This article here talks about dunes visible on comet 67P:
In August, the European Space Agency (ESA) achieved a major success when its Rosetta probe rendezvoused with comet 67P. The spacecraft has been returning spectacular images of this strange little world, located midway between the orbits of Mars and Jupiter. Among its latest photographs is the image below showing what appear to be dunes like those found on the deserts and beaches of Earth.
The comet would acquire an extremely thin atmosphere when it is close to sun. But this atmosphere would have very low density and would not produce winds as on earth.
So how can there be sand and small rocks on 67P?
