Ripples occur on Earth and Mars in a range of sizes. From terrestrial studies, ripple size is known to depend on grain‐size frequency, wind duration, wind strength (including stronger winds that can flatten ripples), and fundamental environmental factors that differ between the two planets. Here we use computational fluid dynamics (CFD) experiments to model boundary layer shear stresses applied to aeolian ripple surfaces, to investigate how these stresses might differ between Earth and Mars. CFD experiments used ANSYS Fluent, with inlet wind speeds of 10 and 15 m/s for both planetary environments. Ripple profiles for Earth and Mars were developed using saltation and reptation properties modeled by the numerical saltation model COMSALT (Kok & Renno, 2009, https://doi.org.10.1029/2009JDO11702) to develop ripple profiles using the numerical technique of Yizhaq et al. (2004, https://doi.org.10.1016/j.physd.2004.03.015). Although the CFD experiments using these inputs could not include the effects of a saltation cloud, results are robust enough to indicate clearly that for similar modeled wind speeds on Earth and Mars, boundary layer shear stress applied to ripple surfaces is greater on Earth under conditions for which sand transport is expected. These results indicate that ripples can grow larger on Mars than on Earth, because for typical Martian wind speeds the shear velocity at the ripple crests is below the fluid threshold.

Aeolian ripples are abundant in arid regions on Earth and on the surface of Mars. On Earth, wind of sufficient strength can begin mobilizing sand grains into "saltation" (bouncing) trajectories that disturb other grains on the bed with every rebound, quickly leading to a cascading mobilization of many other grains. In fully developed saltation, shear stress at the bed is reduced by momentum exchange between the boundary layer and numerous grains in flight, so that continued grain mobilization from the bed is due primarily to impact effects from high-speed saltating grains (e.g., Bagnold, 1941, pp. 31-33). Ripples formed during this process are referred to as "impact ripples" or "ballistic ripples" (Figure 1) to distinguish them from ripples forming underwater by a different, fluid drag process. Impact ripples in relatively well-sorted desert sands Abstract On Mars, large aeolian ripples with wavelengths typically 1-3 m but lacking very coarse sand at crests have been encountered by rovers and observed from orbit. These bedforms have no terrestrial counterpart and several hypotheses for origins have been proposed. This work reports results of Computational Fluid Dynamics (CFD) experiments with ANSYS Fluent under terrestrial and Martian boundary layer conditions, using the k − ω SST turbulence model to evaluate shear stress along a topographic profile of large Martian ripples at different boundary layer wind speeds. Results indicate that, compared with Earth conditions: (1) boundary-layer flow along large ripples under Martian conditions is less turbulent due to higher kinematic viscosity; (2) reverse-flow vortex regions from crests at ripple lee flanks are larger; and (3) shear stresses at crests of large ripples are relatively low, so ripple flattening is less likely at high wind speeds. These results indicate Martian ripples formed by the saltation impact splash mechanism should be less constrained by shear stress effects limiting growth of exposed ripple crests, because the low-density Martian atmosphere applies relatively low wind-related shear stress to ripple surfaces. Other origins for the large Martian ripples are not excluded, however. On Earth, very large ripples with crests unprotected by very coarse grains do not develop due to higher wind-related shear stresses. Plain Language Summary Ripples of wind-blown sand are ubiquitous in terrestrial sandy deserts, and also on the arid surface of Mars. On Earth, aeolian ripples in typical dune sands have wavelengths <30 cm and develop when surface grains are disturbed by numerous rebounding impacts of high-speed, wind-driven grains that bounce rapidly downwind. On Mars, rovers have observed aeolian ripples in similar sands but of much greater size, including numerous examples with wavelengths >2 m. There is no consensus explanation why much larger ripples develop on Mars than on Earth. Work reported here describes Computational Fluid Dynamics (CFD) numerical flow experiments using a ground profile modeled after large Martian ripples. Experiments with this model...

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