Unsteady saltation on Mars

Saltation is an important process on Mars, as it contributes to dust raising, bedform dynamics, and aeolian abrasion. Lander measurements and mesoscale meteorological models suggest that winds in the Martian atmosphere rarely exceed the fluid threshold value that is necessary to aerodynamically initiate saltation, a fact in stark contrast to the existence of dunes and ripples on the planet, many of which are in an active state of migration. In an attempt to reconcile these observations, we perform an unsteady simulation with a simple turbulence model to calculate the saltation transport rate. Sinusoidal wind variations are imposed on the saltation layer. The numerical simulations verify that gusty transport is one of the main manifestations of Martian sediment transport events. A formula for the saltation transport rate is reported, Qm∼(u∗-u∗it)p, where u∗ and u∗it are the friction velocity and impact threshold friction velocity. The power p varies in the range of 0.7-1.8 on Mars and ∼1.5 on Earth, depending on the period and amplitude of the gusty inflow wind. Our results show that the law of Martian and terrestrial transport rate are not universal, and hence one should be cautious when trying to extrapolate existing terrestrial results to Mars.