The formation and dynamics of the ice cap of the South Pole of Mars finally explained

Although it has been studied for decades, the red planet still contains mysteries. For many years, a particular structure has caught the attention of planetologists, its origin remaining a mystery: an ice cap, composed of ice water and CO2, covering the South Pole of Mars. Recently, a team of NASA astrophysicists, using simulations, was able to confirm the generally advanced hypothesis to explain the formation and dynamics of this ice structure.

The main assumption  is that these layers stacked on top of each other during variations in the tilt of the axis of Mars during its orbit around the Sun, and new simulations published in the journal confirm this idea. The ice cap in question is about a kilometer deep and is believed to contain as much CO2 as the Martian atmosphere today, and a combination of factors has produced this unusual layer pattern.

“Usually when you run a model, you don't expect the results to match as closely as you observe. But the thickness of the layers, determined by the model, perfectly matches the radar measurements of satellites in orbit,” explains Peter Buhler, planetologist at NASA's Jet Propulsion Laboratory.

Structural graph of the ice cap of the South Pole of Mars. Credits: PB Buhler et al. 2020

CO2 ice stabilized thanks to the dynamics of Mars

What makes the South Pole ice cap so strange is that it really shouldn't be there - water ice is more thermally stable and darker than CO2 ice, so planetologists would expect that the CO2 ice is destabilized when it is trapped under the water ice.

CO2 ice cycle on the ice cap of the South Pole of Mars. The dynamics of this ice are subject to climatic and atmospheric variations on the planet. Credits: PB Buhler et al. 2020

According to the new model, three factors prevented this from happening: the changing tilt of Mars as it orbits the Sun, the differences in how these two types of ice reflect sunlight, and the change in atmospheric pressure. which occurs when the CO2 ice turns to gas.

Oscillations of the obliquity of Mars responsible for the CO2 ice cap

The "oscillations" of Mars on its axis of rotation would change the amount of sunlight reaching the South Pole, forming CO2 ice during certain periods and sublimating it (by passing it from a solid to a gas) during other periods.

During periods of ice formation, water ice would be trapped alongside the CO2. As sublimation occurs, this more stable ice would remain behind, forming the layers now present at the South Pole of Mars.

(A and D): Graphs showing the variations in mass of CO2 as a function of the variations in obliquity of Mars. (E) Image showing the structure of the ice cap of the South Pole of Mars with comparison between the observations and the authors' model. Credits: PB Buhler et al. 2020

Over time, climate change on the red planet has not resulted in a systematic sublimation of CO2, piling up successive layers of CO2 ice and water ice. Models show that this process changes atmospheric pressure - between a quarter and twice the level it is today - just as Leighton and Murray predicted in the 1960s.

This has been going on for around 510,000 years, suggest the authors - since the last period of extreme solar illumination, when all of the CO2 had been sublimated in the Martian atmosphere. " Our determination of the history of the large pressure fluctuations of Mars is fundamental to understanding the evolution of the planet's climate, including the history of the stability and habitability of liquid water near the surface  " Buhler concludes.


Coevolution of Mars’s atmosphere and massive south polar CO2 ice deposit

P. B. Buhler, A. P. Ingersoll, S. Piqueux, B. L. Ehlmann & P. O. Hayne

Nature Astronomy (2019)

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