Martian polar ice caps
A 1995 photo captured the approximate size of Mars's polar ice caps, revealing two permanent features made of water ice and dry carbon dioxide. Above kilometer-thick layers of water ice permafrost, slabs of dry ice deposit during a pole's winter. These slabs lie in continuous darkness, causing 25 to 30 percent of the atmosphere to be deposited annually at either pole. When sunlight returns, the frozen CO2 sublimes back into gas. The northern cap spans about 1000 km in diameter during summer and holds roughly 1.6 million cubic kilometers of ice. This volume compares to 2.85 million cubic kilometers for Earth's Greenland ice sheet. The southern cap measures 350 km across with a thickness of 3 km. Its total volume including adjacent layered deposits also reaches 1.6 million cubic kilometers. Both regions display spiral troughs formed by katabatic winds spiraling due to the Coriolis Effect.
Research tracking spacecraft orbits over 16 years found that each winter, 3 trillion to 4 trillion tons of carbon dioxide freeze onto the winter hemisphere cap. This mass represents 12 to 16 percent of the entire Martian atmosphere. Scientists measured tiny changes in Mars's gravity field caused by this movement of carbon dioxide. Each year, up to one-third of Mars's thin CO2 atmosphere freezes out during winter in both hemispheres. The north polar cap grows by adding 1.5 to 2 meters of dry ice every winter. In summer, this seasonal layer sublimates directly into the atmosphere. The south polar cap behaves similarly but maintains a permanent dry ice cover about 8 meters thick. During spring, sunlight warms subsurface layers and pressure from subliming CO2 builds under slabs. This process elevates and ruptures the ice, creating geyser-like eruptions of gas mixed with dark basaltic sand. These geological changes occur within days, weeks, or months, a rate unusual for planetary surfaces.
The southern polar cap displays larger pits, troughs, and flat mesas giving it a Swiss cheese appearance compared to the northern cottage cheese texture. Observations from 2001 showed scarps and pit walls retreating at an average rate of 3 meters per Mars year. Over time, these round depressions merge into plains while mesas become buttes that eventually vanish. Sunlight angle aids formation since the sun moves around the sky 24 hours above the horizon in summer. Walls receive more intense light than floors, causing them to melt far faster. Later research revealed pits exist within a 1 to 10 meter thick layer of dry ice sitting atop a much larger water ice cap. Starburst channels radiate outward as feathery extensions caused by escaping gas and dust. Typically 500 meters wide and 1 meter deep, these spiders undergo observable changes in just a few days. Gas flows toward cracks occurring at weak points in the ice. As soon as sunrise occurs, gas blows out dust which winds carry into dark fan shapes. Some dust gets trapped in channels until frost covers all features again.
In July 2018, ESA discovered indications of liquid salt water buried under layers of ice and dust using radar pulses from Mars Express. Measurements taken in 2018 suggested a subglacial lake exists below the southern polar layered deposits rather than directly under the visible permanent ice cap. If confirmed, this would be the first known stable body of water on the planet. The radar reflections might instead show solid minerals or saline ice instead of liquid water. Scientists analyzed data showing that if massive CO2 deposits changed entirely into gas, atmospheric pressure on Mars would double. Three distinct layers of these deposits each feature a 30-meter layer of water ice preventing CO2 sublimation. These three layers link to periods when the atmosphere collapsed due to climate change. Research published in April 2011 described a large deposit of frozen carbon dioxide near the south pole. Most of this deposit likely enters the atmosphere when planetary tilt increases, thickening the air and strengthening winds.
Both polar caps show layered features resulting from seasonal melting and deposition of ice mixed with dust from Martian storms. Information about past climates may eventually emerge from these layers just as tree rings reveal Earth's history. High-reflectivity zones alternate with lower reflectivity zones in cross-sectional views produced by SHARAD radar instruments. Patterns correlate to models of changes in Mars's axial tilt. Since the top zone of north-polar layered deposits is strongly radar-reflective, researchers propose such sections correspond to periods of relatively small swings in planetary tilt. Dustier layers appear deposited during times when the atmosphere contains more dust. A paper published in January 2010 found understanding layers is more complicated than previously believed. Brightness depends not only on dust amount but also on sun angle and spacecraft viewing angles. The roughness of surfaces greatly changes albedo while wind can erode features. In February 2017, ESA released a new view of Mars's North Pole created from 32 individual orbits of the Mars Express. Research reported in 2009 showed ice-rich layers match models for Martian climate swings.
Evidence obtained from measuring HDO to H2O ratios over the north polar cap suggests Mars once held enough water to create a global ocean at least 137 meters deep. An international team published results in March 2015 showing polar cap ice is about eight times as enriched with deuterium as Earth's oceans. This means Mars has lost a volume of water 6.5 times larger than that stored in today's polar caps. Water may have formed an ocean covering low-lying Vastitas Borealis and adjacent regions like Acidalia, Arcadia, and Utopia planitiae. If all water were liquid and on the surface, it would have covered 20 percent of the planet and reached almost a mile deep in places. Scientists used ESO's Very Large Telescope along with instruments at W.M. Keck Observatory and NASA Infrared Telescope Facility to map isotopic forms over six years. Deuterium is a heavier hydrogen isotope less prone to being carried into space by stellar wind compared to protium. Measurements support predictions from the Mars Global Reference Atmospheric Model 2010 regarding atmospheric mass changes.
Common questions
What is the composition of Mars's polar ice caps?
Mars's polar ice caps consist of permanent features made of water ice and dry carbon dioxide. Above kilometer-thick layers of water ice permafrost, slabs of dry ice deposit during a pole's winter.
How large are the northern and southern polar ice caps on Mars?
The northern cap spans about 1000 km in diameter during summer and holds roughly 1.6 million cubic kilometers of ice. The southern cap measures 350 km across with a thickness of 3 km and also reaches a total volume of 1.6 million cubic kilometers including adjacent layered deposits.
When did scientists discover evidence of liquid salt water under Mars's south pole?
In July 2018, ESA discovered indications of liquid salt water buried under layers of ice and dust using radar pulses from Mars Express. Measurements taken in 2018 suggested a subglacial lake exists below the southern polar layered deposits rather than directly under the visible permanent ice cap.
Why do geyser-like eruptions occur at the Martian south pole?
Sunlight warms subsurface layers during spring causing pressure from subliming CO2 to build up under slabs. This process elevates and ruptures the ice creating geyser-like eruptions of gas mixed with dark basaltic sand within days, weeks, or months.
What does the deuterium ratio reveal about Mars's past water history?
Evidence obtained from measuring HDO to H2O ratios over the north polar cap suggests Mars once held enough water to create a global ocean at least 137 meters deep. An international team published results in March 2015 showing polar cap ice is about eight times as enriched with deuterium as Earth's oceans.