  The linear stability analysis of the equations governing the evolution of a
flat sand bed submitted to a turbulent shear flow predicts that the wavelength
$\lambda$ at which the bed destabilises to form dunes should scale with the
drag length $L_{\rm drag} = \frac{\rho_s}{\rho_f} d$. This scaling law is
tested using existing and new measurements performed in water (subaqueous
ripples), in air (aeolian dunes and fresh snow dunes), in a high pressure
CO$_2$ wind tunnel reproducing conditions close to the Venus atmosphere and in
the low pressure CO$_2$ martian atmosphere (martian dunes). A difficulty is to
determine the diameter of saltating grains on Mars. A first estimate comes from
photographs of aeolian ripples taken by the rovers Opportunity and Spirit,
showing grains whose diameters are smaller than on Earth dunes. In addition we
calculate the effect of cohesion and viscosity on the dynamic and static
transport thresholds. It confirms that the small grains visualised by the
rovers should be grains experiencing saltation. Finally, we show that, within
error bars, the scaling of $\lambda$ with $L_{\rm drag}$ holds over almost five
decades. We conclude with a discussion on the time scales and velocities at
which these bed instabilities develop and propagate on Mars.
