Mars’ 100,000-Year ‘Deep Breaths’ Are Even Deeper Than Expected

Category: Cover Story

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Mars’ seasonal south polar cap is a 1-meter thick layer of CO2 ice (‘dry ice’) that deposits directly out of the atmosphere every winter, draping the terraced form of the up to 1-kilometer-thick south polar massive CO2 ice deposit in a crisp, white blanket. Slight imbalances in annual wintertime deposition and summertime ablation of seasonal CO2 cause the massive CO2 ice deposit to grow and shrink as part of a 100,000-year “breathing” cycle that shunts CO2 between the atmosphere, pole, and soil. A few degrees of latitude away from the pole, rusty swirls of windblown dust begin to impinge onto this pearly expanse of dry ice, yielding a vista reminiscent of a hundreds-of-kilometers-wide cappuccino. This image was taken by the High Resolution Stereo Camera, onboard the European Space Agency’s Mars Express mission. Credit: ESA/DLR/FU Berlin/Bill Dunford.

Mars’ seasonal south polar cap is a 1-meter thick layer of CO2 ice (‘dry ice’) that deposits directly out of the atmosphere every winter, draping the terraced form of the up to 1-kilometer-thick south polar massive CO2 ice deposit in a crisp, white blanket. Slight imbalances in annual wintertime deposition and summertime ablation of seasonal CO2 cause the massive CO2 ice deposit to grow and shrink as part of a 100,000-year “breathing” cycle that shunts CO2 between the atmosphere, pole, and soil. A few degrees of latitude away from the pole, rusty swirls of windblown dust begin to impinge onto this pearly expanse of dry ice, yielding a vista reminiscent of a hundreds-of-kilometers-wide cappuccino. This image was taken by the High Resolution Stereo Camera, onboard the European Space Agency’s Mars Express mission.
Credit: ESA/DLR/FU Berlin/Bill Dunford.

PSI’s Peter Buhler is the lead author of a paper that studies the atmospheric pressure cycle of Mars. Mars’ atmospheric pressure is a key determinant for supporting liquid surface water and thus surface life.

Mars’ atmosphere is primarily CO2, which exchanges with Mars’ polar CO2 ice cap and a vast reservoir of CO2 molecules coating the grains in the Martian soil, in a cycle of “deep breathing” every 100,000 years. Exchange between these reservoirs causes substantial variation to Mars’ surface pressure, driven by changes in the global distribution of sunlight as Mars wobbles on its spin axis over hundred-thousand-year cycles. Mars’ CO2 exchange “breathing” cycle is similar, yet alien, to the way wobbles in Earth’s spin axis cause Earth’s ice age glacial cycles.

As CO2 moves between Mars’ atmosphere, polar cap, and soil, it leaves a record: alternating CO2 ice (i.e., “dry ice”) and water ice layers in the polar cap. The thickness of the CO2 ice layers indicate how much CO2 moves through each cycle, allowing calculation of the total CO2 inventory. This record is important because there are no direct measurements of how much total CO2 is in Mars’ soil, which contains far more CO2 than the current atmosphere and polar cap combined.

Buhler and his co-author Sylvain Piqueux of NASA’s Jet Propulsion Laboratory constructed a thermophysical numerical model that predicts how thick the CO2 layers would be for various total CO2 inventories. They then compared the model results to the observed polar layers to determine the most likely amount of total CO2.

They found that Mars’ soil holds approximately four times more CO2 than previously thought. This means that Mars’ recent peak surface pressure can only reach about 60 percent as high as prior predictions, because the soil has more capacity to drawn down the atmosphere. Lower peak surface pressure makes liquid surface water difficult to achieve in Mars’ recent history. However, the soil’s larger capacity to hold CO2 also means that Mars has more total CO2, which may have helped to support liquid surface water more easily on ancient Mars.

Polygonal undulations in the water ice layer overlying Mars’ massive CO2 ice deposit betray the slow ablation of the underlying CO2 ice. Mars is currently in a part of its orbital cycle when the south pole is receiving gradually more sunlight each year, causing the CO2 to sublime (like “melt,” but going directly from a solid to a gas, with no liquid) into the atmosphere. Although the details of how the polygons and undulations form are not yet understood fully, collapse of the overlying water ice into voids formed by sublimation of the underlying CO2 ice, thermal reworking, and even glacial flow may all play a role. The polygons are typically about 5-to-10 meters (15-30 feet) across.
This image was taken by the High Resolution Imaging Science Experiment camera onboard NASA’s Mars Reconnaissance Orbiter mission. Credit: NASA/JPL/University of Arizona

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