Experiment Reveals What Is Truly Burrowing Beneath Mars's Dunes Each Spring

Strange, sinuous gullies etched into the dunes of Mars are finally giving up their secrets.

These gouges, new experiments reveal, are carved by the explosive sublimation of dry ice, burrowing down the slopes like a mole and blasting away sand as it goes. The new results finally account for all the curious features of Mars' mysterious squiggles.

"It felt like I was watching the sandworms in the film Dune," says Earth scientist Lonneke Roelofs of Utrecht University in the Netherlands.

Related: NASA Can Finally Explain Why Creepy 'Spiders' on Mars Keep Appearing

The gullies rippling down the sand dunes of Mars have puzzled scientists since they were first spotted back in 1999. Initially, scientists linked their formation to seasonal water flows. However, as we now know, there's no evidence that liquid water runs across the surface of Mars, nor are we likely to get any such evidence anytime soon.

However, in 2013, scientists demonstrated an alternative explanation: the runnels could have been carved by carbon dioxide ice, or dry ice, sublimating as it slid down the dunes.

Initial experimentation showed that this mechanism was plausible, but the tests didn't reproduce all the features observed in Martian gullies, especially the snake-like sinuosity.

Roelofs and her colleagues weren't satisfied with that gap. In previous work, they used the Open University's Mars chamber – a sealed environment simulating Martian conditions – to show how CO₂ can drive mass outflows from steep crater walls.

To address the problem of the gullies, the researchers turned to the Mars chamber again.

Adjusting the equipment to mimic Mars's thin, cold air, the team dropped blocks of dry ice onto sand slopes across a range of inclinations and sand grain sizes, recording the results with high-speed cameras.

The results showed that, on slopes sharper than 25 degrees, the ice slides down on a cushion of gas, leaving an indistinct track, as seen in previous experiments. However, when the angle of the slope was gentler, below 22.5 degrees, the ice partially burrowed into the sand.

When the dry ice was thus buried, the sublimation process ballistically ejected the sand, producing the exact features observed in the Mars gullies.

Specifically, these features – never before replicated on Earth – included the sinuosity of the tracks, high, distinct levees, and a little pocket at the end of the trail.

"We tried out various things by simulating a dune slope at different angles of steepness. We let a block of CO₂ ice fall from the top of the slope and observed what happened," Roelofs explains.

"After finding the right slope, we finally saw results. The CO₂ ice block began to dig into the slope and move downwards just like a burrowing mole or the sandworms from Dune. It looked very strange!"

Dry ice is translucent, meaning optical and infrared radiation doesn't just bounce off its surface but penetrates inside. However, the sand or rock underneath the ice is darker, which means it absorbs more of the radiation that reaches it, re-emitting it as thermal (infrared) radiation.

This heat energy is prevented from escaping by the ice block sitting on top of it. As the underside of the ice warms, it transitions directly into a gas in a process called sublimation. This gas, with nowhere else to go, builds until it bursts outwards, flinging the surrounding sand away with significant force.

To test whether their experimental observations could be scaled up to reproduce the observed gullies on Mars, the researchers conducted simulations to account for larger blocks of ice and Mars's gravity.

Their results showed that sublimating blocks up to a meter (3.3 feet) thick can fling sand up to 13 meters on the red planet, easily reproducing the observed gullies. The results even explain why these gullies can only be found on fine-grained slopes: those are the ones for which the mechanism works.

The atmosphere of Mars is rich in CO₂; in winter, it can form a coating over mid-latitude dune fields to an ice thickness of up to 70 centimeters.

"In spring, this ice begins to warm up and sublimate. The last remnants of this ice are located on the shaded side of the dune tops, and that is where the blocks break off from once the temperature is high enough," Roelofs says.

"Once the blocks reach the bottom of the slope and stop moving, the ice continues to sublimate until all the CO₂ has evaporated. What remains is a hollow in the sand at the bottom of the dune."

The researchers now want to run experiments with larger blocks of ice and different sand types to see what happens when they tweak the parameters.

"Mars is our nearest neighbor. It is the only rocky planet close to the 'green zone' of our Solar System. This zone lies exactly far enough from the Sun to make the presence of liquid water possible, which is a prerequisite for life. Questions about the origin of life, and possible extra-terrestrial life, could therefore be solved here," Roelofs concludes.

"Also, conducting research into the formation of landscape structures of other planets is a way of stepping outside the frameworks used to think about the Earth. This allows you to pose slightly different questions, which in turn can deliver new insights for processes here on our planet."

The research has been published in Geophysical Research Letters.