Martian Slope Streaks and Gullies: Origins as Dry Granular Flows

Streaks and gullies (Figs 1,2) are common on Martian slopes, and are geologically young; slope streaks have formed during the last few years of Mars Global Surveyor imaging. Both slope streaks [1] and gullies [2] involve flow of granular material, but it is not clear whether liquid water (or another suspending agent) was involved. The possibility that liquid water was involved makes gullies and slope streaks important for understanding Mars’ recent climate and for the hope of extant life near its surface. Here, we show that significant features of slope streaks and gullies are consistent with dry flows of granular material. Liquid water may not be required. Granular Flows: Recent progress in the theory of granular flows has allowed good quantitative predictions of their behavior [3-5]. The crucial advance was recognition that inelastic collisions and enduring frictional contacts must be included in energy and momentum balances in the flows. These theoretical models [3,4] have replicated well the behaviors of steady granular flows or avalanches on rigid surfaces: slope angles for flow, flow thicknesses, flow rates, and dynamic behaviors (like upward moving waves [3]). Once initiated, granular avalanches propagate by at least two mechanisms [5-7]. Avalanches propagate only downhill on a shallower or less oversteepened slope, covered with a thinner layer of mobile grains. These avalanches develop a triangular plan form, opening downward at a constant angle Ψ [5-8,1]. The angle Ψ increases with slope steepness and oversteepening, to a point at which another mechanism begins. On steeper and more oversteepened slopes, and those with thick layers of mobile grains, avalanches propagate uphill and excavate deeply into the slope [6,7]. If a flow contains grains of different sizes, they can become segregated [9-11]. In unconfined flows over rough surfaces, larger grains rise to the flow tops by several mechanisms including sieving and squeeze expulsion [9,12]. Flow tops move faster than bases, thus transporting larger grains to the flow front and edges, where they tend to remain [10,12,14]. The abundance of larger particles at a flow’s front can cause instability if the larger particles are rougher than the smaller. A retardation in the flow front would then attract larger particles, which would retard that point more, and split the flow front into fingers [10]. Slope Streaks: Slope streaks (Fig. 1) are commonly interpreted as granular flows [1], avalanches of thin layers of bright, wind-deposited dust exposing