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Research suggests limited role for water in Mars recurring slope lineae

Recurring slope linea, or RSL, in an image captured by the HiRISE camera on the Mars Reconaissance Orbiter. (Image: NASA)

Recurring slope lineae as seen in an image captured by the HiRISE camera on the Mars Reconnaissance Orbiter. Image Credit: Lunar and Planetary Laboratory / NASA

Water may play a less significant role than previously thought in recurring slope lineae on Mars according to a paper published in Nature Geoscience on Nov. 20. Recurring slope lineae, or RSL, are dark seasonal slope streaks that occur on slope faces in some regions of Mars.

Previous studies suggested that the seasonal appearance of RSL during warmer periods of the year could indicate liquid water plays a role in their development. The new study casts doubt on this hypothesis, noting the steep slopes on which they occur tend to favor dry rather than wet flows.

The project utilized data from the High-Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter (MRO). Stereo pair images from HiRISE were used to construct 3-D models revealing that RSL are restricted to slopes steeper than 27 degrees. By comparison, liquid water should be able to also flow on slopes shallower than 27 degrees.

Recurring slope lineae (RSL) are visible streaking down the surface a dune near the center of this image included in the new study. Certain regions, such as Coprates Chasma featured here, have a greater occurrence of RSL than others. (Image: NASA/JPL/University of Arizona)

Recurring slope lineae are visible transitioning across the surface a dune near the center left of this image. The slope characteristics, as well as physical characteristics, suggest RSL form by a dry granular rather than a liquid water process. Image Credit: NASA / JPL / University of Arizona

Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center in Flagstaff, Arizona is the lead author of the new paper.

“We’ve thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand,” said Dundas. “This new understanding of RSL supports other evidence that shows that Mars today is very dry.”

The presence of hydrated salts and the seasonal variability of RSL opened the door to water as a potential instigator. Understanding if RSL have aqueous origins is an important step in the exploration of the Red Planet. The possibility of liquid water flows raised concerns surrounding planetary protection and mission landing site selection. The new results supporting dry origins help to alleviate some of those concerns.

Co-author and HiRISE Principal Investigator Alfred McEwen expressed confidence the results point to dry flows as the cause of RSL.

“The RSL don’t flow onto shallower slopes, and the lengths of these are so closely correlated with the dynamic angle of repose, it can’t be coincidence,” said McEwen.

Other papers in recent years have also cast doubt on the liquid water hypothesis. The low atmospheric pressure and temperatures at the surface make the existence of large volumes of liquid water difficult. Additionally, a 2016 paper suggests that RSL originating from topographic high points pose a challenge for groundwater release to play a role in their formation.

The new results don’t totally rule out the existence of small volumes of liquid water on the surface. On Mars, water vapor from the atmosphere can be absorbed into the regolith and form transient liquid water in a process called deliquescence. The new study favors dry processes as the mode of formation, but it leaves the door open for deliquescence to play a role. These dry processes can include seasonal shifts in wind direction and the presence of hydrated salts that lead to increased granular flow downslope.

“RSL probably form by some mechanism that is unique to the environment of Mars,” added McEwen, “so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration.”

MRO Project Scientist Rich Zurek of NASA’s Jet Propulsion Laboratory, who was not a part of the study, addressed the study of RSL going forward.

“Full understanding of RSL is likely to depend upon on-site investigation of these features,” said Zurek. “While the new report suggests that RSL are not wet enough to favor microbial life, it is likely that on-site investigation of these sites will still require special procedures to guard against introducing microbes from Earth, at least until they are definitively characterized. In particular, a full explanation of how these enigmatic features darken and fade still eludes us. Remote sensing at different times of day could provide important clues.”




Paul is currently a graduate student in Space and Planetary Sciences at the University of Akransas in Fayetteville. He grew up in the Kansas City area and developed an interest in space at a young age at the start of the twin Mars Exploration Rover missions in 2003. He began his studies in aerospace engineering before switching over to geology at Wichita State University where he earned a Bachelor of Science in 2013. After working as an environmental geologist for a civil engineering firm, he began his graduate studies in 2016 and is actively working towards a PhD that will focus on the surficial processes of Mars. He also participated in a 2-week simluation at The Mars Society's Mars Desert Research Station in 2014 and remains involved in analogue mission studies today. Paul has been interested in science outreach and communication over the years which in the past included maintaining a personal blog on space exploration from high school through his undergraduate career and in recent years he has given talks at schools and other organizations over the topics of geology and space. He is excited to bring his experience as a geologist and scientist to the Spaceflight Insider team writing primarily on space science topics.

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