Difference in the Wind Speeds Required for Initiation versus Continuation of Sand Transport on Mars: Implications for Dunes and Dust Storms

Much of the surface of Mars is covered by dunes, ripples, and other features formed by the blowing of sand by wind, known as saltation. In addition, saltation loads the atmosphere with dust aerosols, which dominate the Martian climate. We show here that saltation can be maintained on Mars by wind speeds an order of magnitude less than required to initiate it. We further show that this hysteresis effect causes saltation to occur for much lower wind speeds than previously thought. These findings have important implications for the formation of dust storms, sand dunes, and ripples on Mars.

Figure 1

Saltation impact threshold on Earth and Mars. The impact threshold simulated with COMSALT [17] for Earth (thin blue solid line) matches measurements [19] (triangles). On Mars, both numerical simulations with COMSALT [17] (thick red solid line) and recent analytical calculations [21] (thick red dashed line) show that the impact threshold is approximately an order of magnitude below the fluid threshold [1] (dash-dotted green line). Also included is the approximate shear velocity at which particles become suspended (dotted black line; obtained from $u^{*}\approx v_t$ [1, 23], where $v_t$ is the particle’s terminal velocity).

Figure 2

Seasonally reversing dune slip faces indicate active saltation at Proctor Crater, but hourly averaged wind shear velocities simulated by a mesoscale model [9] (thin black solid line) are substantially below the fluid threshold [1] (dash-dotted line) for both southern hemisphere winter (top) and summer (bottom). Sand transport nonetheless occurs because the hysteresis of Martian saltation (see text) causes the probability that saltation transport takes place at any given moment [thick blue solid line and right axis; see Eq. (3)] to be substantial when the hourly averaged wind shear velocity is on the order of the impact threshold (dashed line; from Ref. 21; assumed particle size was $740\mu \mathrm{m}$ [9]).

Figure 3

Mean particle speed and hop dimensions for saltation on Earth and Mars. Thin blue lines and symbols represent terrestrial predictions and measurements, while thick red lines denote predictions under Martian conditions. (a) Average horizontal particle speed at the surface (solid lines) and particle speed averaged over all saltating particles (dashed lines) calculated by COMSALT for Earth ($250\mu \mathrm{m}$ particles, typical of terrestrial dunes [1]) and Mars ($500\mu \mathrm{m}$ particles, typical of Martian dunes 9, 23). The symbols denote measurements of the horizontal particle speed linearly extrapolated to the surface (see Fig. 5 of Ref. [29]; we used measurements within 2 mm of the surface) of respectively $242\mu \mathrm{m}$ (squares [28] and triangles [29]) and $320\mu \mathrm{m}$ particles (diamonds [28]). Error bars were derived from the uncertainty in the fitting parameters. (b) The mean height (solid lines) and length (dashed lines) of saltation trajectories as a function of shear velocity for Earth and Mars. Included for comparison is the average deposition length [34] in field measurements (circles), which is a good measure of the average saltation length and was obtained from Fig. 8a of Ref. 34.