Martian polar ice caps

• 1. Introduction

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  The Martian polar ice caps hold roughly 1.6 million cubic kilometers of water ice at each pole. That is a staggering quantity, yet it represents only a fraction of the water Mars once possessed. Scientists measuring the ratio of two isotopes of hydrogen in the atmosphere above the north polar cap found, in March 2015, that Mars has lost a volume of water 6.5 times larger than what sits frozen at the poles today. Had all of that ancient water been liquid and spread across the surface at once, it would have covered twenty percent of the planet and reached nearly a mile deep in places. What happened to it? And what is left behind? The ice caps of Mars are not static ruins. They breathe with the seasons, erupt like geysers in the spring, record climate history in their layers, and may conceal a lake of liquid water beneath the southern pole. The story written in this ice is still being read.

• 2. Seasonal Rhythms

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  Each Martian winter, somewhere between 3 trillion and 4 trillion tons of carbon dioxide freezes out of the atmosphere and piles onto whichever pole lies in darkness. That represents 12 to 16 percent of the total mass of the Martian atmosphere, which is thin by Earth standards but still capable of shifting the planet's gravity measurably as the ice builds and retreats. Scientists have detected those tiny gravitational changes from spacecraft in orbit.

  Mars has seasons comparable to Earth's because its rotational axis tilts at 25.19 degrees, close to Earth's 23.44 degrees. During northern winter, the north polar cap grows by adding 1.5 to 2 meters of dry ice. When sunlight returns, that dry ice sublimates directly from solid to gas. The same cycle plays out at the southern pole. In the process, enormous quantities of dust and water vapor are moved across the planet, generating Earth-like frost on the surface and large cirrus clouds in the sky.

  The seasonal frozen layer in the north is about one meter thick. The south keeps a permanent dry ice cover roughly 8 meters thick year-round, even after the seasonal veneer has come and gone.

• 3. Two Very Different Poles

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  The northern polar cap stretches to a diameter of about 1,000 kilometers during the Martian summer and sits at a relatively low elevation, with its base at roughly 5,000 meters below the Martian datum and its top at about 2,000 meters below it. Because it is warmer, all of its frozen carbon dioxide disappears each summer. What remains is a residual cap of water ice believed to reach as much as three kilometers in depth. Radar measurements published later put the volume of water ice in the layered deposits at 821,000 cubic kilometers, equal to 30 percent of Earth's Greenland ice sheet.

  The southern cap is smaller and sits higher and colder. Its permanent residual cap measures 350 kilometers in diameter and about 3 kilometers thick, compared with the northern cap's 1,000 kilometer width. Its base sits at roughly 1,000 meters above the Martian datum, its top at around 3,500 meters. Unlike the northern cap, which is symmetrical around the geographic pole, the southern residual cap is displaced off-center. Research has traced this asymmetry to a low-pressure weather system generated by the Hellas Basin, which pushes more snow onto one side of the pole than the other. Snow reflects more sunlight and sublimates less; rougher frost traps sunlight and sublimates more, making those areas comparatively warmer.

  One large valley called Chasma Boreale cuts halfway across the northern cap, running about 100 kilometers wide and up to 2 kilometers deep, which is deeper than Earth's Grand Canyon.

• 4. Geysers and Spiders

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  Near the southern cap each spring, sunlight reaches frozen ground covered by a transparent slab of dry ice about 1 meter thick. The subsurface warms, and pressure from subliming carbon dioxide builds underneath until the slab ruptures. The result is a geyser-like eruption of carbon dioxide gas mixed with dark basaltic sand. This process is rapid by planetary standards, observed unfolding over a span of days, weeks, or months.

  As the gas rushes toward the rupture point, it carves channels beneath the ice that radiate outward in feathery extensions. These formations, typically 500 meters wide and 1 meter deep, are called spiders or starburst channels. One model explains their origin through dust grains warmed by sunlight: the grains sink through the ice, leaving clear channels behind. Sunlight then reaches the dark base of the slab, converting the solid carbon dioxide directly to gas, which flows toward any weak point and bursts out carrying dust with it. Surface winds shape the escaping dust into dark fans visible from orbit. The physics behind this process resembles the mechanism proposed to explain dark plumes observed erupting from the surface of Triton.

  Research using HiRISE images published in January 2010 found that some spider channels grow wider as they run uphill, because gas rather than liquid is doing the erosion. Frost buries the fans and channels each autumn, and the cycle begins again the following spring.

• 5. Layers as Climate Archives

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  Both polar caps preserve layered deposits built up over countless seasons of ice accumulation and dust deposition from Martian dust storms. Scientists compare these layers to tree rings or ice cores on Earth: records of past climate that may eventually be decoded to reveal how Mars changed over time.

  NASA's Mars Reconnaissance Orbiter carried a radar instrument that measured contrasts in electrical properties between layers, producing a cross-sectional view of the north polar layered deposits. High-reflectivity zones with multiple contrasting layers alternate with zones of lower reflectivity. The pattern correlates with models of changes in Mars's axial tilt over time. Dustier periods produced darker, more absorptive layers; the topmost zone, the most recently deposited, is strongly radar-reflective, suggesting the Martian axis has not varied greatly in recent history.

  Research published in January 2010 using HiRISE images complicated this picture, showing that layer brightness depends not only on dust content but also on sun angle, spacecraft viewing angle, surface roughness, and fresh frost cover. The HiRISE camera did not reveal layers thinner than those already seen by the Mars Global Surveyor, but it resolved more detail within the existing layers.

  A paper published in Nature in 2023 found an abrupt brightness increase in the northern ice cap layers that occurred at roughly 0.4 million years ago. That change may have shifted wind directions in ways that are now recorded in dune orientations studied by China's Zhurong rover in the Utopia Planitia region.

• 6. The Possibility of Hidden Water

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  In July 2018 the European Space Agency reported that radar data from the Mars Express spacecraft showed indications of liquid salt water buried beneath layers of ice and dust near the south pole. Italian scientists had separately reported in 2018 that radar reflections suggested a subglacial lake 1.5 kilometers below the surface of the southern polar layered deposits, roughly 20 kilometers across. If confirmed, it would be the first known stable body of liquid water on Mars. The radar reflections may, however, be produced by solid minerals or saline ice rather than liquid water, so the finding remains contested.

  A large field of eskers, long ridges left by meltwater flowing under glaciers, exists around the south pole in a region called the Dorsa Argentea Formation. Researchers believe this formation is the remnant of a giant ice sheet that once covered about 1.5 million square kilometers, an area twice the size of the state of Texas.

  Nearby, research published in April 2011 described large deposits of frozen carbon dioxide buried close to the south pole. Each deposit is capped by a 30-meter layer of water ice that prevents the carbon dioxide from sublimating. Analysis showed that if these three buried layers were fully converted to gas, the atmospheric pressure on Mars would double, potentially allowing liquid water to exist across larger areas of the surface.

• 7. Deuterium and a Lost Ocean

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  Deuterium is a heavier isotope of hydrogen and is harder for stellar wind to strip away from a planet than ordinary protium, the common isotope. Over billions of years, Mars lost most of its water to space, but deuterium stayed behind in proportionally greater amounts. By measuring the ratio of deuterium-bearing water to ordinary water in the atmosphere above the north polar cap, scientists built a record of how much water Mars once had.

  An international team used three major observatories over a six-year period: ESO's Very Large Telescope, instruments at the W. M. Keck Observatory, and the NASA Infrared Telescope Facility. Their results, published in March 2015, showed that the polar ice is about eight times as enriched with deuterium as Earth's ocean water. From this enrichment, the team calculated that Mars lost a volume of water 6.5 times greater than what remains in the polar caps today.

  That lost water may once have pooled in the low-lying northern plains of Vastitas Borealis and adjacent lowlands including Acidalia, Arcadia, and Utopia Planitiae. Separate evidence for an ancient global ocean at least 137 meters deep comes from those same isotopic measurements. The surviving polar ice, for all its scale, is the residue of something far larger.