  It is now an accepted fact that the size at which dunes form from a flat sand
bed as well as their `minimal size' scales on the flux saturation length. This
length is by definition the relaxation length of the slowest mode toward
equilibrium transport. The model presented by Parteli, Duran and Herrmann
[Phys. Rev. E 75, 011301 (2007)] predicts that the saturation length decreases
to zero as the inverse of the wind shear stress far from the threshold. We
first show that their model is not self-consistent: even under large wind, the
relaxation rate is limited by grain inertia and thus can not decrease to zero.
A key argument presented by these authors comes from the discussion of the
typical dune wavelength on Mars (650 m) on the basis of which they refute the
scaling of the dune size with the drag length evidenced by Claudin and
Andreotti [Earth Pla. Sci. Lett. 252, 30 (2006)]. They instead propose that
Martian dunes, composed of large grains (500 micrometers), were formed in the
past under very strong winds. We show that this saltating grain size, estimated
from thermal diffusion measurements, is not reliable. Moreover, the microscopic
photographs taken by the rovers on Martian aeolian bedforms show a grain size
of 87 plus or minus 25 micrometers together with hematite spherules at
millimetre scale. As those so-called ``blueberries'' can not be entrained by
reasonable winds, we conclude that the saltating grains on Mars are the small
ones, which gives a second strong argument against the model of Parteli et al.
