  Explaining the unexpected presence of dune-like patterns at the surface of
the comet 67P/Churyumov-Gerasimenko requires conceptual and quantitative
advances in the understanding of surface and outgassing processes. We show here
that vapor flow emitted by the comet around its perihelion spreads laterally in
a surface layer, due to the strong pressure difference between zones
illuminated by sunlight and those in shadow. For such thermal winds to be dense
enough to transport grains -- ten times greater than previous estimates --
outgassing must take place through a surface porous granular layer, and that
layer must be composed of grains whose roughness lowers cohesion consistently
with contact mechanics. The linear stability analysis of the problem, entirely
tested against laboratory experiments, quantitatively predicts the emergence of
bedforms in the observed wavelength range, and their propagation at the scale
of a comet revolution. Although generated by a rarefied atmosphere, they are
paradoxically analogous to ripples emerging on granular beds submitted to
viscous shear flows. This quantitative agreement shows that our understanding
of the coupling between hydrodynamics and sediment transport is able to account
for bedform emergence in extreme conditions and provides a reliable tool to
predict the erosion and accretion processes controlling the evolution of small
solar system bodies.
