimes, and their infrared signature extended towards the ground.

NASA called the observation the first definitive detection of carbon-dioxide snow clouds. The snowfall was especially vigorous over the residual south polar cap, the one place where exposed carbon-dioxide ice survives on the surface throughout the year.

Snow is only part of the deposition. Carbon dioxide can also pass directly from gas to solid on sufficiently cold ground. NASA imagery shows the resulting seasonal frost extending from a winter pole towards the middle latitudes. A 2022 HiRISE image, for example, documented patchy dry ice on a pole-facing slope at 37 degrees south, well beyond the permanent cap.

Weighing a polar cap through a spacecraft’s orbit

No scale can be placed beneath the Martian winter cap. Its mass can nevertheless be measured because moving trillions of tonnes of carbon dioxide changes Mars’s gravity by a very small amount.

Antonio Genova and colleagues analysed 16 years of radio tracking from Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. Tiny changes in spacecraft motion allowed the team to separate a time-varying gravity signal from the planet’s static gravity field.

Their 2016 Icarus paper estimated that roughly three to four trillion tonnes of carbon dioxide are deposited at a winter pole, equivalent to about 12 to 16 percent of the entire atmosphere’s mass.

The geography is important. This does not mean 16 percent of the air freezes wherever winter is occurring. Atmospheric circulation transports carbon dioxide into the cold hemisphere, where it is concentrated into an expanding seasonal cap. The cap can reach several metres in thickness in some areas and extend towards roughly 50 degrees latitude at its greatest reach.

The seasonal layer also should not be confused with the permanent polar deposits below it. Much of the long-lived northern cap is water ice. The south includes a residual carbon-dioxide deposit, but both poles acquire a far broader temporary dry-ice covering during winter.

Why pressure changes everywhere, not only at the pole

Surface pressure measures the weight of atmosphere above a location. When carbon dioxide leaves the gas phase and joins a polar cap, the total atmospheric mass decreases. The effect therefore establishes a changing global baseline even though weather, topography and daily thermal tides create additional local variations.

Curiosity’s Rover Environmental Monitoring Station has watched that baseline rise and fall from Gale Crater near the equator. A JPL report after its second Martian seasonal cycle described variations of about 25 percent as the caps alternately captured carbon dioxide in winter and released it in spring.

It may look inconsistent for a cap holding no more than 16 percent of atmospheric mass to accompany a 25 percent pressure range. The apparent mismatch largely comes from how the quantities are defined. The gravity result estimates the cap’s deposited mass relative to the atmosphere. The pressure figure compares a seasonal high with a seasonal low at a measurement site, and the two hemispheres produce unequal cycles. Local circulation and weather ride on top of the planet-wide mass change.

The safe reading is that Mars moves a substantial minority of its atmosphere into and out of solid storage every year. Neither percentage should be treated as an exact universal value for every location and every Martian year.

Spring releases the atmosphere without melting it

When the polar Sun returns, ordinary Martian surface pressure is too low for carbon dioxide to form stable liquid pools. The ice sublimates, changing directly from solid to gas. The seasonal cap recedes and atmospheric pressure climbs.

Where sunlight penetrates translucent slab ice, the ground below can warm first. Gas becomes trapped, builds pressure and escapes through cracks, carrying dark dust onto the bright surface. Repeated jets carve branching features informally known as spiders and leave fan-shaped deposits that record the wind direction at the time of eruption.

Not every pale Martian coating belongs to this carbon-dioxide system. SpaceDaily recently examined morning frost across four giant Martian volcanoes. That equatorial frost is water ice only about 0.01 millimetres thick and vanishes within hours of sunrise. The winter polar cap is a far larger exchange dominated by frozen carbon dioxide.

Winter changes the inventory of the planet

On Earth, winter reorganises heat and water while the mass of the atmosphere remains effectively unchanged. Mars has a different kind of seasonal engine because its main atmospheric gas can freeze under naturally occurring surface conditions.

The extreme cold, dry-ice snowfall, expanding polar cap and pressure swing are therefore not separate curiosities. They are observable stages of one cycle. Carbon dioxide migrates towards the dark pole, becomes part of the landscape, and returns to the air when sunlight comes back.

That is what winter means on Mars: a season long and cold enough for the planet to put a measurable fraction of its atmosphere on the ground.