  The illuminated dusty surface of Mars acts like a gas pump. It is driven by
thermal creep at low pressure within the soil. In the top soil layer this gas
flow has to be sustained by a pressure gradient. This is equivalent to a
lifting force on the dust grains. The top layer is therefore under tension
which reduces the threshold wind speed for saltation. We carried out laboratory
experiments to quantify the thickness of this activated layer. We use basalt
with an average particle size of 67 $\mu$m. We find a depth of the active layer
of 100 to 200 $\rm \mu m$. Scaled to Mars the activation will reduce threshold
wind speeds for saltation by about 10%.
