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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 modeled surface were conducted in Earth as well as Mars conditions to compare levels of turbulence and shear stress applied by wind to the ripple surfaces. Results indicate that (1) wind‐related stresses would be much weaker on Mars than on Earth and (2) wind flow is less turbulent compared with terrestrial conditions. These results indicate that aeolian impact ripples could grow larger on Mars because lower wind stresses should allow ripple crests to reach greater heights (with corresponding greater wavelengths) than on Earth. Key Points: Computer Fluid Dynamics (CFD) numerical simulations of wind over profiles of large ripples were conducted under Earth and Mars conditionsCFD results indicate larger reverse‐flow vortex regions from crests at ripple lee flanks as well as less turbulence on Mars, compared with EarthCFD results indicate lower shear stresses along ripple surfaces under Mars conditions than Earth conditions [ABSTRACT FROM AUTHOR]