Modeling aeolian sediment transport thresholds on physically rough Martian surfaces: A shear stress partitioning approach

This paper explores the effect that large roughness elements (0.30 m x 0.26 m x 0.36 m) may have on entrainment of sediment by Martian winds using a shear stress partitioning approach based on a model developed by Raupach et al. (Raupach, MR., Gillette, D.A., Leys, J.F., 1993. The effect of roughness elements on wind erosion threshold. Journal of Geophysical Research 98(D2), 3023-3029). This model predicts the shear stress partitioning ratio defined as the percent reduction in shear stress on the intervening surface between the roughness elements as compared to the surface in the absence of those elements. This ratio is based on knowledge of the geometric properties of the roughness elements, the characteristic drag coefficients of the elements and the surface, and the assumed effect these elements have on the spatial distribution of the mean and maximum shear stresses. On Mars, unlike on Earth, the shear stress partitioning caused by roughness can be non-linear in that the drag coefficients for the surface as well as for the roughness itself show Reynolds number dependencies for the reported range of Martian wind speeds. The shear stress partitioning model of Raupach et al. is used to evaluate how conditions of the Martian atmosphere will affect the threshold shear stress ratio for Martian surfaces over a range of values of roughness density. Using, as an example, a 125 mu m diameter particle with an estimated threshold shear stress on Mars of approximate to 0.06 N m(-2) (shear velocity, u(*)approximate to 2 m s(-1) on a smooth surface), we evaluate the effect of roughness density on the threshold shear stress ratio for this diameter particle. In general, on Mars higher regional shear stresses are required to initiate particle entrainment for surfaces that have the same physical roughness as defined by the roughness density term (lambda) compared with terrestrial surfaces mainly because of the low Martian atmospheric density. (C) 2009 Elsevier B.V. All rights reserved.