The planet Mars has two large ice caps at its poles made of water ice and some frozen carbon dioxide (CO₂). During winter, thick layers of water ice cover the poles, and thin layers of dry ice form on top. These layers stay in the dark for long periods, causing about 25–30% of the atmosphere to settle at the poles each year. When sunlight returns, the frozen CO₂ turns directly into gas, a process called sublimation. This movement of gas and dust creates frost and clouds similar to those on Earth.
Both polar ice caps are mostly made of water ice. In the northern winter, a thin layer of dry ice about 1 meter thick forms on the northern ice cap. The southern ice cap has a thicker layer of dry ice, about 8 meters thick. During the northern summer, the northern ice cap is about 1,000 kilometers wide and holds about 1.6 million cubic kilometers of ice. If spread evenly, this ice would be 2 kilometers thick. This volume is smaller than the Greenland ice sheet, which holds about 2.85 million cubic kilometers of ice. The southern ice cap is about 350 kilometers wide and 3 kilometers thick. The total ice volume in the southern cap and nearby layers is also estimated at 1.6 million cubic kilometers. Spiral-shaped grooves on both ice caps are caused by strong winds that spiral due to the planet’s rotation, as shown by radar studies.
In areas near the southern ice cap, dry ice forms clear slabs about 1 meter thick on the ground during winter. When spring arrives, sunlight warms the ground, and gas from subliming CO₂ builds up under the slabs. This pressure lifts and breaks the slabs, causing eruptions of CO₂ gas mixed with dark sand or dust. These events happen quickly, over days, weeks, or months, which is unusual for geological processes on Mars. The gas flowing under the slabs creates spider-like patterns of channels beneath the ice.
In 2018, Italian scientists reported that radar data might show a lake of water 1.5 kilometers below the surface of the southern polar layers, not under the visible ice cap. The lake is about 20 kilometers wide. If confirmed, this would be the first known stable body of water on Mars. However, the radar signals could also indicate solid minerals or salty ice instead of liquid water.
Shared features
Research over 16 years showed that each Martian winter, about 3 trillion to 4 trillion tons of carbon dioxide freezes from the atmosphere and forms part of the polar cap in the winter hemisphere. This amount is 12 to 16 percent of the total mass of Mars's atmosphere. These findings match predictions made by the Mars Global Reference Atmospheric Model—2010.
Both polar caps on Mars have layered structures called polar-layered deposits. These layers form from the seasonal melting and buildup of ice mixed with dust from Martian dust storms. These layers may one day provide information about Mars's past climate, similar to how tree rings and ice cores on Earth reveal Earth's history. Both polar caps also have grooved features, likely shaped by wind patterns. The amount of dust affects the grooves: more dust makes the surface darker, which increases melting because dark surfaces absorb more light. Other theories also try to explain the grooves.
China's Zhurong rover studied the Utopia Planitia region and found dunes pointing in different directions. Bright barchan dunes and dark longitudinal dunes suggest that the main wind direction changed by about 70 degrees. Scientists think this wind shift happened when Mars's tilt changed, which also caused changes in the layers of the northern ice caps.
Deuterium is a type of hydrogen that is heavier than the most common type, called protium. Because of its weight, deuterium is less likely to escape into space compared to protium. Scientists measured the ratio of HDO (a form of water with deuterium) to H₂O (normal water) in Mars's north polar cap. In 2015, researchers found that the polar cap ice has about eight times more deuterium than Earth's oceans. This suggests Mars lost enough water to fill an ocean at least 137 meters deep. If all this water had once been liquid on the surface, it would have covered 20% of Mars and reached nearly a mile in depth in some areas, such as the Vastitas Borealis and nearby lowlands.
An international team used the ESO's Very Large Telescope, along with instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility, to study different forms of water in Mars's atmosphere over six years.
North polar cap
The main part of the northern ice cap on Mars is made of water ice. It also has a thin layer of dry ice, which is frozen carbon dioxide. Every winter, the ice cap grows by adding 1.5 to 2 meters of dry ice. In summer, the dry ice turns directly into gas and disappears into the atmosphere. Mars has seasons similar to Earth's because its axis is tilted at about 25.19 degrees, close to Earth's tilt of 23.44 degrees.
Each year on Mars, up to one-third of the planet's thin carbon dioxide atmosphere freezes during winter in both the northern and southern hemispheres. Scientists have measured small changes in Mars's gravity field caused by the movement of carbon dioxide.
The northern ice cap is lower in elevation than the southern ice cap. The northern cap's base is at −5000 meters, and its top is at −2000 meters, while the southern cap's base is at 1000 meters and its top is at 3500 meters. The northern cap is also warmer, so all the frozen carbon dioxide disappears each summer. The part of the cap that remains after summer is called the north residual cap, and it is made of water ice. This water ice may be up to three kilometers thick. A thinner layer of ice, called the seasonal cap, begins forming in late summer or early fall when clouds form. These clouds drop precipitation that thickens the cap. The northern polar cap is symmetrical around the pole and covers the surface down to about 60 degrees latitude. High-resolution images from NASA's Mars Global Surveyor show that the northern polar cap has a "cottage cheese" appearance due to pits, cracks, and small bumps. These pits are closely spaced compared to the larger depressions on the southern cap.
Both polar caps have layers formed by seasonal melting and the buildup of ice and dust from Martian dust storms. These layers lie beneath the permanent polar caps. These layers may one day reveal information about Mars's past climate, similar to how tree rings and ice cores show Earth's climate history. Both caps also have grooves, likely caused by wind patterns and sunlight, though other theories exist. The amount of dust affects the grooves: more dust makes the surface darker, which increases melting because dark surfaces absorb more light. A large valley called Chasma Boreale runs across the northern polar cap. It is about 100 kilometers wide and up to 2 kilometers deep, deeper than Earth's Grand Canyon.
Changes in the tilt of Mars's axis affect the size of the polar caps. When the tilt is greatest, the poles receive more sunlight for longer periods each day. This extra sunlight causes ice to melt, potentially covering parts of the surface with up to 10 meters of ice. Evidence suggests glaciers may have formed during past climate changes caused by shifts in Mars's tilt.
Research from 2009 shows that layers of ice in the polar caps match models of Mars's climate changes. NASA's Mars Reconnaissance Orbiter uses radar to detect differences in the electrical properties of layers. The radar reveals alternating zones of high and low reflectivity, which can be linked to changes in Mars's tilt. The top layer of the northern polar cap, which is the most recent, is highly reflective, suggesting it formed during periods of small tilt changes. Dustier layers may have formed during times when the atmosphere had more dust.
A 2010 study using HiRISE images found that the brightness of layers depends on more than just dust. The angle of sunlight and the spacecraft's position affect how bright the layers appear. Surface roughness and frost cover can also change brightness. Wind can erode surfaces, making some features appear as layers when they are not. HiRISE did not detect layers thinner than those seen by the Mars Global Surveyor but showed more detail within layers.
Radar measurements of the northern polar cap found that the water ice in its layered deposits totals 821,000 cubic kilometers, equal to 30% of Earth's Greenland ice sheet. This includes ice beneath the layered deposits. The radar is part of the Mars Reconnaissance Orbiter.
SHARAD radar data combined into 3D models reveal buried craters, which can help date certain layers. In February 2017, the European Space Agency released a new view of Mars's North Pole, created from 32 images taken by the Mars Express.
A 2023 study in Nature found a sudden increase in brightness in the northern ice cap layers about 0.4 million years ago. This change may have altered wind directions observed in areas explored by the Zhurong rover.
South polar cap
The south polar permanent cap is much smaller than the one in the north. It is 400 kilometers wide, while the northern cap is 1,100 kilometers wide. Each southern winter, the ice cap covers the surface to a latitude of 50°. Part of the ice cap consists of dry ice, which is solid carbon dioxide. Each winter, the ice cap grows by adding 1.5 to 2 meters of dry ice from precipitation from a polar-hood of clouds. In summer, the dry ice turns directly into gas and enters the atmosphere. During each year on Mars, as much as a third of Mars’s thin carbon dioxide (CO₂) atmosphere "freezes out" during the winter in the northern and southern hemispheres. Scientists have measured tiny changes in Mars’s gravity field due to the movement of carbon dioxide. This means the winter buildup of ice changes the planet’s gravity. Mars has seasons similar to Earth’s because its rotational axis is tilted at an angle close to Earth’s (25.19° for Mars, 23.45° for Earth). The south polar cap is higher in elevation and colder than the one in the north.
The residual southern ice cap is not centered on the south pole. However, the south seasonal cap is centered near the geographic pole. Studies show the off-center cap forms because more snow falls on one side than the other. On the western hemisphere side of the south pole, a low-pressure system forms due to winds changed by the Hellas Basin. This system produces more snow. On the opposite side, there is less snow and more frost. Snow reflects more sunlight in summer, so it melts or sublimates less. Frost has a rougher surface and traps more sunlight, causing more sublimation. This means areas with more frost are warmer.
Research published in April 2011 described a large deposit of frozen carbon dioxide near the south pole. Most of this deposit likely enters Mars’s atmosphere when the planet’s tilt increases. This causes the atmosphere to thicken, winds to strengthen, and larger areas on the surface to support liquid water. Analysis showed that if these deposits turned into gas, Mars’s atmospheric pressure would double. There are three layers of these deposits, each capped with a 30-meter layer of water ice that prevents the CO₂ from sublimating. These layers are linked to times when the atmosphere collapsed due to climate changes.
A large field of eskers, called the Dorsa Argentea Formation, exists around the south pole. It is believed to be the remains of a giant ice sheet that covered about 1.5 million square kilometers. That area is twice the size of the state of Texas.
In July 2018, ESA found signs of liquid salt water buried under layers of ice and dust by analyzing radar pulse reflections from the Mars Express spacecraft.
The north polar cap of Mars has a flat, pitted surface resembling cottage cheese, while the south polar cap has larger pits, troughs, and flat mesas that give it a Swiss cheese appearance. The upper layer of the south polar residual cap has been eroded into flat-topped mesas with circular depressions. Observations from the Mars Orbiter Camera in 2001 showed that the scarps and pit walls of the south polar cap had retreated at an average rate of about 3 meters per Mars year. In some places, the scarps retreat less than 3 meters per year, while in others, they retreat as much as 8 meters per year. Over time, south polar pits merge into plains, mesas turn into buttes, and buttes disappear. The round shape of the depressions is likely influenced by the angle of the sun. In summer, the sun moves around the sky, sometimes staying above the horizon for 24 hours. This causes the walls of a round depression to receive more sunlight than the floor, melting the walls more quickly.
Later research using HiRISE showed that the pits are in a 1–10 meter thick layer of dry ice sitting on a larger water ice cap. Pits begin as small areas along faint fractures. Circular pits have steep walls that focus sunlight, increasing erosion. For a pit to form, a steep wall of about 10 centimeters and a length of over 5 meters is needed.
The surface of the south polar cap resembles Swiss cheese, as shown in images. Differences over a two-year period can also