Abundant Recurring Slope Lineae (RSL) Following the Great Martian Dust Storm of 2018

Post contributed by Dr. Alfred S. McEwen, Lunar and Planetary Laboratory, University of Arizona, USA.

Recurring slope lineae (RSL) are dark linear markings on steep slopes of Mars that regrow annually and likely originate from the flow of either liquid water or dry granular material. Following the great dust storm (or planet-encircling dust event) of Mars Year 34 (in 2018), the High Resolution Imaging Science Experiment (HiRISE; McEwen et al., 2007) on Mars Reconnaissance Orbiter (MRO)  has seen many more candidate RSL than in typical Mars years (Image 1). These RSL sites show evidence for recent dust deposition and dust devil activity, so dust lifting processes may initiate and sustain RSL activity on steep slopes.

Image 1:  RSL and dust devil tracks on a hill in the southern middle latitudes (41.1ºS, 187.4ºE).  Inset shows some of the RSL at higher resolution.  Hundreds of dark dust devil tracks are seen on the full image as the more diffuse lines that cut across topography.  HiRISE image ESP_058122_1385, acquired after the 2018 dust storm.  Credit: NASA/JPL/University of Arizona

RSL recur in multiple Mars years (by definition) over the same slopes and often the exact same locations. In typical cases, the lineae grow incrementally or gradually over a period of several months, usually during the warmest time of year for the particular latitude and slope aspect, then fade (and typically disappear) when inactive.  This pattern repeats over multiple years, with varying degrees of interannual variability. RSL are often associated with pristine small gullies or channels that are otherwise rare on equatorial slopes (see Image 2).  Hundreds of individual lineae may be present over a local slope, and thousands in single images captured by MRO/HiRISE.  The origin of RSL has been much debated in the science community, including wet, dry, and hybrid models.

Image 2: Animated GIF of RSL on a gullied mound in Juventae Chasm in equatorial Mars (4.7º S, 298.6º E).  It is not clear if the RSL activity itself formed the gullies over time or if they just follow gullies formed by other processes.  Images reduced to 1 m/pixel scale. Compression has been applied for the purposes of this post. Full-resolution original: https://www.uahirise.org/sim/2013-12-10/gifs/ESP_030373_1755_RED_C_01_ORTHO-N-hill_Labeled.gif. Credit: NASA/JPL/University of Arizona

Although early interpretations of RSL favored seeping water on Mars today, a startling possibility, several recent studies have favored dry flows of sand or dust.  There were prior hints of an association between dust storms and RSL activity, but in 2018 Mars performed a grand experiment with a planet-encircling dust storm that began in the southern spring.  As the atmosphere cleared, HiRISE observed about five times as many RSL sites and covering a wider latitude range than seen previously in southern summer (Image 3).  There are also many new dust devil tracks over most of these regions (Image 1), which form by lifting dust from the surface via several mechanisms, some of which are especially effective in the surface environment of Mars.  When such dust lifting occurs on steep slopes, it may destabilize the dust and sand-size particles to initiate and sustain many small avalanches where the slope exceeds the dynamic angle of repose (stopping angle, ~28-32º) for cohesionless grains.  This motion may remove dust and/or roughen the surface along the flow path to produce the low albedo lineae.  The otherwise puzzling recurrence and year-to-year variability of RSL can now be explained by variable yearly dust fallout.

Image 3:  RSL on the steep slopes of a well-preserved crater inside Baldet Crater (22.4º N, 66.1º E).  Image acquired in middle southern summer after the great dust storm, when RSL are normally not active at this northern latitude.  Red arrows point to tips and initiation points of several lineae.  ESP_057559_2025.  Credit: NASA/JPL/University of Arizona

Further Reading

Dundas, C. M., McEwen, A. S., Chojnacki, M., Milazzo, M. P., Byrne, S., McElwaine, J. N., Urso, A. (2017), Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water.  Nature Geoscience, 10, 903–907.  https://doi.org/10.1038/s41561-017-0012-5

McEwen, A. S., et al. (2007). Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE). J. Geophys. Res., 112, E05S02.  https://doi.org/10.1029/2005JE002605

McEwen, A.S. et al. (2021), Mars: Abundant Recurring Slope Lineae (RSL) following the Planet-Encircling Dust Event (PEDE) of 2018, J. Geophys. Res. Planets, 126(4), e2020JE006575. https://doi.org/10.1029/2020JE006575

Schaefer, E. I., McEwen, A. S., Sutton, S. S. (2019). A case study of recurring slope lineae (RSL) at Tivat crater: Implications for RSL origins.  Icarus, 317, 621-648. https://doi.org/10.1016/j.icarus.2018.07.014

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