  The wind-driven hopping motion of sand grains, known as saltation, forms
dunes and ripples and ejects fine dust particles into the atmosphere on both
Earth and Mars. While the wind speed at which saltation is initiated, the fluid
threshold, has been studied extensively, the wind speed at which it is halted,
the impact threshold, has been poorly quantified for Mars conditions. We
present an analytical model of the impact threshold, which we show to be in
agreement with measurements and recent numerical simulations for Earth
conditions. For Mars conditions, we find that the impact threshold is
approximately an order of magnitude below the fluid threshold, in agreement
with previous studies. Saltation on Mars can thus be sustained at wind speeds
an order of magnitude less than required to initiate it, leading to the
occurrence of hysteresis. These results confirm earlier simulations with a
detailed numerical saltation model, and have important implications for the
formation of sand dunes, ripples, and dust storms on Mars.
