Mystery of the Martian polar cap explained

Mystery of the Martian polar cap explained
Mystery of the Martian polar cap explained

In 1966, two scientists at the California Institute of Technology pondered the effects of Martian's thin carbon dioxide (CO2) atmosphere, first discovered by NASA's Mariner IV spacecraft, built and operated by JPL. They hypothesized that Mars, with such an atmosphere, could have long-term stable polar ice deposition from CO2, which in turn would control global atmospheric pressure.

A new study from the California Institute of Technology (Caltech) suggests that the theory developed by physicist Robert B. Layton and planetary scientist Bruce K. Murray may indeed be correct.

Carbon dioxide makes up over 95 percent of Mars' atmosphere, which has a surface pressure of only 0.6 percent of Earth's. One of the predictions of Leighton and Murray's theory - with enormous implications for climate change on Mars - is that its atmospheric pressure will fluctuate as the planet rotates on its axis as it moves around the Sun, exposing its poles to sunlight. … Direct sunlight on CO2 ice deposited at the poles leads to its sublimation (direct transition of material from solid to gaseous state). Leighton and Murray predicted that when solar radiation changes, atmospheric pressure could fluctuate from one quarter of today's Martian atmosphere to two times in cycles of tens of thousands of years.

Now, a new model by Peter Buhler, JPL Ph. D., run by Caltech for NASA, and colleagues at Caltech, JPL, and the University of Colorado, provides key evidence to support this model. The model was described in an article published in the journal Nature Astronomy on December 23.

The team investigated the existence of a mysterious feature at the South Pole of Mars: a massive deposition of CO2 ice and water ice in alternating layers like cake layers that extend 1 kilometer deep, with a thin glaze of CO2 ice on top. Puff cake deposits contain the same amount of CO2 as is found in the entire atmosphere of Mars today.

In theory, such delamination should not be possible, because water ice is more heat resistant and darker than CO2 ice; CO2 ice, scientists have long believed, would quickly destabilize if buried under water ice. However, a new model by Buehler and his colleagues shows that the deposit could have formed as a result of a combination of three factors: 1) a change in the angle of inclination (or tilt) of the planet's rotation, 2) a difference in the way sunlight is reflected by water ice and CO2 ice, and 3) an increase in atmospheric pressure arising from the sublimation of CO2-ice.

“Usually, when you run a model, you don't expect the results to match what you observe. But the layer thicknesses determined by the model are in excellent agreement with radar measurements from orbiting satellites,”says Buehler.

The researchers speculate how the deposit formed: when Mars has been wobbling on its rotational axis for the past 510,000 years, the South Pole received varying amounts of sunlight, allowing CO2 ice to form when the poles received less sunlight. When the CO2 ice formed, small amounts of water ice were trapped along with the CO2 ice. As the CO2 sublimated, the more stable water ice remained below and consolidated into layers.

But the layers of water do not completely seal the sediment. Instead, the sublimated CO2 raises the atmospheric pressure on Mars, and the CO2 ice layer cake evolves in equilibrium with the atmosphere. When the flow of sunlight begins to decrease again, a new layer of CO2 ice forms on top of the water layer, and the cycle repeats.

Since the intensity of sublimation episodes usually decreased, some of the CO22 ice remained between the layers of water - thus the alternation of CO2 and water ice was formed. The deepest (and therefore the oldest) CO2 layer was formed 510,000 years ago after the last period of extreme solar flux, when all CO2 was sublimated into the atmosphere.

“Our definition of the history of Large Pressure Drops on Mars is fundamental to understanding the evolution of Mars' climate, including the history of liquid water stability and habitability near the surface of Mars,” says Buehler. This work was part of Buhler's graduate work at the California Institute of Technology. He continued his research in his current role as a postdoctoral researcher at JPL. It is co-authored by his former advisors Andy Ingersoll and Bethany Elmann, both professors of planetary science at the California Institute of Technology, Sylvain Piquet of JPL, and Paul Hein of the University of Colorado at Boulder.

The study is titled "Co-evolution of the atmosphere of Mars and massive south polar CO2 ice deposits." The study was funded by NASA.