Scientists wore their Sherlock Holmes caps to solve a mystery that has lasted for decades: the origins of a huge layered expanse of ice and water ice in the south pole of Mars and its link with CO2 in the Martian atmosphere. A main hypothesis is that these layers were superimposed on each other as the axis of Mars slopes towards and away from the Sun and the simulation models published in a new study support that idea. The ice cap in question is about a kilometer deep (two thirds of a mile) and is believed to retain the amount of CO2 present in the Martian atmosphere present today, and a combination of factors produced this unusual layered pattern. “Usually, when running a model, the results are not expected to match so closely to what is observed,” says Peter Buhler, a planetary scientist at NASA’s Jet Propulsion Laboratory. “But the thickness of the layers, as determined by the model, matches beautifully with radar measurements from satellites in orbit.” What makes the ice cap in the south pole so strange is that it really shouldn’t be there: water ice is more thermally stable and darker than CO2 ice, so scientists would expect CO2 ice to destabilize if trapped under the water ice. According to the new model, three factors prevented this from happening: the changing inclination of Mars as it orbits the Sun, the differences in how these two types of ice reflect sunlight and the change in atmospheric pressure that occurs when CO2 ice turns into a gas.
The “oscillations” of Mars on its rotational access would change the amount of sunlight reaching the south pole – forming CO2 ice in some periods and sublimating it (going from a solid to a gas) in other periods. During periods of ice formation, water ice would have been trapped together with CO2. As sublimation took place, this more stable ice would remain behind, forming the layers now present on the south pole of Mars. Over time, the changing climate of the Red Planet meant that not all CO2 ice was sublimated every time, accumulating successive layers of CO2 ice and water ice. Models show this process by changing atmospheric pressure – from a quarter to twice the level today – just as predicted by Leighton and Murray in the 1960s. This has been going on for about 510,000 years, scientists suggest – from the last period of extreme solar sunlight, when all CO2 would have been sublimated into the Martian atmosphere. Being more confident in the story behind the ice cap in the south pole of Mars means that researchers can potentially understand more about the long-term history of the planet, peering over billions of years. “Our determination of the history of the great pressure swings of Mars is fundamental to understanding the evolution of the climate of Mars, including the history of liquid water stability and habitability near the surface of Mars,” says Buhler.