Dark, spider-like systems called araneiforms have been revealed from basins found only in the Antarctic regions of the Red Planet, the like of which do not exist on Earth.
This makes it difficult to understand what creates it, but scientists have obtained the first physical evidence to support the more common model, known as the Kieffer hypothesis. According to this idea, spider-like shapes are formed through direct sublimation of frozen carbon dioxide (CO2).
“This research provides the first set of experimental evidence for a surface process thought to alter the polar landscape on Mars. Kieffer’s hypothesis has been well accepted for more than a decade,” said planetary scientist Lauren McCune, who previously worked at Trinity College Dublin, Ireland, and is now at the Open University. “From time, but until now, they have been placed in a purely theoretical framework. Experiments directly show that the spider patterns that we observe on Mars from orbit can be carved through the direct conversion of dry ice from solid to gas.”
(NASA/JPL/University of Arizona)
Mars’ axial tilt is very close to Earth, which means that seasonal temperature changes are very similar to those of Earth as well (although the year, and thus the seasons, is twice longer on Mars). This means a significant drop in temperatures in the fall and winter seasons, and a rise again in the spring and summer.
On the other hand, the Martian atmosphere is very different from the Earth’s atmosphere. It is much thinner, and is mostly composed of carbon dioxide (about 95%). The planet is also much farther from the earth than the sun, so it is cooler there. And when winter sets in, carbon dioxide from the atmosphere freezes over on Earth, especially at higher latitudes.
In 2006 and 2007, geophysicist Hugh Kiefer and his colleagues proposed that in the spring, frozen carbon dioxide sublimates – that is, it moves from ice to gas, without a fusion step to the liquid in between – trapped under translucent sheets of surface ice.
As the gas heats up and expands, pressure builds up until the plate cracks, creating a vent for the gas to escape. As it flows down the hole, the gas forms a system of spider-like channels on the surface of Mars, and it carries with it the excavated material.
The gas and materials are ejected together in a high velocity wave. And when the chunk of ice on top finally melts, what remains is an araneiform shape.
Kiefer noted that this hypothetical process is unlike anything observed on Earth. It was also never observed on Mars – we only saw the anomalous shapes in satellite images – so McCune and her team designed an experiment to simulate the process in a lab environment.
They took advantage of a phenomenon you can observe in your kitchen, called the Leidenfrost effect: If you place a drop of water on a surface hotter than the point of evaporation of water, the drop will rise (which is why water in a very hot skillet will dance like mercury).
In a special chamber with low pressure into the Martian atmosphere, the team placed a sheet of carbon dioxide ice with a single hole drilled through it on a surface covered with tiny grains of glass that simulated dirt or regolith.
When the ice touched the surface, it began to sublimate, as gas was seen escaping from the crater. Once the team raised the ice, they found a spider-like system etched into the glass sand as gas flowed through it to escape from the hole.
McKeown et al., Sci Rep, 2021
In fact, the process was so vigorous, with material being dumped all over the room, indicating that sublimation rates on Mars could be higher than those on Earth.
The team repeated the experiment with different sized granules to see how the different regolith textures affected the result. They found that the finer the grain size, the greater the bifurcation of the pattern.
And although the experiment is the first time scientists have actually proven that a hypothesized Kieffer process can actually happen – and it has a lot of weight in its favor in terms of shapes on Mars.
The results indicate that the geomorphological processes on Mars still hold some secrets, and also that the sublimation of carbon dioxide may be an explanation for other strange surface features on Mars.
The team hopes that studying the waterforms over many years on Mars will help shed more light on the planet’s remarkable seasonal processes.
The research is published in a journal Scientific Reports.