A new way to look at Recurring Slope Lineae

Since their discovery in 2011, recurring slope lineae (RSL) have been one of the most hotly debated features on Mars. These dark seasonal streaks appear on the slopes of crater walls and have yet to be definitively explained. In 2015, Lujendra Ojha and coauthors concluded that the streaks could be formed by briny water melting in the warmer seasons. With enough perchlorates mixed into the sediments, it was possible that liquid water could persist even under Martian conditions.

This conclusion was challenged just last year when Colin Dundas and coauthors argued that RSL were only occurring on particularly steep slopes (slopes greater than 27 degrees). Further, as these slopes level off near the bottom and reach a “dynamic angle of repose”, the RSL streaks stop. Both these angles correlate very closely with our understanding of dry flows, suggesting that RSL are not wet like we previously thought.

Here at the Lunar and Planetary Science conference, Jan Raack from the Open University presented some new experimental data that might help explain. Similar to Mc Keown’s work with dry blocks, Raack suggests that boiling of water could create a cushion of air underneath the sediment. This levitation effect could result in a huge increase in the amount of sediment that flows in the presence of water.

To test it, Raack created a 90-by-40-by-50-centimeter sediment bed inside the same Large Mars Chamber, and inclined it at 25 degrees. He set pressure to 9 millibars. Then, he introduced water at the top of the sediment to examine how much it could displace downslope. He conducted the test under two distinct temperatures, once cooling the sediment to 278 K and once to 297 K. These temperatures are important because they represent the surface temperature differences from winter to summer.

The results were remarkable. In the cold environment, the sediment flow was small, and lines up well with what we would expect here on Earth under terrestrial pressures. In this test, water carried most of the sediment flow along with it. However, in the warm environment, which would simulate the surface of Mars as the sun bathes it in the spring and summer, the flow dramatically increased. Water carried some of the sediment overland. It carried even more downslope through a process called percolation. Additionally, it beaded some sediment into pellets that tumbled downward. Finally, the water triggered dry avalanches resulting in even more flow. The warm test resulted in 9 times as much sediment flow as the cold test. Raack estimates that Martian gravity would enhance the levitation effect, resulting in nearly 48 times as much sediment flow.