Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water

Abstract

. 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...

Observational evidence for a dry dust-wind origin of Mars seasonal dark flows

“…In this paper, we have reviewed the observational evidence previously derived from visible imagery and near-IR spectral imagery in favour of a contribution of liquid water to the initiation and/or development of RSL still considered in recent studies (Dundas et al, 2017). We showed in section 2 that instrumental biases have been misinterpreted as evidence for salts at RSL.…”

Geologic Constraints on Early Mars Climate

“…Runoff at Lyot crater -either ice-melt or snowmelt -marks the youngest definite climate-driven water runoff on Mars. Shallow <100°C groundwater at 0.1-1.7 Ga is recorded by meteorites, but it is not clear if this shallow groundwater required a wet climate (Borg & Drake 2005, Swindle 2000, Nemchin et al 2014 (Dundas et al 2017a(Dundas et al , 2017bLeask et al 2018).…”

The Polygonal Surface Structures in the Dalangtan Playa, Qaidam Basin, NW China: Controlling Factors for Their Formation and Implications for Analogous Martian Landforms

“…The geomorphology of younger craters with fluidized ejecta blanket (Carr et al, 1977;Mouginis-Mark, 1987) and more recently recurring slope lineae (Chevrier & Rivera-Valentin, 2012;Levy, 2012;McEwen et al, 2011McEwen et al, , 2014Ojha et al, 2014Ojha et al, , 2015 all indicate that the ice-rich and/or salt-ice mixture materials (producing enough brine to account for the recently observed flow features) still exist in today's Mars surface/subsurface. Though recurring slope lineae were recently interpreted as dry mass wasting (granular flows; Dundas et al, 2017), the strong seasonality associated with warm slopes and the detection of hydrated salts are still consistent with some role for water in their initiation (Dundas et al, 2017). Therefore, the remelting of the salt-ice mixture materials and subsequent evaporation would have contributed to the formation of PSS landforms 10.1029/2018JE005525…”