New lab experiments suggest the sublimation of carbon dioxide -- which freezes in the winter and thaws in the spring -- is leaving unique marks on Mars' sand dunes.

Much science has been devoted to the events and processes that shaped the Martian surface millions of years ago, but a team of scientists at the University of Dublin, in Ireland, wanted to better understand how the Red Planet's surface continues to be altered today.

To find out, researchers tested how CO2 phase changes influence different types of surfaces.

"We've all heard the exciting news snippets about the evidence for water on Mars," Lauren Mc Keown, Ph.D student at Trinity College Dublin, said in a news release. "However, the current Martian climate does not frequently support water in its liquid state -- so it is important that we understand the role of other volatiles that are likely modifying Mars today."

Because Mars' atmosphere is 95 percent CO2, researcher figured it was the most logical volatile to experiment with. Unlike water, Mars' changing seasons move carbon dioxide through several phase changes.

"Mars has seasons, just like Earth, which means that in winter, a lot of the CO2 in the atmosphere changes state from a gas to a solid and is deposited onto the surface in that form," Mc Keown said. "The process is then reversed in the spring, as the ice sublimates, and this seasonal interplay may be a really important geomorphic process."

The experiments were inspired by patterns first observed on Martian sand dunes several years ago by Dublin researcher Mary Bourke.

"I called them 'sand furrows' as they were elongated shallow, networked features that formed and disappeared seasonally on Martian dunes," Bourke said. "What was unusual about them was that they appeared to trend both up and down the dune slopes, which ruled out liquid water as the cause."

To see whether CO2 could be responsible for the sand furrows, Bourke and her research partners place blocks of frozen CO2 on miniature sand dune models. They conducted the experiments inside a low humidity chamber. The tests showed sublimating CO2 can leave a wide variety of uniquely shaped furrows on the sand surface.

"The difference in temperature between the sandy surface and the CO2 block will generate a vapor layer beneath the block, allowing it to levitate and maneuver downslope, in a similar manner to how pucks glide on an ice-hockey table, carving a channel in its wake," Mc Keown said. "At the terminus, the block will sublimate and erode a pit. It will then disappear without a trace other than the roughly circular depression beneath it."

Similar formations, called gullies, are formed by water and ice migrating down rocky slopes on Earth, but the terminuses are usually marked by debris deposits, not a depression, which suggests Martian gullies and furrows are indeed formed by CO2, not water.

The lab tests showed under certain conditions, frozen CO2 blocks can sublimate fast enough enough to burrow directly into the sand and become swallowed up in less than a minute, leaving only a slight depression behind.

"This process is really unlike anything seen to occur naturally on Earth -- the bed appears fluidized and sand is kicked up in every direction," Mc Keown said. "When we first observed this particular effect, it was a really exciting moment."

The researchers described the effects of sublimating CO2 on sand in a new paper published this week in the journal Scientific Reports.

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Study shows how water could have flowed on 'cold and icy' ancient Mars
Providence RI (SPX) Oct 18, 2017
For scientists trying to understand what ancient Mars might have been like, the red planet sends some mixed signals. Water-carved valleys and lakebeds leave little doubt that water once flowed on the surface. But climate models for early Mars suggest average temperatures around the globe stayed well below freezing. A recent study led by Brown University geologists offers a potential bridge ... read more

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