Numerical modeling of basaltic sand ripples on Eagle crater as indirect evidence for the hysteresis effect in Martian saltation

Aeolian ripples, which form regular patterns on sand beaches and desert floors and also on Mars, indicate the instability of flat sand surfaces under the wind-induced transport of sand grains. The opportunity rover documented small normal basaltic sand ripples at the bottom of Eagle crater in Meridinai planum. These ripples are composed of fine basaltic sand (100 micron) and their average wavelength and height are 10 cm and 1 cm respectively. Such light particles are thought to be easily suspended by turbulence at the fluid threshold, such that the wind speed at which these bedforms developed must be substantially below the fluid threshold. The occurrence of these bedforms on the Martian surface thus requires the impact threshold to be substantially smaller than the fluid threshold. Recently, it was suggested that saltation on Mars can be maintained at much lower wind speeds than the fluid threshold which is needed to initiate it (Kok, 2010). We used simulations of the steady state saltation model COMSALT together with a dynamic model for sand ripples (Yizhaq et al., 2004) to show that the small basaltic ripples can develop under wind speeds below the threshold for suspension. We used COMSALT to give the basic values of the parameters that used by the ripple model for saltation on Mars with and without cohesion: 1. The average number of reptating grains per impact of one saltating grain. 2. The number density of impact saltating grains on flat surface. 3. The probability distribution of reptation lengths. We used COMSALT results to calculate the sand flux on Mars for different shear velocities and used GCM models simulations for prediction of the sand flux under predicted wind regime and compare it with recent estimations (Bridges et al., 2012). Our numerical simulations (Fig. 1) show that ripples like the basaltic ripples on Eagle crater can be developed by shear velocity much below the fluid threshold by the impact mechanism. These findings can be regarded as an indirect proof of the unique saltation mechanism on Mars. References Bridges, N. T. et al. 2012. Earth-like sand fluxes on Mars. Nature 485, 339-342. Kok, J. F. 2010. Differences in the wind speeds required for initiation versus continuation of sand transport on Mars: Implications for dunes and sand storms. Physical Review Letters, 104, 074502. Yizhaq, H., Balmforth, N.J and Provenzale, A. 2004. Blown by wind: Nonlinear dynamics of aeolian sand ripples. Physica D, 195, 207-228. Model simulation of normal ripples on Mars with parameters computed by COMSALT for different shear velocities with and without cohesion after one hour. The inset shows the final ripples profile for u*=0.65 m/s.