For years, we have seen images of strange grooves in the dunes of Mars that seemed to have been carved by liquid water, feeding the hopes of finding conditions for life. But the reality, as often happens in these fields, is much stranger and fascinating as shown a new study published in Geophysical Research Letters which not only confirms that the culprit is carbon dioxide ice, but has discovered a completely new mechanism that seems straight out of science fiction.
The study. Dr. Lonneke Roelofs of Utrecht University, lead author of the study, described it in a way that will be familiar to many of us: “I felt like I was watching the sandworms from the movie Dune.” And it’s no wonder. In his experiments, he saw blocks of dry ice not only slide, but literally burrow and dig into the sand with explosive force, a phenomenon never before observed.
Recreating Mars. To solve the mystery, the research team used the martian simulation chamber ‘George’. Inside this two-meter cylinder, they recreated the conditions of Mars’ thin atmosphere, which has a pressure of only 700 pascals compared to Earth’s 100,000 pascals.
The experiment was simple: place a tray with dune sand, adjust the inclination and drop blocks of CO₂ ice from the top. Here the process that was being sought was that of sublimationthe direct passage from solid to gas.
On Earth, this is a calm process, but on Mars it is extremely violent. The enormous temperature difference between the ice and the sand on which it is located, combined with the low pressure, causes the CO₂ to expand explosively and generate immense force.
Results. In this case, the team discovered that the ice blocks moved very differently depending on the steepness of the slope. On steep slopes (>22.5º) the ice block slid rapidly, at about 0.8 m/s, over a layer of gas, almost floating. This movement created straight, shallow channels with almost imperceptible ridges. This is something that coincides with the channels seen in the highest parts of the Martian dunes.
In the case of gentle slopes is where the real magic happened. The block moved very slowly, at about 0.0003 m/s, and instead of sliding it was partially buried in the sand. Explosive sublimation threw grains of sand ballistically in all directions, carving a deep channel beneath the block and with high ridges on its sides.
This ‘digging’ movement perfectly explains the deep channels, high ridges and sinuous curves that for so long They baffled scientists. But finally, when the block finally stops at the foot of the dune, the sublimation effect continues to occur and generates the characteristic pits.
The importance. These findings are very relevant to understanding Mars as a planet. First of all, the results confirm that one of the most active and striking phenomena is driven by CO₂ processes, without the need for have liquid water. In addition, it gives us a physical model that explains all the strange characteristics of the ravines. Sharp curves, for example, are not due to the flow of liquid, but to an excavating block that changes course due to small irregularities in the terrain.
Finally, the formation of these ravines requires very specific conditions, such as sufficient accumulation of CO₂ ice in winter and sufficient solar radiation in spring to heat the sand and cause violent sublimation.
In short, the mystery of the grooves in the Martian dunes has been solved, and the answer is not the water we long to find, but a violent and exotic physical process, more typical of an alien planet.
Images | Daniele Colucci POT
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