Evaluation of an Efflorescence, Saltation, and Grain Flow Mechanism for Martian Recurring Slope Lineae (RSL)

RSL are low-albedo features that incrementally lengthen down steep slopes seasonally and are widespread on Mars. Our preliminary atmospheric modeling shows that active RSL occur only when the relative humidity (RH) never approaches the deliquescence RH (DRH) of Ca(ClO4)2. Additionally, mesoscale atmospheric modeling found that upslope winds at many RSL sites are common throughout the year, and thus lack a specific correlation with RSL active seasons if only dry processes are considered. Therefore, we present an RSL formation mechanism where upslope winds are unable to saltate a sufficient volume of sand grains during the RSL inactive season, because the grains are sticky due to deliquesced brines. As temperatures rise and RH decreases, efflorescence occurs causing the sand to lose cohesion. This could trigger RSL-like flows, but it does not explain why RSL always start at the top of slopes or incrementally lengthen. With the loss of cohesion, the upslope winds now saltate more sand grains upslope until a topographic barrier (typically an outcrop of bedrock) prevents further saltation, creating a pile of sand grains at the bedrock-regolith interface. As this sand pile grows it will eventually reach the dynamic angle of repose, resulting in slope failure and a subsequent grain flow that disrupts the thin veneer of dust on the surface, darkening the surface. Throughout the RSL active season additional volumes of saltated sand are able to accumulate near the bedrock-regolith interface, allowing grain flows to continue to advance/lengthen downhill. RSL activity ceases when the RH increases to the DRH, thus increasing the cohesion of the sand, which results in decreased saltation rates. Decreased sand emplacement shuts off the RSL triggering mechanism, allowing the RSL to fade as atmospheric dust settles on the slope. In our presentation we will discuss the advantages and disadvantages of this RSL mechanism and compare it to RSL observations and models in Palikir, Krupac, Garni, and Rauna crater.