Numerical Modeling of Martian Recurring Slope Lineae as Wind-cached, Overprinting Grain Flows

Content Provider	Semantic Scholar
Author	Grimm, R. E. Barth, E. L. Egan, A. Stillman, D. E.
Copyright Year	2020
Abstract	Introduction. Recurring Slope Lineae (RSL) dark, narrow streaks on steep slopes that appear annually in warm seasons, appear to grow incrementally, and fade in cold seasonshave been considered the best evidence for contemporary liquid water flowing at the surface of Mars [e.g., 1-3]. However, groundwater discharge is problematic due to RSL occurrence on isolated topographic highs [4], lack of concentrated salt deposits that would be left by evaporating brines, and an incomplete conceptual understanding of how fluid would be transported from stable aquifers at depth to a seasonally melted layer. Furthermore, we have been unable to find consistent brine composition (freezing temperature) or geological substrate even for RSL that occur within a few km of each other (Garni crater, [5]). Alternatively, a granular-flow mechanism is best supported by termination of RSL near the expected angle of repose [6-8]. In this case, it is uncertain how seasonal triggering occurs or how RSL continue to form thereafter [9]. The apparent incremental advance of RSL is much more easily explained as liquid flow in a porous medium [10,11] than as a slowly advancing grain flow. Building on the prior literature [12], we propose a new model for RSL as wind-cached, overprinting grain flows (Fig. 1). RSL onset is due to seasonally favorable winds that loft fine-grained sand into crevices and alcoves on steep slopes. Our preliminary meteorological modeling indicates that RSL formation is consistent with seasons of upslope winds. Grains are temporarily cached either dynamically or statically, i.e., until the wind stops blowing or slope failure occurs. The discharged sand creates a dark linea as it displaces dust. This occurs repeatedly, with overprinting of the RSL track both by smaller flows that do not go as far and by larger ones that extend the dark linea (Fig. 1). We are modeling grain flows using the Discrete Element Method (DEM) in order to assess the minimum flow volumes that can displace dust yet not build resolvable levees or toes. In this model, RSL onset at surface temperatures broadly comparable to briny ice melting is a coincidence, and the apparent incremental advancement of RSL is an illusion of grain-flow and orbital-imaging history. Additionally, any aeolian features are below the resolution of HiRISE [6,12]. In this view, RSL are wind-driven sedimentological phenomena and may hold no implications for water in adsorbed, hydrated, or liquid states. Wind Modeling. We use the Mars Regional Atmospheric Modeling System (MRAMS, [13]) to assess surface-wind direction and velocity at RSL sites as a function of solar longitude Ls. MRAMS is a nonhydrostatic atmospheric modeling system capable of simulating mesoand microscale wind flows over complex topography. Our preliminary investigation focused on Palikir crater (41.6 S, 202.3E, 16-km dia.), one of the original type locations for RSL [1]. Here RSL grow dominantly on W-facing slopes from Ls 240-315 (with peak activity around Ls 270) and are completely faded by Ls 15 [15]. We used 2 nested grids to capture the crater, with the finest grid 50-km across with 1-km spacing. The outputs of a global General Circulation Model (GCM, [14]) form the top-level MRAMS inputs and the model is run for 4 sols at the beginning of each martian month to obtain a uniform distribution of Ls. This procedure is repeated for each level. The results for Palikir (Fig. 2) demonstrate that winds are dominantly upslope during RSL activity and downslope at other times. This supports the hypothesis that sand is blown upslope into temporary caches.
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