An Earth scientist solved a Mars mystery about what dug these weird ditches

Strange, narrow channels threading through some of Mars' sand dunes have intrigued scientists for years. 

These Martian features don't look like anything on Earth. Early theories proposed that flowing water created these gullies, perhaps when the Red Planet had a warmer, wetter climate billions of years ago. That idea thrilled scientists because it meant Mars might once have supported life. 

But newer space images have revealed that these gullies aren't relics of the past but forming and changing with the seasons now, making them all the more perplexing. Though Lonneke Roelofs is an Earth scientist at Utrecht University in the Netherlands, she was determined to solve the extraterrestrial mystery. She describes herself as "broadly interested," and that curiosity has led her to study surface processes far beyond our own planet. 

"Mars is currently the only planet on which we have observed these types of gullies," she told Mashable, "so they are a pretty special and exciting landform."

Rather than rely on computer simulations as other planetary studies so often do, Roelofs and a graduate student went to a lab at The Open University in the United Kingdom and recreated Mars-like conditions, complete with very low air pressure and fine sand. In the end, their experiments were able to reproduce the gullies — in a surprising way. 

These Martian ditches, called linear dune gullies, often run parallel and end with small pits. They range from about three to 30 feet wide and 65 to hundreds of feet long, Roelofs said, depending on the size of the dune.

Despite the name, many of the ditches squiggle instead of forming straight lines. With the help of NASA's Mars Reconnaissance Orbiter images, scientists have observed these surface features changing during the Martian spring. 

A closer view of the linear dune gullies at Hellas Planitia, taken by NASA's Mars Reconnaissance Orbiter and shown at the top of this story, reveals how these channels twist and curve through the sand. Credit: NASA / JPL-Caltech / UArizona

"How quickly they are eroded is dependent on how quickly the dunes move," Roelofs said. "Over the time span of 10 years, we have seen gullies fade. But some exist for longer."

Since the Red Planet has no running water on its surface today, scientists have considered other possible explanations. One idea has pointed to chunks of frozen carbon dioxide, aka dry ice. 

During the Martian winter, frost and dry ice accumulate on top of desert dunes in the southern hemisphere, sometimes forming a layer over two-feet thick. But when spring sunlight returns, the ice begins to warm and break. Pieces then slide down the slopes. 

In short, some scientists have wondered if the gullies are caused by avalanches. But it was unclear how an avalanche of dry ice under the thin Martian air would differ from how it works with snow on Earth. 

In the lab's Mars chamber, the team released blocks of dry ice onto slopes of different angles and watched how they moved. The experiment yielded two distinct behaviors. Their findings appear in the journal Geophysical Research Letters.

On steep slopes, the dry ice just slid, creating thin, shallow etches. But on gradual slopes, the blocks did something wild: The bottom of the ice touching the dune converted directly into gas, skipping the liquid phase, and blasted surrounding sand away, due to its high pressure. The ice burrowed downward like a mole, digging deeper and more twisted trenches. 

Suddenly, a simple chunk of ice seemed to move more like an animal. That process caused ridges, called levees, to pile up on either side. Video recordings of the experiments, shared above, demonstrate how it works. Roelofs likens the strange behavior to sandworms in Dune.

The tests also revealed that the ditches only develop on dunes with very fine, evenly sized sand grains. Coarser or more jagged material prevents the burrowing motion, which could explain why the gullies only appear in certain Martian regions, according to the paper.

Many planetary scientists have assumed that undulating channels always indicate a past presence of liquid water, streams rushing over and carving out the surface. But, when it comes to Mars at least, that's not necessarily true.