Saltation and Martian sandstorms

Abstract

A brief topical review of current knowledge available for predicting the characteristics of sandstorms is presented. Particular emphasis is placed on extrapolations to Mars where saltation of loose surface material appears to be a necessary condition for initiation of observed dust storms. Considerable data are now available to predict threshold winds under earth's environmental conditions, but techniques for predicting the quantity of material transported and the height and velocity to which it travels are still crude. Extrapolation of current knowledge to Mars is probably sufficient only for estimating purposes and landing craft design. The upcoming Viking landing mission on Mars will, it is hoped, shed additional light on the problem. Owen [1964] presents a solution, qualitatively valid for the 135 136 MAEGLEY: SALTATION AND MARTIAN SANDSTORMS Altitude, z Em] 0.012F

Aeolian sand transport: a wind tunnel model

“…Sand transport rate increases with an increase in wind velocity, but decreases with an increase in grain size. This, in a qualitative sense, agrees with Kawamura's (1951), Kind's (1976), and White's (1979) conclusions, but disagrees with Bagnold's (1941), Zingg's (1953), Hsu's (1971), Maegley's (1976, and Lettau and Lettau's (1978) results. The first attempt here is to relate transport rate to wind velocity at the centerline height of the wind tunnel.…”

“…This type of equations have relatively sound theoretical support but a very severe limitation that they output meaningless sand transport when the shear velocity is less than the threshold. To overcome this drawback, the threshold shear velocity is included in the modified Bagnold-type equations (Kawamura, 1951;Owen, 1964;Iversen et al, 1976;Kind, 1976;Maegley, 1976;Lettau and Lettau, 1978;White, 1979). In fact, Bagnold's equation also has an implicit threshold term that makes the equation used only when the sand is moving.…”

“…The effect of grain size on transport rate differs greatly between models. In some models (Zingg, 1953;Maegley, 1976;Lettau and Lettau, 1978), transport rate increases with an increase in grain size, while in some other models (Kawamura, 1951;Kind, 1976;White, 1979), transport rate decreases with an increase in grain size. In Hsu's (1971) model, the 0.325-mm sand has the least transport rate.…”

“…Ever since, many successors have attempted to evaluate the previous or develop new models. Consequently, many models have been published (Kawamura, 1951;Zingg, 1953;Kuhlman, 1958;Owen, 1964;Kadib, 1965;Hsu, 1971;Kind, 1976;Maegley, 1976;Radok, 1977;Lettau and Lettau, 1978;Nakashima, 1979;Takeuchi, 1980;Horikawa et al, 1984;Sarre, 1987;Sherman and Hotta, 1990;Werner, 1990;Sherman et al, 1998). All of the published models are based on a set of ideal conditions.…”

Wind tunnel Study on Aeolian Saltation Dynamics and Mass flow

“…From these results, it is argued that Eqn (4) has important implications for roughness height prediction in winds of different traction. The significance of Eqn (4) can be considered from the fact that it accounts for the U * /U *t ratio, which defines the concentration of sand in the saltation layer (Maegley, 1976;Sarre, 1988;Li & Martz, 1995;Spies et al, 1995), thus satisfying the basic law of aeolian saltation dynamics and mass flow: Z 0 0 BZ 0 for U * pU *t . In contrast, Eqn (3) suffers from the limitation that it tends to predict unrealistic values of roughness height in the saltation cloud when the friction velocity is well below the surface threshold value.…”