Numerical modeling of wind-blown sand on Mars

Abstract

Recent observation results show that sand ripples and dunes are movable like those on Earth under current Martian climate. And the aeolian process on Mars therefore is re-attracting the eyes of scientific researchers in different fields. In this paper, the spatial and temporal evolution of wind-blown sand on Mars is simulated by the large-eddy simulation method. The simulations are conducted under the conditions of both friction wind speed higher and lower than the "fluid threshold", respectively. The fluid entrainment of the sand particles, the processes among saltation sand particles and sand bed, and the negative feedback of sand movement to flow field are considered. Our results show that the "overshoot" phenomenon also exists in the evolution of wind-blown sand on Mars both temporally and spatially; impact entrainment affects the sand transport rate on Mars when the wind speed is smaller or larger than the fluid threshold; and both the average saltation length and height are one order of magnitudes larger than those on Earth. Eventually, the formulas describing the sand transport rate, average saltation length and height on Mars are given, respectively.

Unsteady saltation on Mars

“…0 and T ? 1) transport coefficients are smaller than those (p = 2.0 and A = 223.7) in Almeida's general expression but somewhat close to those (p = 2.0 and A = 26.4) of Huang et al (2014). It is not surprising because the continuous super-fluid threshold transport is dominant on Earth.…”

“…Additional research is thus required to determine the frequency, characteristic timescale and fluctuating intensity of Martian gusty wind by in-situ measurements and high resolution models. Unfortunately however, the universal scaling law of the transport rate on both planets cannot be obtained as previous studies (Almeida et al, 2008;Huang et al, 2014) suggested. Eq.…”

“…Those for Earth are q = 1.20 kg/m 3 , m = 1.78 Â 10 À5 kg/m s, g = 9.81 m/s 2 and q p = 2650 kg/m 3 respectively. The height of the simulation region is H = 1 m for Earth and H = 10 for Mars in consideration of the huge differences in the trajectory height (Greeley et al, 1976;Almeida et al, 2008;Huang et al, 2014). Regardless of the complexity in threshold friction velocity, we adopt Bagonld's expression with A 0 = 0.1 for a better comparison between the results on Mars and those on Earth.…”

“…Indeed, as shown by Mars missions, much of the observed soil erosion on Mars could have occurred in a few to several tens of seconds (Arvidson et al, 1983;Moore, 1985), while the simulations of Huang et al (2014) had reported a response time of 7-8 s. This means that steady state transport could not last for a long time on Mars. In the extreme case, saltation might begin to attenuate before reaching the ''equilibrium state''.…”

“…By considering the hypothesis that once saltation is initiated by a localized wind gust, particle entrainment succeeds mainly through particle -soil collisions and saltation can be sustained at a smaller impact threshold u it (Claudin and Andreotti, 2006;Almeida et al, 2008;Kok, 2010), Kok (2010) proposed a model to calculate the substantial probability of saltation transport by summing the probability of wind speed larger than fluid threshold u ft needed to initiate saltation and the probability of u it < u < u ft . Subsequently, based on the model of Kok (2010), Huang et al (2014), simulated the spatial and temporal evolution of wind-blown sand on Mars. They found that both the average saltation length and height are consistent with those calculated by Kok (2010), but substantially smaller than found by Almeida et al (2008).…”