Gas dynamics model and results. Upper right: surface density boundary condition used for the calculation. Upper left: resulting density in the x–z plane at a y position of +900 m providing a slice directly above the putative aeolian ripples. The white area indicates that the slice cuts through the nucleus to the left of the ripples. Lower left: gas speed in the domain. Note the high speed just to the right of the nucleus at the approximate position of the ripples. Lower right: y velocity of the gas speed. This illustrates that the gas is mostly directed downward, in good agreement with the ripple orientation at velocities of around 500 m/s.