Wind has been an enduring geologic agent throughout the history of Mars, but it is often unclear where and why sediment is mobile in the current epoch. We investigated whether eolian bed-form (dune and ripple) transport rates are depressed or enhanced in some areas by local or regional boundary conditions (e.g., topography, sand supply/availability). Bedform heights, migration rates, and sand fluxes all span two to three orders of magnitude across Mars, but we found that areas with the highest sand fluxes are concentrated in three regions: Syrtis Major, Hellespontus Montes, and the north polar erg. All regions are located near prominent transition zones of topography (e.g., basins, polar caps) and thermophysical properties (e.g., albedo variations); these are not known to be critical terrestrial boundary conditions. The two regions adjacent to major impact basins (Hellas and Isidis Planitia) showed radially outward upslope winds driving sand movement, although seasonally reversing wind regimes were also observed. The northern polar dunes yielded the highest known fluxes on the planet, driven by summer katabatic winds modulated by the seasonal CO 2 cap retreat-processes not known to affect terrestrial dunes. In contrast, southern dune fields (<45°S) were less mobile, likely as a result of seasonal frost and ground ice suppressing sand availability. Results suggest that, unlike on Earth, large-scale topographic and thermophysical variabilities play a leading role in driving sand fluxes on Mars.

. Atmospheric water vapor is unlikely to condense on warm 62 slopes, while groundwater is unlikely to emerge on all sides of isolated peaks 11 . These 63 challenges suggest that we should consider alternative models for RSL. 64 65 Evidence for Granular Flow Processes 66We measured the terminal slopes of 151 RSL at ten well-studied sites (Table S1). 67The results (Fig. 1a) show that in nearly all cases the mean slope near the end of a linea is 68 between 28°-35°. This range matches that of slipfaces for active Martian and terrestrial 69 dunes 23 , interpreted as the range of critical angles where granular flows of sand can 70 terminate (often called the dynamic angle of repose), and is similar to earlier 71 measurements of overall RSL slopes 6, 11, 17, 24 . We avoided clear artifacts or interpolated 72 areas (Methods); the few points outside this slope range are likely due to artifacts in the 73 topographic data. RSL slopes (or fans) are straight to slightly concave (Fig. 1b, Fig. S1), 74 consistent with dry granular flows such as sand dune slipfaces, and unlike the strongly 75 concave slope profiles produced by repeated debris flows or fluvial gullies 25 . Figure S2 76 shows RSL on weakly concave slopes, beginning at >35° and terminating near 30°. 77The terminal slopes of RSL, identical to sand dunes, suggest that movement on 78 those slopes is by dry grainflows. Aqueous flows could occur on such slopes and small 79 volumes of liquid might only produce short lineae and prevent runout onto lower slopes. 80However, it is unlikely that water is only produced near the tops of slopes at these angles 81 or that, if so, it is never able to flow onto lower slopes. RSL at a single site in Eos 82Chasma with widely varying lengths all terminate on similar slopes (Fig. S2). It is 83 unlikely that liquid volume is the controlling variable-this would require the volume of 84 liquid to correspond to the length of slope available, producing more liquid on longer 85 slopes. (If RSL deposit material they could build their own slopes, but saturated flows 86 should be more mobile than dry sandflows.) We therefore consider the primary 87 mechanism of RSL motion to be dry granular flow. 88Flows on a dune slipface at 27ºN provide a useful comparison (Animation S1). The dune slipface setting suggests that they are dry grainflows, particularly since they 96 occur when aeolian transport is strongest (perihelion 26 ) but northern-hemisphere 97 temperatures are low and northern RSL are inactive 10 . We attribute the visibility of these 98 lineae to the presence of a small amount of dust on the surface, as shown by dust devil 99 tracks. The lineae are initially present at the same time as dust devil tracks, and both fade 100 seasonally although the lineae require longer to fade as the dust is removed or 101 redistributed. These tracks and lineae can fade much faster than crater blast zones or 102 slope streaks 27 because they involve only superficial dust on a low-albedo surface. A few 103 microns of dust can markedly brighten a dar...

Aeolian processes have likely been the predominant geomorphic agent for most of Mars' history and have the potential to produce relatively young exposure ages for geologic units. Thus, identifying local evidence for aeolian erosion is highly relevant to the selection of landing sites for future missions, such as the Mars 2020 Rover mission that aims to explore astrobiologically relevant ancient environments. Here we investigate wind‐driven activity at eight Mars 2020 candidate‐landing sites to constrain erosion potential at these locations. To demonstrate our methods, we found that contemporary dune‐derived abrasion rates were in agreement with rover‐derived exhumation rates at Gale crater and could be employed elsewhere. The Holden crater candidate site was interpreted to have low contemporary erosion rates, based on the presence of a thick sand coverage of static ripples. Active ripples at the Eberswalde and southwest Melas sites may account for local erosion and the dearth of small craters. Moderate‐flux regional dunes near Mawrth Vallis were deemed unrepresentative of the candidate site, which is interpreted to currently be experiencing low levels of erosion. The Nili Fossae site displayed the most unambiguous evidence for local sand transport and erosion, likely yielding relatively young exposure ages. The downselected Jezero crater and northeast Syrtis sites had high‐flux neighboring dunes and exhibited substantial evidence for sediment pathways across their ellipses. Both sites had relatively high estimated abrasion rates, which would yield young exposure ages. The downselected Columbia Hills site lacked evidence for sand movement, and contemporary local erosion rates are estimated to be relatively low.