  Dust and sand motion are a common sight on Mars. Understanding the
interaction of atmosphere and Martian soil is fundamental to describe the
planet's weather, climate and surface morphology.
  We set up a wind tunnel to study the lift of a mixture between very fine sand
and dust in a Mars simulant soil. The experiments were carried out under
Martian gravity in a parabolic flight. The reduced gravity was provided by a
centrifuge under external microgravity. The onset of saltation was measured for
a fluid threshold shear velocity of 0.82$\pm$0.04 m/s. This is considerably
lower than found under Earth gravity.
  In addition to a reduction in weight, this low threshold can be attributed to
gravity dependent cohesive forces within the sand bed, which drop by 2/3 under
Martian gravity. The new threshold for saltation leads to a simulation of the
annual dust cycle with a Mars GCM that is in agreement with observations.
